Creating a Masterpiece. The Road to (and Beyond) Woodward and Hoffmann's 1969 Angewandte Chemie Treatise

Abstract

This publication presents the history and key scientific content of Woodward and Hoffmann's publications from 1966 to 1973. It was during the writing of the Angewandte Chemie treatise, published in December 1969, that the Woodward‐Hoffmann collaboration became intimate and reached its apex. Highlights in the development of this publication include letters and notes exchanged between Woodward and Hoffmann, excerpts from the drafts of the publications, and early stage, hand drawn chemical graphics. Several figures contain a series of excerpts from Woodward and Hoffmann's drafts that illustrate the evolution of their ideas, text, and chemical pictography.

This publication describes how R. B. Woodward and Roald Hoffmann crafted their masterpiece publications. Illustrations include Woodward's first draft of the famous “Violations There are none. Nor can violations be expected of so fundamental a principle of maximum bonding.” Original but discarded text shows the stepwise paths toward the W‐H masterpieces.

1.

Table of Contents

• 1.Introduction…4
• 2.A Hint at Understanding the Organization of This Publication and the Background and Context for the Woodward‐Hoffmann rules…7
• 3.Invitation from the Editor of Angewandte Chemie to Write “a Somewhat More Elaborate Review”…8
• 4.
  W‐H 11a: On the Invitation from Angewandte Chemie and the First Drafts by Hoffmann…9

  • 4.a.
    Why Hoffmann and not Woodward?…9
  • 4.b
    Hoffmann's First Drafts of the Angewandte publication: Content…10
  • 4.c
    Hoffmann's First Drafts of the Angewandte publication: Chronology…13
• 5.
  W‐H 10: Accounts of Chemical Research: “The Conservation of Orbital Symmetry”…15

  • 5.a
    The Invitation…15
  • 5.b
    Content From Hoffmann's First Angewandte Draft Used in the Accounts Publication…16
  • 5.c
    Accounts Final Dteps: Authorship, Order of Names, and Dubmission…18
  • 5.d
    From Hoffmann's Laboratory Notebooks…19
  • 5.e
    What is New in the Accounts Publication?…21
  • 5.f
    What is Not in the Accounts publication?…23
  • 5.g
    Summary…23
  • 5.h
    Onward…23
• 6.
  W‐H 11a: Angewandte Chemie (International Edition in English and German‐language Edition): “The Conservation of Orbital Symmetry” and “Die Erhaltung der Orbitalsymmetrie”…24

  • 6.a
    Angewandte Chemie's Invitation and the Fulfillment…24
  • 6.b
    Writing the Angewandte Chemie Manuscript…26

    • 6.b.1
      An Overview of Hoffmann's and Woodward's Writing of the Angewandte Chemie Manuscript. A treatise Without an Organizational Plan…26
    • 6.b.2
      Dating the Drafts and Documents Relevant to the Angewandte Chemie Publication…26
    • 6.b.3
      Topic Trajectories in the Drafts. The Table of Contents…27
    • 6.b.4
      Major Modifications, Even at the Very End…28
    • 6.b.5
      Hoffmann Often Urged Woodward to Write…31
    • 6.b.6
      Woodward and Hoffmann's working sessions at Harvard in 1967 and 1968…33
  • 6.c
    Hoffmann's Contributions to the Drafts…34

    • 6.c.1
      Hoffmann Sharing New Inserts with Woodward…34
    • 6.c.2
      Theory New to W‐H…35
    • 6.c.3
      The Generalized Selection Rule…37
    • 6.c.4
      Hoffmann Brought Literature References to Woodward…37
    • 6.c.5
      Hoffmann Corrected Errors in the Structures…39
    • 6.c.6
      Hoffmann predicted new chemistry…39
    • 6.c.7
      From Hoffmann's Laboratory Notebooks to the W‐H Treatise…39
    • 6.c.8
      Hoffmann's Additional Inserts to Later Drafts of the Angewandte manuscript…40
    • 6.c.9
      Hoffmann Seeking Woodward's Guidance (and Hoffmann Making a Prediction)…42
  • 6.d
    Woodward's Contributions to the Erafts…43

    • 6.d.1
      Woodward as the “Secretary”…43
    • 6.d.2
      The Table of Contents…44
    • 6.d.3
      The Introduction…44
    • 6.d.4
      On MO theory, Orbitals and Bonding, Correlation Diagrams…50
    • 6.d.5
      On the Photochemical Transformation of 1,3‐Butadiene to Bicyclobutane…50
    • 6.d.6
      Sigmatropic Reactions…50
    • 6.d.7
      On Ketenes…51
    • 6.d.8
      “Violations. There are None!”…53
    • 6.d.9
      The Generalized Selection Rule…53
    • 6.d.10
      The “Conclusion” Section…53
    • 6.d.11
      Woodward's Words…56
    • 6.d.12
      Woodward's Influence on Hoffmann's Words…61
    • 6.d.13
      References and Figures…61
    • 6.d.14
      Woodward Seeking Hoffmann's Guidance…61
  • 6.e
    What is New in the Angewandte treatise?…61

    • 6.e.1
      New Nomenclature…61

      • 6.e.1.a
        Preface…61
      • 6.e.1.b
        “Conservation of Orbital Symmetry”…64
      • 6.e.1.c
        “Pericyclic” and “Annulotropic”…64
      • 6.e.1.d
        “Cheletropic”…66
      • 6.e.1.e
        “Cycloreversion”…67
      • 6.e.1.f
        “Concerted”…68
      • 6.e.1.g
        Words That Were Not Adopted by the Community…68
      • 6.e.1.h
        W‐H's “Device of Denominating the Type of Orbital Involved in a Concerted Reaction”…68
    • 6.e.2
      New Chemistry…68

      • 6.e.2.a
        Preface…68
      • 6.e.2.b
        Following the Molecular Orbitals. Pedagogy in MO Theory…68
      • 6.e.2.c
        All Pericyclic Reactions are Cycloadditions. Trans Addition of 1,3‐Butadiene to Cyclobutene…69
      • 6.e.2.d
        A Common Mechanism and Its Consequence, the Generalized Selection Rules…71
      • 6.e.2.e
        Ketene Cycloadditions…71
      • 6.e.2.f
        Nonlinear Reaction Trajectories…72
    • 6.e.3
      Some Odd Topics…72
  • 6.f
    What is not in the Angewandte Chemie Treatise?…72

    • 6.f.1
      The “Catalog of Concerted Reactions”…72
    • 6.f.2
      Nomenclature…72
    • 6.f.3
      Allowedness via Transition State Aromaticity; H FC;ckel Versus Möbius Transition States…73
    • 6.f.4
      Whether Many of the Stated Reactions Are Actually Concerted or Not…74
    • 6.f.5
      What to Do ‘If There is No Relevant Symmetry Element’ in the Reaction?…75
    • 6.f.6
      Sigmatropic Reactions: Following the Orbitals…75
    • 6.f.7
      Concerted S _N 2 Reactions…75
    • 6.f.8
      On Pedagogy…75
    • 6.f.9
      Photochemistry…76
    • 6.f.10
      Credit Attributions…77

      • 6.f.10.a
        Authorship Order and Credit…77
      • 6.f.10.b
        Credit to Others…77
    • 6.g
      Production and Distribution of the Treatise…80

      • 6.g.1
        Graphics…81
      • 6.g.2
        Color…81
      • 6.g.3
        On Woodward's Distribution of the Submitted Manuscript…82
    • 6.h
      Major Accomplishments of the 1969 Treatise…82
    • 6.i
      Woodward's Pride in “The Conservation of Orbital Symmetry”…83
  •  
    7. W‐H 11b: The Wiley Monograph (the Teprint of the Angewandte Chemie Publication (W‐H 11a))…84
  •  
    8. W‐H 12: Science…84
  •  
    9. W‐H 13a and W‐H 13b: Chemie in unserer Zeit and Kemisk Tidskrift…86

    • 9.a
      Invitation and Publication Details…86
    • 9.b
      Substance of Chemie in unserer Zeit (W‐H 13a) and Kemisk Tidskrift (W‐H 13b)…87
    • 9.c
      Conclusions…87
    • 9.d
      Coda…87
  •  
    10. W‐H‐14: R. B. Woodward's “The Arthur C. Cope Award Lecture”…89
  •  
    11. Woodward‐Hoffmann and the Social Construction of Science…91

    • 11.a
      The Effect of Quantum Chemistry on the Sociology of Chemistry…91
    • 11.b
      The Matthew Effect…91
    • 11.c
      The Consequences…92

      • 11.c.1
        On Woodward…92
      • 11.c.2
        On Hoffmann…92
      • 11.c.3
        On Cornell…92
      • 11.c.4
        On Angewandte Chemie…92
  •  
    12. Overall “Master” Conclusions…92
  •  
    Acknowledgments…94
  •  
    References…94
  •  
    Supporting Information…105

“It is now in the third and final draft, and 3/4 done. The longer we wait, the longer the paper gets” [1].

‐‐ Roald Hoffmann, in a 1968 letter to his friend Lionel Salem

2. Introduction

This essay is the story of Woodward and Hoffmann's “Heart and Soul” articles that were published in in the late 1960s. But this story begins much earlier.

In 1965, Woodward and Hoffmann published a series of five communications in the Journal of the American Chemical Society (JACS). These five publications rather concisely presented an orbital symmetry‐based analysis for electrocyclizations [2] cycloadditions [3], and sigmatropic reactions [4]. They also postulated secondary molecular orbital effects that control the stereochemistry of certain Diels‐Alder [5] and Cope reactions [6]. I refer to these as Woodward and Hoffmann's five “Core Publications” (Table 2) [7].

TABLE 2.

| Tabulation of the Woodward‐Hoffmann publications and not submitted manuscripts, including lectures given by Woodward and Hoffmann on the topic that were published in their entirety (NP = not published).

	Date received (and/or accepted or published) or date completed	Publication
The “Core” Publications
W‐H 1	Received Nov. 30, 1964 Published January 20, 1965	R. B. Woodward, R. Hoffmann, “Stereochemistry of Electrocyclic Reactions,” Journal of the American Chemical Society 87 (1965): 395–397.
W‐H 2	Received March 22. 1965 Published May 5, 1965	R. Hoffmann, R. B. Woodward, “Selection Rules for Concerted Reactions.” Journal of the American Chemical Society 87 (1965): 2046–2048.
W‐H 3	Received April 30, 1965 Published June 5, 1965	R. B. Woodward, R. Hoffmann, “Selection Rules for Sigmatropic Reactions,” Journal of the American Chemical Society 87 (1965): 2511–2513.
W‐H 4	Received August 16, 1965 Published October 5, 1965	R. Hoffmann, R. B. Woodward, “Orbital Symmetries and endo‐exo Relationships in Concerted Cycloaddition Reactions,” Journal of the American Chemical Society 87 (1965): 4388–4389.
W‐H 5	Received August 16, 1965 Published October 5, 1965	R. Hoffmann, R. B. Woodward, “Orbital Symmetries and Orientational Effects in a Sigmatropic Reaction,” Journal of the American Chemical Society 87 (1965): 4389–4390.
The “Treading‐Water” Publications
W‐H 6NP	Summer to Autumn 1965. Not submitted. Sections inserted in other W‐H publications.	R. Hoffmann, “Some Further Selection Rules for Concerted Cycloadditions”
W‐H 7	Published in February 1966.	R. Hoffmann, “The Electronic Structure of Some Transition States and Intermediates in Organic Reactions,” Transactions of the New York Academy of Sciences 28 (Series II (1966): 475–479.
W‐H 8NP	Late 1965 to early 1966. Not submitted but inserted into early drafts of the 1969 Angewandte Chemie treatise, then deleted prior to submission.	R. Hoffmann, “A Catalogue of Concerted Reactions” (See the Section S10)
W‐H 9	Based on Woodward's lecture presented on July 8, 1966. Published 1967	R. B. Woodward, “The Conservation of Orbital Symmetry” in “Aromaticity, Publication No. 21”, The Chemical Society, 1967, pp. 217–249.
The “Heart and Soul” Publications
W‐H 10	Received July 3, 1967 Published January 1, 1968.	R. Hoffmann, R. B. Woodward, “The Conservation of Orbital Symmetry,” Accounts of Chemical Research 1 (1968):17−22.
W‐H 11a	Received January 7, 1969 Published November 1969	R. B. Woodward, R. Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Ed. 8 (1969): 781–853; and R. B. Woodward, R. Hoffmann, “Die Erhaltung der Orbitalsymmetrie,” Angewandte Chemie 21 (1969): 797–870.
W‐H 11b	W‐H 11a, reformatted and reprinted in monograph form, hardback (and soft cover editions), in English and in German, 1970.	R. B. Woodward, R. Hoffmann, “The Conservation of Orbital Symmetry,” Verlag Chemie, 1970; and R. B. Woodward, R. Hoffmann, “Erhaltung der Orbitalsymmetrie,” Verlag Chemie, 1970.
The “Looking Back” Publications
W‐H 12	Published February 6, 1970	R. Hoffmann, R. B. Woodward, “Orbital Symmetry Control of Chemical Reactions,” Science 167 (1970): 825–831.
W‐H 13a	Published in 1972 in Germana	R. Hoffmann, R. B. Woodward, “Das Konzept von der Erhaltung der Orbitalsymmetrie,” 6 (1972): 167–174.
W‐H 13b	Published in 1973 in Swedishb	R. Hoffmann, R. B. Woodward, “Den reaktionskontrollerande orbitalsymmetrin,” Kemisk Tidskrift 85 (1973): 28–30, 33−34, 37.
W‐H 14	Lecture given on August 28, 1973. Published in 2001	R. B. Woodward, “The Arthur C. Cope Award Lecture,” In Robert Burns Woodward. Architect and Artist in the World of Molecules, (Eds.: O. T. Benfey, P. J. T. Morris), Chapter IV, Chemical Heritage Foundation, Philadelphia, PA, 2001.
“A Claim of Plagiarism” Response Publication
W‐H 15	Published in 2004	R. Hoffmann, “A Claim on the Development of the Frontier Orbital Explanation of Electrocyclic Reactions,” Angewandte Chemie International Edition 43 (2004): 6586–6590.

^a Written in English by Hoffmann and translated into German by the publisher.

^b Written in English by Hoffmann and translated into Swedish by the publisher.

During the next 15 months, Woodward and Hoffmann—individually and jointly—wrote four more manuscripts [8, 9, 10, 11] and published two of these [9, 11]. The two published articles contained some new chemistry and Woodward's personal history as to his pathway into pericyclic chemical research. I refer to these as Woodward and Hoffmann's “Treading Water” publications [7].

Woodward and Hoffmann published two articles between 1968–1973, a period which the Woodward‐Hoffmann collaboration reached its apex [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. The first of these two articles (W‐H 10 in Table 2) appeared in the inaugural issue of Accounts of Chemical Research (January 1968) [12]. This essay was essentially an abbreviated summary of their five JACS communications with concise pointers to some new chemistry. The highlight of the heart‐and‐soul publications is their 1969 treatise published in Angewandte Chemie in English [13] and in German [14] (W‐H 11a). This treatise was republished in 1970 as monographs, also in English [15] and German (W‐H 11b) [16] (as these publications are essentially identical, in this publication I will typically refer only to the English version of the 1969 Angewandte edition [13] though I am actually referring to all four, including the softback editions published later).

The major scientific advances provided in these heart‐and‐soul publications included:

• •Definitions and nomenclature for new concepts;
• •Expanded pedagogy;
• •The concept of antarafacial cycloadditions;
• •The identification and mechanism of cheletropic reactions, group transfers and eliminations;
• •The mechanism of cycloaddition reactions of ketenes;
• •The unification of electrocyclizations, cycloadditions, sigmatropic reactions, and other reactions within the rubric of pericyclic reactions;
• •A mechanism based on orbital symmetry considerations and MO bonding characteristics for all pericyclic reactions;
• •A set of generalized selection rules for all pericyclic reactions; and ultimately
• •The precipitation or tipping point of a revolution in chemistry [22, 23].

Then came three perspectives published in English (W‐H 12) [17], in German [18] (W‐H 13a), and in Swedish (W‐H 13b) [19], written for more general audiences. The latter two were identical except for language and formatting. And there was one more publication in this grouping, singly‐authored by Woodward, on the occasion of his receipt (jointly with Hoffmann) of the first Cope Award in 1973. I refer to these three as their “Looking Back” publications.

The very last publication listed in Table 2 was written by Hoffmann in 2004. That essay responds to E. J. Corey's assertion, published in Chemical and Engineering News [24] and The Journal of Organic Chemistry [25], that Woodward plagiarized his (Corey's) explanation of the mechanism of electrocyclic reactions (note, that while Corey never used the word “plagiarize,” the actions he ascribed to Woodward coupled with the lack of credit to Corey by Woodward and Hoffmann fit the definition of plagiarism [26, 27, 28, 29]). Corey's experiences and claims and Hoffmann's experiences and position are discussed in the following four articles in this series [30, 31, 32, 33]. Woodward's relevant experiences and statements have been reported in his autobiographical essay discussed very briefly in Section 10 and in detail in a previous publication in this series [34].

The lengthy publication you are now reading tells the story of all the Woodward‐Hoffmann publications from 1968 to 1973 (W‐H 10 through W‐H 15). Each has its own story. You can read the following sections one‐at‐a‐time to get the story of a specific W‐H publication. You can do this in any order, skipping and jumping, as if each section were its very own short publication, for indeed each section could very well have been just that. Or you can read these in chronological order. Lastly, you can skip to the end and read about Woodward‐Hoffmann and the “social construction of science,” followed by the conclusions that come from all the above.

3. A Hint at Understanding the Organization of This Publication and the Background and Context for the Woodward‐Hoffmann Rules

Table 2 suggests that this publication should first discuss Woodward and Hoffmann's Accounts of Chemical Research article (W‐H 10 published January 1, 1968) and then the Angewandte Chemie publication (W‐H 11 published November 1969). But life's processes and especially research in science are often nonlinear and generally not as expected. In July 1965, Woodward received an invitation to publish a major review on the W‐H rules in Angewandte Chemie [35]. Hoffmann began to work on that publication shortly thereafter but for Woodward, progress was slow but steady. His Muse, and I capitalize the word “Muse” because Woodward frequently referred to his “Muse” [36, 37], had not been activated. In March 1967, Hoffmann received an invitation to publish an essay in the inaugural issue of Accounts of Chemical Research with a short deadline of early July 1967 [38]. To meet the Accounts deadline, Hoffmann cannibalized parts of the Angewandte draft he had written and used these in an Accounts draft. Thus, the historical, chronological order in this publication is Angewandte then Accounts, and then back to Angewandte.

To simplify the text, the Accounts of Chemical Research article will be referred to as the Accounts essay. The Angewandte Chemie article will be referred to as the Angewandte treatise. With that summary explanation, the stories begin.

4. Invitation from the Editor of Angewandte Chemie to Write “a Somewhat More Elaborate Review”

It was early in July 1965, shortly after Woodward and Hoffmann published their third 1965 communication in the JACS [4] on sigmatropic reactions (W‐H 3; Table 2). Addressed to Woodward only was an invitation from the editor in chief of Angewandte Chemie Helmut Grünewald (Figure 1) and his assistant Gerlinde Kruse to publish an article in their journal. Grünewald and Kruse wrote:

FIGURE 1.

On the right, Helmut Grünewald, editor in chief of Angewandte Chemie in the 1960s, then published by Verlag Chemie. On the left is Friedel Hess, then a production manager of Verlag Chemie. They were honoring Eduard Kruezhage on the occasion of his 65th birthday, Frankfurt, Germany, May 1, 1966. Kreuzhage was the Managing Director of Verlag Chemie at the time. A brief biography of Grünewald is found in the (Figure S32 in the Supporting Information). Photograph courtesy Wolfgang Gerhartz and Peter Gölitz.

“Your communication on the ‘Stereochemistry of Electrocyclizations’ seems to be so much of general interest that we wish to invite you and Dr. Hoffmann to write a somewhat longer review on the same subject . . . ” (Figure 2) [35]

FIGURE 2.

Invitation from Dr. Helmut Grünewald, editor in chief of Angewandte Chemie, and his assistant Gerlinde Kruse to R. B. Woodward, to write a review on electrocyclizations, Heidelberg, West Germany, July 1, 1965 [35].

While Grünewald and Kruse referred only to Woodward and Hoffmann's first communication on electrocyclizations [2], by the date of the construction of this letter, the second and third W‐H publications on cycloadditions [3] and sigmatropic reactions [4] had also been published—the third a month earlier on June 5, 1965. That Grünewald and Kruse did not know of those two publications was a fact that surely was not overlooked by Woodward. Still, that omission did not faze Woodward and was not going to veto a welcome invitation. At that time, Angewandte Chemie was a predominantly German journal. The first issue of the “International Edition in English” had appeared just 3 years earlier, in 1962. And Angewandte had not yet reached the level of prestige that it would attain under the editorship of Peter Gölitz (from 1982 to 2017). But at that time, Woodward was a sensitive judge of the future of chemical publishing. He was likely to have favored Angewandte Chemie over other possible venues for the full account of the W‐H story for several reasons. One was the carte blanche, no‐word‐limit invitation. Second was Woodward's chemical and nonchemical European cultural affection and affectation. Third was the fact that the friends and colleagues Woodward admired had previously published major perspectives in Angewandte Chemie (William von E. Doering [39], Rolf Huisgen [40, 41], Emanuel Vogel [42], Rudolf Criegee [43], and Robert S. Cahn, Christopher Ingold and especially Vladimir Prelog [44]).

Nearly 3 months later, on September 29, 1965, Woodward responded affirmatively but without a promised time commitment (Figure 3). And there the matter lay, or so Woodward thought. He had not considered what Hoffmann might do. Grünewald was not so patient. In his response to Woodward, on October 20, 1965, Grünewald was polite, persistent, and perhaps even a bit too proactive if not presumptuous. Grünewald and Wolfgang Jung [46] wrote:

FIGURE 3.

Letter to Helmut Grünewald from R. B. Woodward, Cambridge, MA, September 29, 1965 [45]. Note Woodward's usage of the term “conservation of orbital symmetry” in the very first sentence. This is the first time this phrase was used by either Woodward or Hoffmann publicly.

“We shall be patient and leave it to you to determine the time at which such a review will be available. Nevertheless, we wish to point out that from our standpoint it would be fully satisfactory to receive a paper, which has been prepared in cooperation with a co‐author from your laboratory, for example, Dr. Hoffmann, if this is possible. To us it would seem, that this procedure would take a good deal of work from you and make it possible to have to a review available somewhat earlier” [47].

Woodward hardly felt the sting of Grünewald's boldness. He was very capable of responding quite pointedly to much more pushy overtures [48]. Perhaps Woodward was even mildly amused at Grünewald's suggestion for several reasons. First, Woodward was surely pleased with the invitation and opportunity. Second, Woodward knew that he couldn’t and wouldn’t begin writing any time soon, if ever. Third, he understood that Grünewald could not know the relationship that he (Woodward) had with Hoffmann. Hoffmann wasn’t Woodward's student. Rather, Hoffmann was an independent scholar, by then an Associate Professor (without tenure) at Cornell University. And lastly, Woodward was very selective as to whom he would take offense [48, 49, 50, 51, 52]. There the matter would lie for some time in the distant future, perhaps even for several years, at least from Woodward's perspective.

As for Hoffmann, we know that he was and is always on high alert for any pleasing opportunity. Hoffmann is a self‐described scavenge [53]. By the time of Woodward's receipt of Grünewald's invitation (Figure 2), Hoffmann had barely arrived at Cornell (in mid‐June 1965). He could not begin writing an Angewandte manuscript right away. But as we shall soon see, he was to begin that task shortly.

5. W‐H 11a: On the Invitation from Angewandte Chemie and the First Drafts by Hoffmann

5.1. Why Hoffmann and Not Woodward?

In his rather negative August 30, 1966, peer review of a Kenichi Fukui submission to JACS (later published in the Bulletin of the Japanese Chemical Society in 1967) [54]. Woodward wrote:

“Hoffmann and I both feel very strongly that a detailed and extended exposition of the principles of conservation of orbital symmetry, methods of applying the principles, and extended abundant exemplification, is very much needed, and we hope to find the time to prepare such a paper at some time in the not too remote future” [55].

Nonetheless, Woodward was focused on other matters. In 1964, his major experimental project—the total synthesis of vitamin B₁₂—had hit a brick wall [56]. Indeed, up to 1964, this project's greatest success was providing a new lead into the Woodward‐Hoffmann rules [34, 57]. And Woodward had his mind on other very pressing matters. In mid‐1965, Woodward entered into a collaboration with Albert Eschenmoser at the ETH Zürich on the total synthesis of vitamin B₁₂. This required frequent travels for Woodward to Switzerland [58, 59, 60]. Woodward very much favored Switzerland. The Woodward Research Institute in Basel, Switzerland, was in its earliest days [61, 62], further adding to Woodward's professional occupations and personal predilections. And of course, he had his own research group at Harvard that relied on him. Furthermore, Woodward had received the Nobel Prize in Chemistry in December 1965, further distracting him, including writing his Nobel lecture [63]. Also, Woodward was to give two major lectures on orbital symmetry control, one at the 1966 Bürgenstock Conference [64, 65, 66] and the second at the International Symposium on Aromaticity [67] sponsored by the Chemical Society (London) in July 1966, The latter lecture ultimately led to his mostly autobiographical essay on orbital symmetry (W‐H 9) [11]—which he, not Hoffmann, would have to write. Woodward was simply not much available in 1965, 1966, or even in 1967 to contribute to writing another orbital symmetry article.

In 2024, Hoffmann pointed out to me that, as a matter of very practical philosophy, if you want a manuscript written, ask the junior member of the collaboration [68]. Apparently Grünewald, brought up in a more hierarchical European system, did not know that principle, or he would have written directly to Hoffmann—perhaps. Shortly, as we shall see in Section 5a, Joseph F. Bunnett, the founding editor of Accounts of Chemical Research, did follow that principle with an invitation to Hoffmann to write an essay, not an invitation to the more senior and newly‐Nobel laureate Woodward [38].

5.2. Hoffmann's First Drafts of the Angewandte Publication: Content

No grass was growing under Hoffmann's feet. An invitation from Angewandte Chemie's editor in chief to write an essay was magic to him. Furthermore, Hoffmann liked to write. Many of his orbital symmetry results obtained at Harvard were still unpublished, burning a hole in his briefcase, so to speak. Hoffmann was just beginning to develop a propensity for always publishing his results—either immediately or after some short digestive time‐period. He was and is a salvager and a scavenger.

Immediately after arriving at Cornell in mid‐June 1965, Hoffmann began a multiphase research program on the electronic structure of organic intermediates [69, 70, 71, 72, 73, 74, 75], these being research projects not involving Woodward. But on the W‐H side of the ledger, Hoffmann and eventually two undergraduate summer interns began working on a project, the Catalogue of Concerted Reactions (W‐H 8). Simultaneously, Hoffmann devised a further not‐so‐subtle plan to encourage Woodward to work on the Angewandte publication (“further” because Hoffmann had been urging Woodward in this direction previously [76]).

On January 21, 1966, Hoffmann sent Woodward his 18‐page Catalogue of Concerted Reactions (W‐H 8) [10] (which is discussed in detail in the previous publication in this series [7]). This is reproduced in its entirety in the (Figure S34 in the Supporting Information). By sending Woodward the Catalogue, Hoffmann hoped to entice Woodward into working on the Angewandte submission. Hoffmann wrote:

“Perhaps this enumeration could serve as the basis of a long paper [the term ‘long paper’ became Hoffmann's moniker or nickname for the Angewandte treatise]” [77].

Woodward did not bite.

So, Hoffmann did the next best thing. He began drafting the Angewandte treatise himself. Table 3 summarizes the pertinent chronology of these activities.

TABLE 3.

| Events in the Woodward‐Hoffmann story reveal the nonlinearity of the drafts and eventual publication of their Accounts of Chemical Research essay and their Angewandte Chemie treatise. See Table 2 for the dates of publication of the entire set of W‐H articles.

	Date	Relevant W‐H publication	Event
1	July 1, 1965	W‐H 11	Editors of Angewandte Chemie invites Woodward (and Hoffmann) to write an essay about electrocyclizations
2	Mid‐June 1965	‐‐‐	Hoffmann leaves Harvard and arrives at Cornell in Ithaca, NY
3	ca. Autumn 1965	W‐H 6	Hoffmann sends Woodward a manuscript entitled Some Further Selection Rules for Concerted Cycloadditions. No response from Woodward was received.
4	January 21, 1966	W‐H 8	Hoffmann sends Woodward the Catalogue of Concerted Reactions and suggests this may be useful for the Angewandte submission
5	Mid‐November 1966 to early 1967	W‐H 11	Hoffmann sends Woodward a 119‐page typewritten draft for the Angewandte Chemie submission
6	March 16, 1967	W‐H 10	Joseph F. Bunnett invites Hoffmann to submit a manuscript for the inaugural issue of Accounts of Chemical Research
7	June 28, 1967	W‐H 10	Hoffmann extracts text from W‐H 6 (row 3) and from drafts of W‐H 11 (row 5) and submits a manuscript for Accounts of Chemical Research (W‐H 10)
8	1966—1968	W‐H 11	Hoffmann and Woodward draft Angewandte Chemie manuscript and submit manuscript on December 28, 1968

Writing came easy to Hoffmann, even in English—not his native language [78]. By the spring or summer of 1966, Hoffmann produced a substantial handwritten draft of the Angewandte manuscript, having 99 pages [79]. We are fortunate in that the draft survives in Hoffmann's Cornell files.

Hoffmann's first draft initially lacked an introduction. Page 1 jumped right into a 24‐page detailed explanation of cycloadditions [79] (Figure 4). Either planned or not, Hoffmann later added a substantial though short, four‐page introduction which he numbered page 0000 through page 0 (Figure 5A) [79].

FIGURE 4.

Page 1 of Hoffmann's first handwritten draft of the Angewandte treatise [79]. Subsequently, Hoffmann inserted a four‐page introduction that began with page 0000 (Figure 5A).

FIGURE 5.

(A) Page “0000” of Hoffmann's draft manuscript of the Angewandte submission, 1966 or early 1967 [79]. There are four handwritten draft pages found in Hoffmann's papers numbered “0000,” “000,” “00,” and “0.” These four pages were almost certainly written after the manuscript draft itself, whose first page is numbered “1.” Total number of pages in this draft exceeds 95 pages [79]. (B) Page 1 from Hoffmann's typewritten draft of 119 pages that was sent to Woodward in late 1966 or early 1967 [80]. (C) Page 1 of Hoffmann's 28‐page handwritten draft of the Accounts of Chemical Research manuscript [81]. (D) First two paragraphs of W‐H's Accounts of Chemical Research publication [12]. The yellow highlight, added by Seeman, comes from (A) and (B). The text within the red box, also added by Seeman, comes from (C). For (D), Adapted with permission from: R. Hoffmann and R. B. Woodward, Accounts of Chemical Research. “The Conservation of Orbital Symmetry, 1 (1968): 17–22. Copyright 1968 American Chemical Society.

Hoffmann's 119‐page second draft was typewritten (see Figure 5B for page 1 of that draft), and it was that draft that was ultimately sent to Woodward. Both drafts had the same double‐title,

“The Conservation of Orbital Symmetry

or

The Theory of Concerted Reactions” [80]

Six comments regarding this draft are relevant at this point.

First, Hoffmann was overreaching in the subtitle, “The Theory ofConcerted Reactions.” Even in the mid‐1960s, there were many well‐known organic chemistry reactions that were (or thought to be) concerted and were not governed by orbital symmetry considerations. Primarily, these were noncyclic concerted reactions like the S _N 2 and E2 reactions [82], the Eschenmoser‐Grob‐Wharton fragmentation [83, 84], squalene‐type cyclizations [85], and so forth. This being said, Hoffmann did cite the “no‐mechanism reaction” [86] in Figure 5A,B (though neither the Aromaticity publication [11] nor the Angewandte treatise [13] refers to that specification).

Second, consider Hoffmann's use of the term “the conservation of orbital symmetry.” Where did that phrase originate? It appears that it was first used in Woodward's letter to Grünewald, September 29, 1965, (Figure 3) [45], a private communicaion which Woodward may or may not have shared with Hoffmann; it is not in Hoffmann's papers. Woodward was surely pleased to see Hoffmann's draft title, or at least the first portion of it.

Third, we know today—and Hoffmann told me several times [53]—that with time, he was not so pleased with the term “the conservation of orbital symmetry.” To him, it seemed boastful, even as it served Woodward's apparent desire to uncover a fundamental principle of nature. Still, the draft phraseology is real; Hoffmann proposed that title—which ultimately was used in the next two W‐H publications (W‐H 10 and W‐H 11).

Fourth, Figure 5A from Hoffmann's first draft of the Angewandte treatise carried over into his second draft (Figure 5B) and into the Angewandte publication itself (Figure 5D). This is just one excerpt of many whose origins come from those first drafts.

Fifth, three excerpts from the four documents in Figure 5 reveal a conceptual and philosophical relationship among all of them. Consider these three excerpts from Hoffmann's drafts:

• •“a fruitful symbiosis of [modern] organic chemistry and molecular orbital theory”
• •“this has been largely the combination of poor theory with good experiment”
• •“tentative conclusions were arrived at on the basis of themes so riddled with approximations that they had no right to work” or “tentative conclusions have been arrived at on the basis of themes which were such a patchwork of approximations that they appeared to have no right to work”

Hoffmann was voicing quite clearly the inadequacies of his extended Hückel theory (eHT). The origin of his reticence no doubt was the theoretical chemistry community's reaction to eHT. Hoffmann's later not‐so‐private ambivalence to computational chemistry is interesting, given that his earliest research and his early series of eHT‐based publications served as a foundational cornerstone for a career enmeshed with theoretical chemistry and computers.

More generally, we see here the workings of a lifelong attribute of Hoffmann's mode of expression in science. He does not make grandiose claims, he is reticent, and he tends to understate. None of this has damaged the chemistry community's appreciation of his research and reputation, as he progressed in his career from chemical physics to organic chemistry to inorganic, organometallic and solid‐state chemistry and physics.

Sixth, chemical pictographs were also carried over from those early drafts. The chemical pictograph in Figure 6C is reproduced from the Accounts essay; this comes directly from Hoffmann's hand‐drawn graphics from his first two drafts of the Angewandte treatise (Figure 6A and Figure 6B). The same sequence of drafts → Accounts essay is shown in Figure 7 for the stereochemical consequences of conrotatory and disrotatory electrocyclizations of 1,3‐butadiene ⇋ cyclobutene.

FIGURE 6.

Depictions of the stereochemical consequences of two‐component cycloadditions. (A) From page 18 of Hoffmann's first handwritten draft of the Angewandte Chemie manuscript [79]. (B) From page 24 of Hoffmann's typewritten draft of the Angewandte Chemie manuscript sent to Woodward, late 1966 or early 1967 [80]. (C) From Woodward and Hoffmann's Accounts of Chemical Research publication [12]. Reprinted with permission from: R. Hoffmann, R. B. Woodward, Acc. Chem. Res. 1968, 1, 17–22. Copyright 1968 American Chemical Society. (D) From Woodward and Hoffmann's Angewandte Chemie publication [13]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

FIGURE 7.

Depictions of the stereochemical consequences of conrotatory and disrotatory electrocyclizations. (A) From page 28 of Hoffmann's first handwritten draft of the Angewandte Chemie manuscript [79]. (B) From page 35 of Hoffmann's typewritten draft of the Angewandte Chemie manuscript sent to Woodward, late 1966 or early 1967 [80]. (C) From Woodward and Hoffmann's Accounts of Chemical Research publication [12]. This graphic was not reproduced in Woodward and Hoffmann's Angewandte Chemie publication [13]. Reprinted with permission from: R. Hoffmann, R. B. Woodward, Acc. Chem. Res. 1 (1968): 17–22. Copyright 1968 American Chemical Society.

5.3. Hoffmann's First Drafts of the Angewandte Publication: Chronology

Sometime between August 1966 and early 1967, Hoffmann sent the typewritten draft to Woodward. In an undated cover letter (Figure 8), Hoffmann rather matter‐of‐factly pointed out to Woodward that he had

FIGURE 8.

Hoffmann's undated two‐page handwritten letter to Woodward [87], informing him of an attached, 119‐page typewritten draft [80] of their Angewandte Chemie manuscript along with brief discussion of other matters, Ithaca, NY, ca. late 1966 or early winter of 1967. The photochemical transformations of 1,3,5‐hexatrienes into their E, Z‐stereoisomers and through pericyclic reactions to, for example, bicyclo [3.2.1]hexenes and cyclobutenes, was also of interest to William G. Dauben, beginning in the late 1950s [88, 89, 90].

“sat down to write about our work, and it turned out to come to 120 pages without footnotes, duly attached. . . . I have also attached about 20 pages of references from my notebooks, as well as a folder with crucial articles containing many more references . . . I hope we can get this out pretty soon; but I’m afraid it's closer to a book than a journal article” [87].

While Hoffmann's cover letter (Figure 8) is undated, it contains some evidence as to when it was written. Hoffmann referred to a publication by Derek Bryce‐Smith and H. Christopher Longuet‐Higgins [91] that appeared November 17, 1966. This indicates a date for Hoffmann's cover letter to be no earlier than mid‐November 1966.

We have other evidence that can help date Hoffmann's cover letter to Woodward [87]. On October 13, 1967, Hoffmann wrote a letter to William G. Dauben at the University of California at Berkeley in which he responded to a question about potential orbital symmetry control of a reaction. In that letter, Hoffmann wrote:

“Our conclusions on the hexatriene to bicyclohexane transformation came out from our general conclusions on cis‐trans [i.e., suprafacial‐antarafacial] and trans–trans cycloadditions. The only place where I have written any about these is in a version of a long paper with Woodward, which I am sending you under separate cover. This is a very preliminary manuscript which I wrote down about a year ago and which Woodward is supposed to be rewriting. . . . ” [92]

The Dauben letter suggests a date for the typewritten draft to be ca. October 1966 (for the curious reader, I repeat the first name and middle initials of “William G. Dauben” in this publication so as not to confuse “William G.” with his brother “Hyp J.” Hyp J. Dauben, Jr. was also a physical organic chemist and taught for many years at the University of Washington and died in 1968 [93].)

On August 16, 1968, Hoffmann wrote to his good friend Lionel Salem [94] on several matters and mentioned:

“As you know, Woodward and I have been trying to write a long paper for some time. I wrote a first draft, now 2 years ago, and I believe you have a copy of that. Woodward is a very hard man to hurry into anything, especially to write papers. However, he really wants to get this out now – there are so many things we have found out which others do not know, and so many aspects of the work subject to misinterpretation, that we must get it out soon. . . . .” [1] [italics added]

The Salem letter suggests the date of Hoffmann's typewritten manuscript to be ca. August 1966.

This collective evidence indicates that Hoffmann sent the typewritten draft to Woodward between mid‐November 1966 and early 1967. Hoffmann concurs with this estimate [95].

A conscientious yet reasonable reviewer has concluded that “the attempts to date the letter in Figure 8 doesn’t belong into the main text. It would suffice to give the letter an approximate date.” There are two reasons that I have gone in such detail in the above several paragraphs. First, placing even an approximate date of Hoffmann's shipment of the first advanced draft of the Angewandte treatise to Woodward is historically important. Second, I wanted to share with the reader the difficulties inherent in historical research—even when the participants in the historical events are alive and cognizant.

6. W‐H 10: Accounts of Chemical Research: “The Conservation of Orbital Symmetry” [12]

6.1. The Invitation

Be prepared for the unexpected. Be prepared to alter one's carefully thought‐out plans!

On March 14, 1967, Bunnett, the founding editor in chief of Accounts of Chemical Research, invited Hoffmann to write an article for the inaugural issue (January 1968) of this new journal. Bunnett wrote:

“In issuing this invitation, I have in mind particularly your work on the Woodward‐Hoffmann Rules, but you should feel free to write on another suitable subject which you may consider more appropriate. . . . Would you like to try for the first issue? If so, I should have your manuscript by early July” [38].

Hoffmann apparently shared this invitation with Woodward, for within Woodward's files is his handwritten, undated note that documents Woodward's response: “Handled with Roald by ‘phone. ‘We’ will submit a paper!” [96].

6.2. Content from Hoffmann's First Angewandte Draft Used in the Accounts Publication

Hoffmann set to work immediately on the Accounts manuscript. He had to, as the deadline was early July 1967, just 3.5 months after the invitation was proffered. Hoffmann was speedy. He sent an advanced draft to Woodward within 3 months of the invitation [97] and mailed a manuscript to Accounts 2 weeks later [98]. This timing is impressive, given that Hoffmann had other pressing duties. By March 1967, his research and his research group at Cornell were just beginning to “rock and roll.” While in 1967 he published only one article, he was to publish 10 articles in 1968 (seven in JACS, one each in The Journal of Chemical Physics, Tetrahedron, and Accounts of Chemical Research). Many of these publications were certainly drafted in 1967.

In order to achieve this rather speedy performance, Hoffmann, the scavenger (where everything is a possibility), turned to an immediate source of ideas, text, and chemical pictography: his unused drafts of previous projects and especially his drafts of the Angewandte submission (W‐H 11) written in 1966 and his unpublished manuscript W‐H 6 from 1965 (see Table 1 and 2; this manuscript is discussed in detail in the previous publication in this series [7]). Within Hoffmann's files is a 28‐page handwritten first draft of the Accounts publication. The first page of this draft is reproduced in Figure 5C. One can readily see content taken from his early drafts of the Angewandte treatise (Figure 5A,B) and used in drafts for the Accounts article (Figure 5C,D, the highlighted portions of Figure 5D coming from the previous drafts in this figure).

TABLE 1.

| The 31 publications in the series “The History of the Woodward‐Hoffmann rules.”

As Hoffmann recently observed:

“This is a handwritten draft (on yellow lined paper) of the Accounts article. It contains almost verbatim what is in the Accounts article text, beginning to end. It is very clearly written to begin with, as you can see. One can imagine that I am copying this from an earlier text. But the draft deteriorates, with paste‐ons from some earlier typed copy, from p.17 on . . . ” [99]

If there are any doubts that Hoffmann wrote the first drafts of the eventual 1969 Angewandte treatise before he wrote the 1968 Accounts essay, the size of those two drafts establishes the point (94 and 119 pages, respectively). Manuscript drafts of those lengths far, far exceeded the world‐count limits of an essay in Accounts of Chemical Research.

6.3. Accounts Final Steps: Authorship, Order of Names, and Submission

Hoffmann did not include the authors’ names in the typewritten draft [80] of the Accounts of Chemical Research manuscript that he sent to Woodward on June 16, 1967 (Figure 5B). In his cover letter, Hoffmann wrote:

“I am attaching a draft of our paper for ‘Accounts of Chemical Research.’ I think I sent along earlier the guidelines for this new journal. I am hoping that you will find it more or less acceptable in this form and let me know very soon (I promised to send it in by July 1) of changes that need to be made. The figures are sketched; they are being drawn neatly by an illustrator here. I also need to know what we should use as a title (Conservation of Orbital Symmetry will do) and in which order you prefer our names on the manuscript. Bunnett wants an abstract but I just don’t see how we can provide one ‐‐ the whole manuscript is an abstract in a sense” [97].

I repeat one sentence from Hoffmann's letter. He asked Woodward:

“. . . in which order [do] you prefer our names on the manuscript” [97].

Remember, Hoffmann had written the document all by himself. In the academic hierarchy, Hoffmann was obviously in a much more junior position relative to Woodward. Indeed, Woodward had just received his Nobel Prize in December 1965. Ultimately, the published Accounts article had Hoffmann's name first (the normal placement of the senior author in the 1960s); Woodward's name appeared second. That must have been Woodward's decision, as Hoffmann surely would not have countered a suggestion by Woodward otherwise, especially after asking Woodward for his opinion.

Regarding the content and style of the draft itself, Hoffmann did not give Woodward much time to suggest revisions—actually less than 2 weeks (but then again, Bunnett had not given Hoffmann much time to write and submit the manuscript). Presumably, Hoffmann anticipated that Woodward would have few, if any, changes. Or Hoffmann was implying that Woodward not make many changes.

It is also worth noting that Hoffmann's goal of providing the manuscript to the Accounts editorial office by the July 1, 1967, deadline must be placed within the context of Hoffmann's travel schedule. In the same letter to Woodward, Hoffmann indicated that he was to begin his summer travels by being in Sweden from June 29 to July 15, 1967. That schedule left little time for Woodward to suggest changes, get these into Hoffmann's hands in Ithaca, and then be inserted by Hoffmann into the final manuscript and mailed.

In fact, on June 28, 1967, Hoffmann wrote to Bunnett:

“I am pleased to enclose a manuscript ‘The Conservation of orbital Symmetry’ by R. B. Woodward and myself, in response to your invitation to write an article for one of the early issues of Accounts of Chemical Research. Having no precedent to guide us, we hope that its format and style meet with the American Chemical Society's expectations of this Journal.

“There is one point in your ‘Notice to Authors’ with which I must take exception. I think that since ‘Accounts’ is to be a ‘journal of concise, critical reviews of recent research developments . . . ’ that the provision of abstracts is inconsistent with its aims. It is impossible to abstract a review since by its nature it is already essentially a lengthy abstract. In the specific case of our paper, the methodology is not amenable to abstraction, and the discussion of results is already in quite concise form. We are thus not able to write an abstract” [98].

On June 28, 1967, if not a day earlier, Hoffmann and his family traveled across the Atlantic Ocean.

As to Woodward's role in writing the manuscript, the above evidence suggests that Hoffmann did all of the writing. Hoffmann agrees, saying:

“I think I did, or most of it. The style is more mine. The language of the article is more mine than his, in the use of colloquial expressions” [100].

On September 8, 1967, Bunnett wrote to Hoffmann, saying in part:

“Your manuscript was sent for review to an organic chemist of high competence in the field of quantum mechanics. A copy of his report is enclosed, as well as one copy of your manuscript. The reviewer is generally appreciative of your manuscript, but he does make some solid suggestions in regard to the manner of presentation of its message. Although you and Professor Woodward are under no obligation to accept the suggestions of the reviewer, his suggestions are perceptive and constructive and they merit your careful attention, I urge that you and Professor Woodward make some modifications in your manuscript to satisfy some of the points he has raised” [101].

Indeed, Woodward and Hoffmann did modify their manuscript (somewhat). Several of the reviewers’ suggestions were not followed, to the disadvantage of the final product. But that judgment is produced with the benefit of hindsight (this was the first time that any of Woodward's and Hoffmann's individual or joint submissions on orbital symmetry had been subjected to peer review).

Even though Bunnett was mindful of Hoffmann's earlier position that an appropriate abstract could not be written and would not be provided by Woodward and Hoffmann [98], Bunnett nonetheless urged Hoffmann to write one. Bunnett wrote:

“It is necessary to have an abstract, which will probably be used verbatim by Chemical Abstracts. Something like ‘The stereochemistry of electrocyclic reactions and reactivity in cyclo‐addition [sic] reactions are extensively controlled by considerations of orbital symmetry’ would be acceptable, but a somewhat longer abstract would perhaps be better” [101].

The abstract that appears with Hoffmann and Woodward's journal article reads,

“Orbital symmetry controls in an easily discernible manner the feasibility and stereochemical consequences of every concerted reaction” [102].

Today, SciFinder includes a 40‐word abstract [102] for this publication, far more than the 17 words that Bunnett had provided” [101] or the 16 words that Hoffmann and Woodward used [102]. The content and style of the SciFinder abstract (reproduced in the, Figure S3) is such that Hoffmann (but not Woodward) could well have written it, though today, Hoffmann cannot confirm that as an actuality, and there is no contemporaneous evidence one‐way or the other. In any event, Hoffmann forwarded a copy of Bunnett's letter [101] and the reviewer's rather extensive comments [103] to Woodward [104] (see Figure 9). Bunnett gave Woodward and Hoffmann full license to do as they chose, and the reviewer's recommendation was strongly positive. Then, among other professional endeavors, Hoffmann turned his attention back to the Angewandte Chemie draft.

FIGURE 9.

Hoffmann note [104] to Woodward enclosing Bunnett's cover letter [101] and the reviewer's comments [103]. Hoffmann is being polite and sensitive. He surely assumed with justification that Bunnett had not sent the cover letter and reviewer's comments to Woodward, given that the letter was addressed solely to him (and that the invitation had gone solely to Hoffmann). Bunnett's cover letter and the reviewer's comments are available in the (Figures S1 and S2).

Also found within Hoffmann's papers at Cornell is a single, undated title page for a manuscript entitled “The Conservation of Orbital Symmetry” handwritten by Hoffmann (Figure 10) [105]. Hoffmann is uncertain [108] as to whether this first page was used for a draft of the 1968 Accounts of Chemical Research article [12] or for the draft for some other W‐H article, e.g., for their 1969 treatise published in Angewandte Chemie in 1970 in monograph format [15] of this same title or the 1970 Science article (W‐H 12, see Table 2) which had a different title (“Orbital Symmetry Control of Chemical Reactions” [17]). Of interest and possibly some importance is the format of the names of the authors. The centering of Hoffmann's name in the byline suggests that Hoffmann originally considered that he would be the sole author of the publication. That is, the text “and R. B. Woodward” was added to the cover page at some time after the cover page had been initially drafted and is offset to the right of center.

FIGURE 10.

Hoffmann's handwritten cover page of an unknown draft manuscript entitled “The Conservation of Orbital Symmetry” [105]. Was this cover page to be for Hoffmann and Woodward's essay of that same title in their 1968 Accounts of Chemical Research article [12] or for their 1969 treatise published in Angewandte Chemie and in 1970 in monograph format [15] of the same title? Hoffmann does not know [106]. The asymmetry of the authors’ names in the byline suggests that Hoffmann originally considered the draft to have just one author, himself. Hoffmann recently mused, “The scribbled numbers are fun—it doesn’t look like my writing. Is someone balancing their checkbook, or calculating number of pages in a printed version?” [107].

How we wish we knew the date of this page (Figure 10) and its connection to the drafts we have discussed!

‐‐‐‐‐‐‐‐‐‐‐

A reviewer of this publication wrote: “The author keeps referring to the Accounts paper as Woodward and Hoffmann's paper or ‘Woodward and Hoffmann said . . .’ although it was . . . written almost entirely by Hoffmann.” My response, perhaps not completely justifiable: This phraseology was chosen because Hoffmann and Woodward jointly asserted authorship, and both shared accountability and responsibility for its contents.

6.4. From Hoffmann's Laboratory Notebooks

The previous sections reveal the nonlinear sequence that led from Hoffmann's drafts of the Angewandte treatise written in 1966 and 1967 to first, the 1968 Accounts essay [12], and, in time to the final version of the 1969 Angewandte treatise itself [13]. I shall now provide one of many possible examples of the evolution of Hoffmann's theoretical derivations from his laboratory notebooks to the Accounts essay. As the record will show, Woodward was not the sole—and likely not even the major—supplier of literature examples whose mechanisms were explained in the Woodward‐Hoffmann publications. It is amusing, indeed instructive, to connect entries in Hoffmann's laboratory notebooks from the literature and from his very own imagination with inserts in the W‐H publications.

Consider the thermal and photochemical valence tautomerism of compounds of empirical formulae (CH) _n where n = 4 and 6 (Figure 11). This was a topic of much interest in the mid‐1960s. As Woodward and Hoffmann wrote in their Accounts essay:

FIGURE 11.

The publication and evolution trajectories cycloaddition to and valence tautomerism of (CH)₄ and (CH)_6. (A) From page 61 of Hoffmann's Laboratory Notebook 16, ca. February 10, 1966. (B) From page 49 of Hoffmann's Laboratory Notebook 17, ca. March 14, 1966. (C) Excerpt from Hoffmann's third draft of his unpublished 1965 manuscript entitled “Some Further Selection Rules for Concerted Reactions” [8]. Similar graphics appear in Hoffmann's first and second drafts of this article, late 1965 [8]. (D) First paragraph from Hoffmann's March 14, 1966, letter to Woodward in which he communicates his results on (CH)₆ valence isomerizations from the above considerations. (E) An excerpt from page 21 of Hoffmann's draft of the Accounts of Chemical Research manuscript, spring of 1967 [109]. The taped in excerpt is from another draft of (C). (F) From Hoffmann and Woodward's 1968 publication in Accounts of Chemical Research. Reprinted with permission from R. Hoffmann, R. B. Woodward, Acc. Chem. Res. 1 (1968): 17–22. Copyright 1968. American Chemical Society. For a leading reference from 1966 that includes symmetry assignments of the excited states of benzene, see the 1966 publication of Bryce‐Smith and Longuet‐Higgins [91].

“We show below [Figure 11F] the pathways of interconversion of the isomers of cyclobutadiene and benzene. The state labels emerging from benzene permit a detailed prediction that different excited states of benzene have different isomerization routes available to them” [12].

Figure 11A,B provide another example [7, 110, 111, 112] where Hoffmann rederived the same chemical results in his laboratory notebooks multiple times. In this case, the derivations are separated by ca. 5 weeks’ time. Was Hoffmann unsure of himself or did he simply forget what he had done previously? Today, he is not sure of the explanation himself [53]. In Figure 11B, Hoffmann produced a set of selection rules for these transformations that he carried on in his subsequent uses of this chemistry (see the other reproductions in Figure 11). Note that the selection rules in Figure 11B do not always alternate in permissiveness (allowed to forbidden; or thermal to photochemical; and the converse in both cases). By March 1966, Hoffmann had not yet identified this alternation characteristic of all pericyclic reactions, though he certainly had examples of it staring him in the face in electrocyclic reactions, cycloadditions, and sigmatropic reactions. Indeed, the organizational classification of “pericyclic reactions” had not yet been formulated and would not be until ca. 1968. Hoffmann's reuse of the graphic and text from his unpublished manuscript [113] (Figure 11E into Figure 11F) is another example of his being a salvager. Making publication‐quality chemical pictographs in the 1960s before ChemDraw was a serious chore not to be underestimated. For Hoffmann, the benzene isomer rearrangements were on his mind. As he told me, it was important for him to understand it.

Some of the transformations recorded in Figure 11 are photochemical reactions. Hoffmann also had a long‐standing interest in organic photochemistry; for example, he attended the International Symposium on Organic Photochemistry in Strasbourg, France, on July 20–24, 1964. Hoffmann had a lengthy and substantive correspondence with the theoretical‐photochemist Derek Bryce‐Smith [91] on some of the benzene isomer interconversions. And benzene and Hückel theory figured prominently in the senior (undergraduate) honors seminar that Hoffmann took with Ronald Breslow at Columbia College in 1958. Hoffmann agreed with the above and added several more explanations:

“Maybe the central role of benzene to organic chemistry. Maybe just the surprise that such strained molecules could exist. Maybe the perturbation on chemistry if these were known at the time of Kekulé's work. Maybe because the special nature benzenes played in the Pauling and Hückel heritage of a theoretician? If there is one thing I could wish for in some other incarnation, it is to have synthesized the isomers of benzene” [114].

My guess: likely all of these.

6.5. What is New in the Accounts Publication?

The following topics are new in the Accounts publication.

• •Woodward and Hoffmann provided certain key theoretical underpinning of the W‐H rules, namely that:

“The characteristic of concerted processes is that in certain well‐defined circumstances, it is possible to transform continuously the molecular orbitals of reactants into those of the product in such a way as to preserve the bonding character of all occupied molecular orbitals at all stages of the reaction” [12].

Hoffmann explained:

“I think the antecedents of this important statement are in Woodward's Aromaticity publication [11] (but not as clear as this statement, in part because we were groping toward understanding) and in the discussion in my notebooks with John Baldwin [115] and Dave Lemal around 1965–1966” [107].

A peer reviewer of this publication commented that

“a shift from the old‐fashioned approach based on reactant structure to one based on transition‐state structure and energy is complicated. I have always found that organic students have difficulty with correlation diagrams but can master aromatic transition state easily. In this context a reference to Evans’ 1939 paper [116] would not be amiss.”

Indeed, Meredith Gwynne Evans, in part with his Leeds, then Manchester collaborator Ernest Warhurst, published several articles in the late 1930s [116, 117] in which a transition state (TS) model was proposed to explain the facility of the thermal Diels‐Alder reaction by an aromatic‐like stabilization of its transition state. These ideas ultimately served as the basis of Howard E. Zimmerman's [118, 119, 120] and Michael J. S. Dewar's [121, 122, 123] Hückel and Möbius TS model for allowed and forbidden pericyclic reactions. In a real sense, Dewar's advocacy of M. G. Evans “having done it all before” thesis [122, 123, 124] was a disservice to Evans and ironically to Dewar, too. It poorly served Dewar's attempt to diminish Woodward and Hoffmann's achievement. But at the same time, it diminished Dewar's novel achievement of bringing a transition state analysis into the pericyclic reaction mechanism story. Furthermore, Evans did very well for his time, when few physical chemists paid any attention to organic chemistry, and when the delineation of aromaticity and antiaromaticity was incomplete. For example, Evans was elected a Fellow of the Royal Society in 1947 at the youthful age of 43 [125, 126]. He died quite prematurely in 1952 at the age of 48.

• •Woodward and Hoffmann discussed suprafacial and antarafacial sigmatropic reactions in their third JACS communication of 1965 (W‐H 3) [4]. The dating of Hoffmann's laboratory notebook pages provides unambiguous evidence that he understood suprafacial and antarafacial reaction trajectories for sigmatropic reactions before Woodward and Hoffmann's second JACS communication on cycloadditions was published in March 1965 (W‐H 2) [110, 111]. But at that time Woodward and Hoffmann had not transferred the suprafacial/antarafacial stereochemical concept from sigmatropic reactions to cycloadditions. In the Accounts essay, Woodward and Hoffmann advanced the chemistry of cycloadditions to include what they then termed cis and trans cycloadditions (Figure 12A) but did not (yet) connect that chemistry to suprafacial and antarafacial sigmatropic reactions, respectively. In their 1969 treatise [13], they would do so.

FIGURE 12.

(A) Enhanced selection rules for cycloadditions, to include cis and trans additions. Table reprinted with permission from R. Hoffmann, R. B. Woodward, Acc. Chem. Res. 1 (1968): 17–22. Copyright 1968. American Chemical Society. (B) Trans addition illustrated for the first time in Hoffmann's laboratory notebook (NB16/40), ca. February 10, 1965, 6 weeks prior to Woodward and Hoffmann's submission of their cycloaddition communication of 1965 to JACS (W‐H 2) (March 22, 1965) [3]. Note the conformation of the substituents on the terminal carbon atoms of the reacting double bond within the allene in the bottom left structure. Hoffmann is apparently not aware of the required orientation of the p‐orbitals for an antarafacial addition which should be perpendicular to the plane of the p‐orbitals of the reacting olefin. He will be soon.

As Hoffmann summarized:

“Even though we could draw an a + a cycloaddition [see Figure 12B for Hoffmann's earliest but‐not‐quite‐right geometric hint at such a reaction], all we saw was that was difficult geometrically, as it might be stereochemically for the relevant transition state to be realized – whether 4a + 2a, or 6a + 4a also came out allowed (if 4s + 2s or 6s + 4s were allowed, as they are). Early on, we were not able to grasp the full complexity or richness of selection rules for cycloadditions until we conceived of a forbidden reaction, forbidden because one component entered in an antarafacial way. It was not easy to draw a reaction path for this s + a situation, nor to give up the crutch of symmetry for drawing a correlation diagram.

I think that in 1965, we needed to let go of symmetry (after I had fallen in love with correlation diagrams), which I had to do for sigmatropic reactions. I was then prepared to see antarafacial cycloadditions, both as geometrical possibilities and as changing selection rules” [127].

• •The Accounts publication also discussed (a) when symmetry is absent, but the MO energy levels can still be followed through the reaction; (b) some basic principles about MO theory, e.g., the effect of MO levels mixing on the energy of the resultant MO levels; that for a symmetry element to be used in analyzing pericyclic reactions, it must “pass through bonds made or broken in the reaction” [12]; and that in a correlation diagram, “an intended [MO] level crossings is just as good (or bad) as a real one; and (c) why HOMOs and LUMOs are generally more important than other MOs in determining the fate of proposed reactions.

• •The Accounts publication corrected previous incomplete or misleading credit attributions by Woodward and Hoffmann to Charles H. DePuy and Longuet‐Higgins and enhanced their attributions to Fukui.

• •The Accounts publication presented some chemistry not discussed in any of the previous W‐H joint publications, including (a) the (CH) _n valence isomerizations; (b) a variety of photochemical cycloadditions including the photochemical Diels‐Alder reaction (which Woodward studied in his Ph.D. research at MIT [128]); (c) trimolecular cycloadditions; and (d) oxygen extractions from N‐oxide among other cheletropic reactions. Hoffmann did discuss some cheletropic reactions in his publication in the Transactions of the New York Academy of Sciences [9] (W‐H 7).

6.6. What is Not in the Accounts Publication

The following topics are shortcomings in the Accounts publication:

• •All pericyclic reactions were provided in the form of Markush [129] generalizations, i.e., structures that specify a core, backbone structure with variable substituents. Consequently, there was not a single literature reference to any specific pericyclic reaction.

• •The Accounts publication claimed:

“By utilizing the most simple but fundamental concepts of molecular orbital theory, we have in the past 3 years been able to rationalize and predict the stereochemical course of virtually every concerted reaction” [12].

Of course, this claim is overreaching. Woodward and Hoffmann meant “in every pericyclic reaction” which, by their definition, is concerted [13]. That is, all pericyclic reactions are concerted but not all concerted reactions are pericyclic, e.g., S _N 2 and E₂ reactions.

• •Only in the course of writing the Angewandte paper did the realization come that all of these valence isomerizations, i.e., electrocyclizations, cycloadditions, sigmatropic reactions, cheletropic reactions, and group transfers and eliminations, were of a single kind. Only later would Woodward and Hoffmann call all of these pericyclic reactions “cycloadditions” [13]. The use of the term “cycloadditions” for multiple purposes is unfortunate in its ambiguity.

• •There was not a mention of Hückel‐Möbius transition states as proposed by Howard E. Zimmerman in 1966 [118, 119] or of aromatic‐antiaromatic transition states as proposed by Michael J. S. Dewar, also in 1966 [121] although he hinted at it in Theorem 24 and Theorem 33 in one [130] of his six JACS publications on perturbation theory in 1952 [130, 131, 132, 133, 134, 135].

• •The reviewer of the Accounts submission suggested a more balanced approach was ignored, to the essay's disadvantage. That reviewer wrote:

“I would rather have seen some space devoted to the some of the more important experimental verification of their predictions . . . I think this ‘growing‐in’ should be stated explicitly with quantum mechanical justification . . . the non‐crossing rule which figures prominently in [the] correlation diagrams of any kind is mentioned almost incidentally and perhaps should be stressed . . . it would seem appropriate to have a sentence or two on the meaning of excited states in terms of level correlation diagrams . . . Although [William G.] Dauben is mentioned, there is no reference to him . . . ” [103].

• •The Accounts publication was devoid of extended Hückel theory calculations. Hoffmann summarized:

“In our work we have relied on the most basic ideas of molecular orbital theory – the concepts of symmetry, overlap, interaction, bonding, and the nodal structure of wave functions. The lack of numbers in our discussion is not a weakness – it is its greatest strength. Precise numerical values would have to result from some specific sequence of approximations. But an argument from first principles or symmetry, of necessity qualitative, is in fact much stronger than the deceptively authoritative numerical result. For, if the simple argument is true, then any approximate method, as well as the now inaccessible exact solution, must obey it” [12].

Interestingly for eHT calculations, as approximate as they were, they figured prominently in the credibility of the analysis of electrocyclic reactions [2, 136], and in Hoffmann's discovery of how different the MO consequences were in [2 + 2] cycloadditions compared to [4 + 2] cycloadditions [3, 110]. It is almost as if Hoffmann discarded his use and dependence of detailed eHT calculations once he had begun to use other MO‐based tools, those being correlation diagrams and qualitative perturbation theory.

• •The article was devoid of major conclusions. The essay just ended, rather abruptly—even though there were more than several inches of white space at the very bottom of the article.

6.7. Summary

The January 1968 Accounts article was essentially the abstract to the eventual November 1969 Angewandte treatise. Indeed, portions of the early drafts of the Angewandte treatise were used in the Accounts submission. Furthermore, the Accounts essay was multiply‐honorific. It was prestigious for Woodward and Hoffmann's essay to appear in the inaugural issue of Accounts of Chemical Research. It was estimable for Accounts of Chemical Research to publish Woodward and Hoffmann's first review of the W‐H rules. It was a feather in Woodward and Hoffmann's caps, as it was for Joseph Bunnett to have captured Woodward and Hoffmann.

6.8. Onward

In his June 16, 1967, cover letter to Woodward that accompanied a copy of their Accounts submission, Hoffmann urged Woodward:

“I have still not given up hope that you will get around to rewriting the long paper” [97].

Woodward would soon heed that call and do so with great intensity. The intimate Woodward‐Hoffmann collaboration was (finally) about to begin.

7. W‐H 11a: Angewandte Chemie (International Edition in English and German‐Language Edition): “The Conservation of Orbital Symmetry” [ 13 ] and “Die Erhaltung der Orbitalsymmetrie” [14]

7.1. Angewandte Chemie's Invitation and the Fulfillment

By mid‐1967, Angewandte editor Helmut Grünewald was getting anxious. It had been more than 2 years since he invited Woodward to write his W‐H story (Figure 2) and somewhat less than that since Woodward had accepted (Figure 3). Grünewald was acutely aware that review‐type articles on the Woodward‐Hoffmann rules topics were already being published and frequently so by other chemists [118, 119, 121, 122, 137, 138, 139, 140, 141, 142, 143, 144, 145], including some in Grünewald's own journal [41, 123, 141, 146, 147, 148, 149, 150, 151, 152], Grünewald surely worried that a much‐delayed article by Woodward and Hoffmann might have limited value and lower journalistic impact. Perhaps he was also aware of Woodward's disinclination to write, a behavioral quality possibly shared by some of Woodward's European friends. Of course, Woodward's commitment back in 1965 was not ironclad. And an individual's commitments change considerably after they have received the Nobel Prize. Woodward's came in December 1965.

On July 6, 1967, Grünewald and colleague Wolfgang Jung wrote to Woodward:

“You will recall our correspondence regarding a review article on the ‘Woodward‐Hoffmann rule’. Your reply was favourable although we did not agree on any date for the delivery of such a manuscript.

It seems to me now that work on this rule and its application has reached a stage which makes a review very timely indeed. My question therefore is whether you, probably in cooperation with Dr. Hoffmann, would like to undertake a review and send us a manuscript either by the end of this year or early in 1968.

I realize that other commitments may make it difficult for you to write such an article in the near future. It is my opinion, however, which is shared by many colleagues here in Germany that a review on the correct application of this rule is badly needed.

May I hope therefore to receive a favourable reply?” [153]

Woodward did not respond, though he certainly could have, and done so in a positive fashion. What Grünewald and Jung could not know was that Woodward—for perhaps six or 7 months—was already in receipt of Hoffmann's 119‐page typewritten draft of the Angewandte manuscript [80]. After 2 months, having not received a reply from Woodward, Grünewald wrote again to him on September 1, 1967:

“I wrote to you on July 6 and asked you whether you would like to write a review on the ‘Woodward‐Hoffmann Rule’ and send us the manuscript either by the end of this year or early in 1968.

Unfortunately I have not received an answer until now. May I therefore repeat my question. The reason is that a young German chemist who is presently working in the United Sates, Dr. Binsch in the Department of Chemistry at Notre Dame University, gave a lecture here recently on the same subject which was so good that I would like to print it if you are not in a position to supply us with a review article. However, I wish to emphasize that I would be much happier to receive a review from the man after whom the rule is named than by any substitute. I would even be prepared to wait for another year or so if I only know that you will contribute a review as indicated” [154].

The name Gerhard Binsch was not yet well‐known in the world of chemistry. Eventually it would be. Binsch received his doctorate with Huisgen in Munich in 1963 and then studied with John D. Roberts at Caltech and Edgar Heilbronner at ETH Zürich, putting together quite a prestigious CV. After a brief tenure at the University of Notre Dame, Binsch returned to Munich where he flourished as a full professor until his premature death in 1993 at the age of 58. His field of expertise became experimental and theoretical nuclear magnetic resonance [155, 156, 157, 158, 159, 160] with important tangents into areas of physical organic chemistry [161].

It took Woodward another 2 months to respond (4 months from Grünewald's initial 1967 probe). On October 25, 1967, Woodward wrote what he could have written in early September 1967:

“May I thank you very much for your letter of September first and apologize for not having been able to reply sooner.

Two points occur to me in connection with the matter which you raised there.

1. Dr. Hoffmann and I have in preparation a long paper on orbital symmetry conservation. It exists in a crude draft form, and we hope to be able to revise and improve it before the end of this year. We cannot at present be certain of its precise length, but it is very likely that it might be too long for publication in Angewandte Chemie, or that at least it would take up an entire issue. In these circumstances I raise the question of whether you might be willing to consider the publication of such an extended article.

2. It is quite possible that the article by Dr. Binsch might well be outstanding and that it might as well or better serve your purposes than the article by Dr. Hoffmann and myself. Perhaps you would value my own opinion on that point, and should it not violate your editorial policy, I should be glad to examine Dr. Binsch's article if you would care to send it along” [162].

Note also that Woodward was apparently not too concerned that Grünewald might rescind his invitation, possibly rendering useless Hoffmann's long draft, which certainly would not have pleased Hoffmann. Hoffmann was not worried for several reasons. One, more than being an optimist, Hoffmann is not a worrier. Second, Hoffmann is a pragmatist. He had no doubt that, after the publication of the five JACS communications of 1965 [2, 3, 4, 5, 6], other journals would be eager to publish a treatise on pericyclic reactions by Nobelist Woodward and Hoffmann [163].

Grünewald responded immediately to Woodward (November 8, 1967):

“When I read the first of your two comments, I was reminded of a story that is told about the famous painter Max Liebermann. A young painter once complained that, in a painting by Cezanne, one of the persons depicted there had one arm which was too long [Figure 13]. Liebermann's reply was: ‘An arm painted by Cezanne cannot be long enough.’

FIGURE 13.

Paul Cézanne's “The Boy in the Red Vest” at the Foundation E. G. Bührle in Zürich was painted in 1889 or 1890 and is the likely work of art to which Max Liebermann was referring, “one arm which was too long” [164] (see the text). See also: http://www.friedrichshainerschule.de/redvest.htmt and https://commons.wikimedia.org/wiki/File:Le_Gar%C3%A7on_au_gilet_rouge,_par_Paul_C%C3%A9zanne.jpg.

Similarly I would say that a paper written by you and Dr. Hoffmann on orbital symmetry conservation cannot be long enough. In other words: Whatever the size of the manuscript which you are presently preparing may be, we shall accept it for publication in ANGEWANDTE CHEMIE, because I trust that it will be an excellent contribution.

I hope very much that you will send me this manuscript as soon as it has reached its final stage, and we shall then proceed with its publication as fast as possible” [164].

No further mention of Binsch's review article on the Woodward‐Hoffmann rules was made by either Grünewald or Woodward. And such a publication does not appear in Binsch's list of publications via my own SciFinder search.

On December 12, 1967, Grünewald wrote again to Woodward, inquiring if Woodward and Hoffmann would consider publishing a monograph instead of an article in Angewandte Chemie [165]. There is no record of any response by Woodward or Hoffmann to this inquiry. I shall return to this matter in Section 7, when the topic arises again with a positive response from Woodward and Hoffmann to a slightly different possibility.

A further year passed with no communication between Grünewald and Woodward. Grünewald surely could not have anticipated what did happen next. On December 30, 1968, Woodward wrote to Grünewald:

“It gives me the very greatest pleasure to transmit to you with this letter two complete copies of the manuscript of the paper by Roald Hoffmann and myself entitled ‘The Conservation of Orbital Symmetry,’ for publication in Angewandte Chemie.

I realize that the length of the paper may well present some unusual problems, but I hope on the positive side you will find considerable satisfaction in the knowledge that it certainly would never have been written without your assurance that whatever the size of the manuscript might be, you would surmount the problems of publishing it in Angewandte; only that assurance has enabled me to write the paper, in my blood, while I have let all of my other responsibilities lapse completely during the past several months.

I don’t think you’ll regret your bold initiative” [166].

On January 14, 1969, Grünewald acknowledged receipt of the manuscript. He wrote:

“Thank you for your letter of December 30 and for the big manuscript on ‘The Conservation of Orbital Symmetry’. It arrived here a few days ago and I enclose the printed notification card, but in addition to this I wish to thank you personally. Although it arrived after December 25, 1 consider this manuscript my finest Christmas present and I shall see to it that it becomes translated and published as fast as possible.

I realize that the preparation of this manuscript meant an unusual amount of work to both you and Professor Hoffmann, but I am sure on the other hand that the response to this paper will be overwhelming and that it will stand out for many years to come as one of the most important contributions to organic chemistry. Needless to say, I am proud to have the ity to publish this manuscript in ANGEWANDTE CHEMIE” [167].

Needless to say, Woodward and Grünewald were both correct in their anticipations. Woodward was right when he characterized the matter as a “bold initiative” [166]. Grünewald was right when he anticipated the publication to be “one of the most important contributions to organic chemistry” [168]. For many years, it was among the most downloaded articles from Angewandte Chemie.

‐‐‐‐‐‐‐‐‐‐‐

For Helmut Grünewald, receiving and reading the Woodward‐Hoffmann treatise in early January 1969 must have been a wonderful, joyous event. It was certainly the crowning achievement in his editorship of Angewandte Chemie. And it also made its mark on Angewandte Chemie, which up to that point was a fine but small and not yet top‐ranked journal, as it would soon become. Woodward and Hoffmann's treatise gave this one‐time Germany‐facing journal an international exposure and an intellectual allure that would continue with time.

7.2. Writing the Angewandte Chemie Manuscript

7.2.1. An Overview of Hoffmann's and Woodward's Writing of the Angewandte Chemie Manuscript. A Treatise Without an Organizational Plan

Woodward and Hoffmann's Angewandte Chemie publication actually consists of four authorship stories that occurred simultaneously in 1967 and 1968. These four stories involve:

• •The development of new ideas and concepts.
• •The evolution of the manuscript itself, how certain topics appeared and disappeared during the writing of the various drafts;
• •The individual contributions made by Woodward and Hoffmann to the various drafts; and
• •The melding of an enormous amount of text and graphics into a single, unform, cohesive, and archival document.

In the following sections, I shall discuss each of these stories.

‐‐‐‐‐‐‐‐‐‐‐

There is no evidence that either Woodward or Hoffmann thought deeply about the goals and objectives of their Angewandte treatise prior to beginning their individual writing contributions. We do know that Hoffmann wrote the first two drafts. Succeeding drafts were built upon Hoffmann's foundational drafts with contributions from both Hoffmann and Woodward. Hoffmann had no plan, in terms of content, order, length, and so forth. He did not need one, either. Throughout his career, Hoffmann has written manuscripts that he has described as “simply written, never using an outline” [169].

Woodward had the well‐deserved reputation for under‐publishing, in spite of his awesome writing capabilities [51, 52, 170, 171]. Style, not volume, was Woodward's forte—a behavioral quality that was also consistent with his self‐described “tranquil, equable, and lethargic temperament” [172]. Many of his students’ research results remain hidden within unpublished reports and Ph.D. dissertations. Some of Woodward's publications appeared many years after the experimental work was completed. For example, a preliminary communication [173] and a general account [174] of the total synthesis of strychnine appeared in 1954 and 1955, respectively; however, the full paper appeared only in 1963 [175, 176]. For another example, the communication [177] and a general account [178] on the total synthesis of chlorophyll appeared in 1960 and 1961, respectively; however, the full paper [176] appeared only in 1990, eleven years after Woodward's death. And there was never a joint communication by Woodward and Albert Eschenmoser on their collaborative total synthesis of vitamin B₁₂ [179]. There likely were varied reasons for these delays or absences, one of which was, in Woodward's own characterization of himself, due to his “lethargic temperament” [172].

Hoffmann rightfully anticipated that if an article for Angewandte were to materialize, he had to write it, or at least, begin a collaborative process by first providing an extensive draft to Woodward. So, as we have seen in Section 4, Hoffmann began writing the first draft within months of his arrival at Cornell [180]. And in late 1966 or early 1967, he sent a rather extensive typewritten draft to Woodward.

7.2.2. Dating the Drafts and Documents Relevant to the Angewandte Chemie Publication

There were, at a minimum, two drafts prepared by Hoffmann at Cornell (herein named CD‐1 and CD‐2) and three Harvard drafts prepared by Woodward and Hoffmann (named HD‐1, HD‐2, and HD‐3, as these were maintained at Harvard though much was written at Cornell) (Table 4). HD‐3 is the final manuscript submitted to Angewandte Chemie. During the 3 years that Hoffmann and then Woodward and Hoffmann produced these drafts, there were numerous inserts, deletions, and modifications. Consequently, there are blurs in the interpretations as to what sections of texts in a somewhat disorganized folder within Woodward's papers at the Harvard Archives belong to which draft. While none of these drafts was dated, there are hints about dates within some of the documents. For example, in his letter to Woodward dated “Tue. Jan. 2,” Hoffmann wrote:

TABLE 4.

| The Angewandte Chemie Drafts.

1965–1968	‐‐‐	Hoffmann drafts various handwritten and typewritten section drafts, of various chronologies.
1966	Cornell Draft 1 (CD‐1)	Hoffmann works on and completes a handwritten draft that is given to a secretary who produced a typewritten version.
Fall of 1966 to early 1967	Cornell Draft 2 (CD‐2)	Hoffmann's typewritten draft sent to Woodward.
1967–1968	Harvard Draft 1 (HD‐1)	Mostly typed, sometimes with red Roman numerals identifying specific sections, sometimes with page numbers, e.g., “K15‐K18.” with some handwritten inserts and sections.
1968	Harvard Draft 2 (HD‐2)	Typed draft with references, often with smaller type font and larger interline spacing. Section numbers and page numbers are sometimes indicated by a four number code.
1967–1968	‐‐‐	Intermediate page drafts and new sections by Woodward and Hoffmann, individually and together.
1968	Harvard Draft 3 (HD‐3)	Typed near‐final draft
December 30, 1968	Final manuscript	Submitted to Angewandte Chemie

“Attached are some further parts of our paper:

1) small changes and references for the first XVI sections . . . ” [181]

Hardly ever were Hoffmann's notes to Woodward dated.

The year must have been 1968, as January 2, 1967, was a Monday but January 2, 1968, was a Tuesday. This then dates the sections in Roman numbers to have been written in late 1967 and early 1968 and are herein classified as in HD‐1. Section identification shown in Table 4 are the result of analyses of Hoffmann's papers by Hoffmann and me and analyses of Woodward's papers held in the archives at Harvard (mostly by me but with substantial help from Hoffmann). The master copies of the last several drafts were typed by Dyer and are found at Harvard. Three “complete” Harvard drafts were found within the Harvard archives, and each of these contained a header page (see, for example, Table S12) written by the individuals who organized Woodward's papers after he died on July 8, 1969. Two of these header pages, written shortly after Woodward's death by the organizers of Woodward's unfiled documents, labeled the bundled documents as:

“Misc. early versions of various sections (some paste‐up, cut‐out, etc.) (Had been just to the right of center on shelf – between next to final version & next to next‐to‐final version)” [182].

and

“Version before next‐to‐final version (Had been spread out on shelf on right side)”

“Sections already typed into final white version (i.e. final version typed from these sections)” [182]. See Figures S9 and S11–S13.

These refer to the drafts named above, namely HD‐1, HD‐2, and HD‐3. There is also a large folder containing miscellaneous and unorganized if not disorganized inserts.

While writing the various Harvard drafts, Woodward and Hoffmann each wrote sections or portions of sections of various lengths, along with making corrections on those drafts and earlier drafts (primarily Woodward). These documents, actually “inserts,” were incorporated into master manuscripts, which had been numbered sequentially from 1, 2, 3, . . . except for CD‐1 which began with 0000, 000, 00, 0, 1, 2, . . . The inserts were numbered in several different styles: sometimes “1, 2, 3, etc.,” or in numerical order beginning with a very high number, e.g., “501, 502, etc.” and sometimes beginning with a capital English letter, e.g., “K1, K2, W01” [183], or in Roman numerals. In some instances, several “inserts” were pasted together. These are identified by the observation that their pages numbers are not in sequence, but the pages were found together in Hoffmann's files and Woodward's archives. More importantly, the text in the Angewandte treatise follows the course of the text in these seemingly separate inserts. As can well be anticipated, HD‐1 and HD‐2 were modified by both Woodward and Hoffmann, pieces of which became inserts into subsequent drafts. Even though HD‐2 was near the end of the long process, it contains much new text and new inserts from previous drafts (see Figure S13) for many examples. One illustration of this practice is shown in Figure 14. Unfortunately, none of these drafts nor any of the inserts were dated. To summarize, flexibility and nonlinearity were rampant in the development of the Angewandte treatise.

FIGURE 14.

Illustration of Woodward's intensive rewriting of Hoffmann's drafts. (Left) Page 40 of Hoffmann's first typewritten draft of the Angewandte manuscript (CD‐2) [80], late 1966 or early 1967. This page was originally page 35 of that draft but updated to page 40 in the version provided to Woodward. Subsequently, this page was inserted within successor drafts, the page becoming page G10 of (likely) HD‐1 [184]. (Right) An excerpt from the 1969 Angewandte treatise [13], in which much of Hoffmann's original text and Woodward's modifications were retained for the text on the left. Compare the structure III at the top left graphic with graphic labelled “(55)” at the bottom right graphic. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.2.3. Topic Trajectories in the Drafts. The Table of Contents

While undated, Woodward's handwritten Table of Contents (TOC) reproduced in Figure 15 helps us date sections of drafts that have different but related numbering systems: capitalized English letters and capitalized Roman numerals. Not quite a Rosetta Stone, this TOC assisted Woodward and Hoffmann in the draft's organizational needs and assisted Hoffmann and Seeman in analyses performed nearly 60 years later. Section I and Section J of HD‐1 are not listed, and the first Roman number is “XI.” Missing are the entries for “I” and “J,” the ninth and tenth letters of the English alphabet. Table 5 displays the chronological trajectories of the major themes within the orbital symmetry umbrella as presented in these drafts and ultimately in the publication. A number of observations and conclusions regarding the writing of the Angewandte treatise can be made based on an analysis of these drafts. More specific observations and aligned, illustrative graphics will be presented in the next sections.

FIGURE 15.

Woodward's handwritten two‐page Table of Contents for the Angewandte treatise, ca. late 1967 or early 1968 [185], likely for HD‐1.

TABLE 5.

| Content in the sections and pages of the major drafts and ultimate Angewandte Chemie publication and monograph (a = absent).

Topic	CD‐1	CD‐2	HD‐1	HD‐2	HD‐3 final
	Pages (section identifiers when reported)				Section
Introduction	0000–0001	1	“Intro 1” and “Intro 2” W01‐W02 (I)	A 1–2 (1)	1
Orbitals and Bonding	16	21	1–9 (II)	B 3–9 (2)	2
Molecular Correlation Diagrams and Cycloadditions	00–0, 17−21, 23, 84–87	2–25	2–10 & 15–39 (III)	Ca ca. 500–513 (3)	3
General Comments on the Construction of Correlation Diagrams	1–16	(a)	B1‐B6, 73–77 & two unnumbered pages, 34–39 as part of Section III (IV)	C1 514–517 (3.1)	3.1
Precautions in the Construction of Correlation Diagrams	48–60	73–76	40–47 (V)	Cc 518–522 (3.2)	3.2
The Conservation of Orbital Symmetry	0000– 000, 82–87	1, 98–107	48–49 (I, VI)	D 523–524 (4)	4
Theory of Electrocyclic Reactions	25–33	32–40	G1‐G10 (VII)	E 525–532 (5)	5
Electrocyclic Reactions Exemplified	a	a	K1‐K25 (VIII)	F 533–551 (5.1)	5.1
Theory of Cycloadditions and Cycloreversions	a	14	1–9 & 52–53 (IX)	G 552‐557a (6)	6.0
Content: Introduction of nomenclature, σ2a and π4s.	a	a	53a‐53b	553, 553c, 554‐557a	6.0
Cycloadditions and Cycloreversions Exemplified	a	41, 109	909–944 (X, XIV)	558–583 (6.1)	6.1
1,3‐Dipolar Cycloadditions	36–39	41–48	41–47, 130 (X)	579–583	6.1
[2 + 2] Cycloadditions in the Photochemistry of the Cyclohexadienones and Cyclohexenones	a	a	D1‐D15 (AA)	688–705 (6.2)	6.2
The [2 + 2 + 2] Cycloaddition Reaction	a	a	1–16 (XI)	1001–1014 (6.3)	6.3
Prismane	a	a	1–9 (XII)	1016–1025 (6.4, XII)	6.4
[2 + 2 + 2 + 2] Cycloadditions	a	a	S1‐S4 and page 51–54 (XIII)	600–602 (6.5, XIII)	6.5
Bicyclobutane	92–94	76–77, 107‐110	M1‐M6 (XIV)	564–566, 913	3.1, 6.1
Theory of Sigmatropic Reactions	40–46	48–60	51–58 (XVI)	603–611 (7.0, XV)	7
Sigmatropic Reactions Exemplified	a	a	J1‐J16 (XVIa) and S1‐S16 * 1–12 (XVIIa)	610–628 (7.1)	7.1
The Vinyl Cyclopropane–Cyclopentene Rearrangement	94–95	110–111	N1‐N3 (XVII)	612‐612b	7.1
1,3,5‐Hexatriene–Bicyclo [3.1.0]hexene rearrangement	a	112–115	35–36, 5‐8 & one numbered page in earlier drafted section	515, 556–569, 693	3.1, 6.1, 6.2
Sequential Sigmatropic Shifts	a	a	S1‐S16 and 1–12 (XVIIa)	628–639 (7.2)	7.2
Group Transfers and Eliminations	45–50	60–64, 69−72, 104	** R1‐R7 (XVIIa, XVIII)	640–645 (8.0)	8
Elimination of Small Molecules	51–52	66–68	H1‐H10 (XVIIIa)	646 (8.2)	10.1
The CO2 Elimination	88	64–66	P1‐P4 (XIX)	Scattered within 1–22 (10.1)	10.1
Abstraction	a	a	Abstraction 1 (4, XX)	16–17 (10.1)	10.1
Secondary Effects	22	26–30, 56‐60	1–12 (XV) and (26–30, 56−62)	656–665 (9.0)	9
Divertissements!	Introduction to Cheletropic Reactions & Cycloadditions			1 (10.0)	10
Cheletropic Reactions	52–55	61–67	(XIX, XX)	1–11 & 271 & three unnumbered pages	10.1
Cycloaddition Reactions of Ketenes	a	a	In Violations section	1–13 (10.2)	10.2
Generalized Selection Rules for Pericyclic Reactions	a	a	a	1–10 (11)	11
Violations	C.f., cheletropic reaction section		1 (Viol)−18 (Viol) and V1‐V24 (XXIII)	666–683 (12)	12
Other Theoretical Work	100	116–118	1–4 (XXII)	684–687 (13)	13
Conclusions	a	a	W03 and Concl. 1‐Concl. 2 (XXIV)	(14)	14
Acknowledgments	99	119	a	a	a
Catalog of Concerted Reactions	62–77	78–97	(XXI)	a	a
Further Precautions	58–60	73–76	(IV)	519a‐521, 607	3.2
Some Further Symmetrical Reactions	48–57	60–72	(XVIIII)	1–11 plus several unnumbered pages (10.1)	8, 10.1

7.2.4. Major Modifications, Even at the Very End

• •Perhaps the most unanticipated observation from the analysis of the various drafts of the Angewandte treatise involved HD‐2, the last (major) draft before the final manuscript was prepared. Most remarkable for a last draft is the degree to which much new text was written, primarily by Woodward. That is, within HD‐2 are pages and pages of text, such as that in Figure 16, which found their way into the Angewandte treatise [13] (several other examples are provided in the Supporting Information). Even in HD‐2, section numbers were being revised and reordered, e.g., the section “[2 + 2 + 2 + 2] Cycloadditions” was numbered “6.5”: and “XIII.“
• •The “Catalogue of Concerted Reactions” was in CD‐2; then, it permanently disappeared. Its contents, however, were described in time, within the pages of the Angewandte treatise [13] and published independently by others [187, 188, 189]. See Figure S34.
• •An “Acknowledgment” section appeared only in CD‐2 and not thereafter.
• •Hardly any literature examples of pericyclic reactions are found in CD‐1 and CD‐2. Rather, the barebones unsubstituted hydrocarbon models for electrocyclizations, cycloadditions, and sigmatropic reactions were reported in these early drafts—just as was found in Woodward and Hoffmann's first five JACS communications of 1965 [2, 3, 4, 5, 6]. There is no implication here that Hoffmann, the author of those two drafts, was averse to citing the literature. We have already seen in earlier discussions and will see again that he enjoyed literature searching and citation. Perhaps Hoffmann did not feel the need, in a first draft of a paper, to include literature examples of pertinent experimental results. In some drafts, the text served as placeholders for subsequently provided references. Or alternatively, Hoffmann envisioned a publication with barebone structures as was common in Woodward and Hoffmann's first publications.
• •The section “Generalized Selection Rules for Pericyclic Reactions” appeared first in HD‐2.
• •The section on “Divertissements” first appeared in HD‐2 and, in the final manuscript, only two topics were placed therein: the entire discussion of cheletropic reactions and elimination of small molecules plus cycloadditions of ketenes. Even though “divertissements” means “diversions,” the section on “Cycloaddition of Ketenes” could well have been included in the “Cycloaddition” section and “Cheletropic Reactions” could have replaced the term “Divertissements” as that would have been the only topic within such a section.
• •As the drafts progressed from one to the next, the MO theoretical underpinnings of the various categories of pericyclic reactions were explained in expanding detail. Nonetheless, the explanation for sigmatropic reactions remained poor, even in the Angewandte treatise, as was the case in all of the earlier W‐H publications.
• •The two Cornell drafts (CD‐1 and CD‐2) included antarafacial cycloadditions, but Hoffmann termed them trans, consistent with the nomenclature in Woodward and Hoffmann's Accounts essay of January 1968 [12]. It was only in the Angewandte treatise that these concepts would be termed “a” (or “antarafacial) and “s” (or “suprafacial”) cycloadditions.
• •As will be shown below, once the drafts were sent to Harvard, Woodward took on the role of “manuscript administrator” in terms of shepherding the various drafts, inserts, graphics, and references.

FIGURE 16.

Woodward's explanation of the principle of conservation of orbital symmetry for symmetry‐forbidden cycloadditions. (Left) From Woodward's notes, page 553c [186] of HD‐2. (Right) From Section 6 of W‐H's Angewandte treatise [13]. Woodward inserted major rewrites, even in the very last draft. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.2.5. Hoffmann Often Urged Woodward to Write

It took until mid‐1967 or early 1968 before Woodward himself engaged in writing his portion of the Angewandte treatise. This was more than 2 years after receiving Grünewald's invitation (Section 3). Woodward's lack of participation in the Angewandte draft did not disturb Hoffmann too much. Hoffmann was busy establishing his own independent research program at Cornell. Hoffmann was also producing his own draft of the Angewandte treatise, and as events would play out, writing his and Woodward's essay for the January 1968 inaugural issue of Accounts of Chemical Research. Recall that in mid‐March 1967, Bunnett had invited Hoffmann to write such an article (see Section 5.a) [38]. “The Conservation of Orbital Symmetry” was received by Bunnett on July 3, 1967 [12].

But Hoffmann was not completely indifferent regarding Woodward's attitude toward Angewandte's invitation. He raised the Angewandte matter to Woodward several times, as evidenced by the following items (in chronological order).

• •On July 27, 1966, Hoffmann sent a multipage letter to Woodward [190] containing his (Hoffmann's) rather negative review of Fukui's submission to JACS on orbital symmetry [191] (that article was later published in its entirety in the Bulletin of the Chemical Society of Japan with Hiroshi Fujimoto, then a member of Fukui's koza and later Hoffmann postdoctoral student [54].) Fukui and Fujimoto revised their manuscript based on Woodward and Hoffmann's anonymous review.) Hoffmann was concerned that Fukui and Fujimoto were right on the heels of Woodward‐Hoffmann and perhaps even ahead of them. This was a very reasonable concern, in that that Fukui actually published orbital symmetry control [192] before Woodward‐Hoffmann [2]. Hoffmann's concern was communicated in the penultimate paragraph of his July 27, 1966, letter (Figure 17):

  “Reading this makes me think we must get a general paper out soon” [190].

FIGURE 17.

Excerpt from Hoffmann's handwritten three‐page letter on July 7, 1966 [190], containing his rather negative peer review of an article submitted by Fukui to JACS [191] and subsequently published in the Bulletin of the Chemical Society of Japan with Hiroshi Fujimoto [191]. The penultimate paragraph of the letter expressed Hoffmann's concern that he and Woodward “Must get a general paper out soon” [190].

• •In a letter to Woodward on June 16, 1967, dealing with the Accounts manuscript (W‐H 10) (see Section 5), Hoffmann expressed his

  “hope that you will get around to rewriting the long paper” [97].

• •Just prior to July 26, 1968, Hoffmann wrote

  “I would like to set some time for the final push at the long paper some time in August . . . ” [193]

• •In early August 1968, Hoffmann wrote to Woodward,

  “Here are two preprints illustrating the troubles one can get into by drawing correlation diagrams (and why we’d better get our paper out quickly)” [194] (Hoffmann does not recall what the preprints were, but thinks they were likely of work by other people [163]).

The Angewandte treatise (W‐H 11) was received in the journal's editorial offices on January 7, 1969, and appeared in the November 1, 1969, issue of the journal [13].

Woodward had his own frustrations and disappointments with the timing of the treatise. For example, in a letter to Paolo Beltrame dated April 25, 1968, in response to a request for information [195]. Woodward wrote:

“Professor Hoffmann and myself are preparing an extensive paper on orbital symmetry conservation for publication in Angewandte Chemie. Although I cannot with certainty predict when it will be completed, it is my hope that a manuscript will be available this summer [of 1968]. I shall be glad to try to get a copy to you when it is available” [196].

And after the appearance of the Angewandte treatise, Woodward wrote to G. B. Gill on January 7, 1970:

“By now you will no doubt know that the rumored paper on orbital symmetry conservation has appeared much later than we had supposed it might. The situation in respect to reprints is that there will be none in the ordinary sense. The memoir will be separately published as a brochure, with identical content, but different format. Although I have not seen any copies, they should be available soon, and when they are, I shall be glad to send you a complimentary copy. In this same connection, may I say that I enjoyed your Quarterly Reviews article [entitled “The Application of the Woodward‐Hoffmann Orbital Symmetry Rules to Concerted Organic Reactions” [140] I very much” [197].

7.2.6. Woodward and Hoffmann's Working Sessions at Harvard in 1967 and 1968

Because neither Woodward nor Hoffmann kept diaries, for information about Woodward and Hoffmann's collaborative efforts, we can only rely only on representations made by Woodward and Hoffmann in the extant documents and especially in Hoffmann's laboratory notebooks. For example, in a letter dated December 22, 1967, to Dieter Seebach, Woodward encouraged Seebach to publish his review on the Woodward‐Hoffmann rules in a book chapter (which Seebach did in 1969 [198]). Woodward also informed Seebach,

“Dr. Hoffmann and I have just been able to spend a week of concentrated work on our rather more extended article for Angewandte Chemie, and I hope that before long we should be able to get it into final form” [199].

In his August 16, 1968, letter to Salem dealing with a question Salem had regarding his own publication strategy, Hoffmann wrote:

“I’ve just come back from my third visit to Harvard this year [1968, or did he mean the 1967 – 1968 academic year?] – we have spent a total of some 12 days and nights working on [the Angewandte Chemie manuscript]” [1].

There surely would be further one‐on‐one meetings for Woodward and Hoffmann. On August 7, 1968, Woodward wrote to Huisgen that

“I am about to go underground, and make a massive effort to complete our mammoth paper on orbital symmetry conservation for Angewandte Chemie” [200].

On July 28, 1969, during the time that Woodward and Hoffmann were working on the galley proofs and other production issues of which there were many, Hoffmann wrote in Woodward's office autograph book:

“Every time I come here I hope it's the last” [201]. (See Figure 18).

FIGURE 18.

Hoffmann's entry in Woodward's office autograph book, Cambridge, MA, July 28, 1969 [201].

As Hoffmann recalled recently:

“Yes, that was a joke, because I had to travel to Cambridge to work on the big Angewandte Chemie paper many times. And Woodward didn’t come to Ithaca” [202].

7.3. Hoffmann's Contributions to the Drafts

This section documents the variety of contributions Hoffmann made to the various drafts of the Angewandte treatise. In addition to the examples discussed in this section, please see the (Figures S4–S8) for additional information.

7.3.1. Hoffmann Sharing New Inserts with Woodward

Likely Hoffmann's most important role in the writing of the Angewandte treatise was the preparation of the first two 90‐plus page drafts (CD‐1 and CD‐2; see Table 4). Once CD‐2 was in hand, both Hoffmann and Woodward prepared “inserts” that would be used in the preparation of updated drafts. Figure 19 presents one example. In his typical undated‐formatted letter, Hoffmann informed Woodward of an attached “new section for the paper” [204]. Hoffmann boasted “everything fits beautifully—particularly some new information from Schaffner” [204] and suggested that “Miss Dyer to type this up for our working session” [204] (Figure 19A) (Dolores “Dodie” Dyer was Woodward's “secretary,” as an “administrative assistant” was then called). One page from Hoffmann's attachment is reproduced in Figure 19A along with an excerpt from Woodward and Hoffmann's 1969 treatise (Figure 19B) which shows this new section was used in the final publication.

FIGURE 19.

(A) One page from a new section handwritten by Hoffmann and sent to Woodward on the photochemistry of β,γ‐unsaturated ketones, ca. 1968 [203]. (B) An excerpt from Section 6.2 of the Angewandte treatise [13] derived from (A). Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

An anonymous journal reviewer for The Chemical Record asked:

“I have always found the ACIE article to be extraordinary evidence of a comprehensive knowledge of an extensive literature, along with a mastery of stereochemistry. I always imagined that Woodward carried a set of molecular models in his head. Did Hoffmann share those capabilities? If so, it is worth commending him. Or did he rely on the ‘two undergraduate summer interns’? Any contribution from them was not acknowledged.”

As discussed in a previous publication in this series [57, 78, 205], for many reasons, Hoffmann was the perfect collaborator on the orbital symmetry project. Of course, Hoffmann brought with him a wide knowledge of theoretical chemistry and his invention, along with others in William Lipscomb's research group at Harvard, of the extended Hückel theory and its computer implementation. Beyond that, Hoffmann had extensive experience in analyzing rearrangements in multiring boron hydrides and their valence isomerizations, compounds being far more geometrically complex than those within the no‐mechanism problem [86]. Hoffmann needed no one to rely upon (as the reviewer questioned), for he was as capable as Woodward in dealing with very complex three‐dimensional stereochemical problems.

Of course, central to the no‐mechanism problem and its solution, the W‐H rules, is stereochemistry. While Hoffmann made great use of eHT, graphics terminals were still more than a decade in the future. Handheld molecular models played an enormous role in understanding stereochemical complexity. The very expensive, elegant, and certainly prestigious stainless steel Dreiding models were useless at this stage of research on pericyclic reactions, while the Framework or Prentice‐Hall jack and straw molecular models were perfect [206].

7.3.2. Theory New to W‐H

Hoffmann's second most important role in developing the Angewandte treatise was in the construction and pedagogy of relevant qualitative MO theory, especially theory that did not appear in any of the previous W‐H publications. Three examples of the latter role are provided in Figure 6 (dealing with suprafacial and antarafacial cycloadditions), Figure 20 (dealing with actual though inconsequential dissymmetry and intended and actual MO level crossings), and Figure 21 (dealing with the use of three‐center bonding in the explanation of sigmatropic reactions). Figure 21A is particularly interesting, in that Hoffmann's thinking can be traced all the way back to his laboratory notebook pages.

FIGURE 20.

The suprafacial–suprafacial [2 + 2] cycloaddition. (A) An excerpt from Hoffmann's first draft (CD‐1) for the Angewandte treatise from 1966 and the underlying theory behind the principle of the conservation of orbital symmetry [79]. (B) From Section 4 of Woodward and Hoffmann's 1969 Angewandte treatise [13]. The use of the phrase “pari passu” (“equal footing”) points to Woodward's hand in revising Hoffmann's draft. Woodward used that phrase in at least three of his publications [20, 21, 207, 208]. Note the qualitative correlation diagrams depicting actual (A, left) and intended (A‐right and B) molecular orbital level crossings of a W‐H forbidden reaction. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

FIGURE 21.

Three‐center bonds and sigmatropic reactions. (A) Excerpt from Hoffmann's Laboratory Notebook 17, pages 26, very early April 1965, illustrating (at the left) the molecular orbitals of a three‐center bonds and (below) the HOMO at the TS in a suprafacial sigmatropic reaction involving inversion via the p‐orbital of a migrating carbon atom. (B) From page 42 of Hoffmann's first draft (CD‐1), ca. late 1965 [79]. (C) From page 45 of Hoffmann's first draft (CD‐1), ca. late 1965 [79]. (D) From Woodward and Hoffmann's 1969 Angewandte treatise [13] and 1970 monograph [15] for a system described in (A–C). Colors exactly per Woodward's design (see Section 6.g). Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition, 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

This was a very important role that Hoffmann undertook, and it had psychological consequences [163]. While RBW had a growing understanding of orbital interactions, frontier orbitals, and perturbation theory in constructing the pedagogical material (as can be seen in some of his chemistry doodles), Hoffmann came to play the master position in a master–apprentice learning situation. Hoffmann believes [53] that he, more than Woodward, cared that readers of their publications understood the simple MO arguments, and where they could go wrong. Why? Because Hoffmann was just then (ca. 1968) teaching introductory chemistry at Cornell, and in doing that, he gained empathy for the learner. According to Hoffmann:

“By then, it was natural for me to worry whether the audience understood what we said” [163, 206].

7.3.3. The Generalized Selection Rule

Hoffmann provided a typewritten, seven‐page section entitled “Cycloadditions with Single Orbital Components” (pages 5100–5106) with a Section headed “AC” [209]. Portions of this section found their way in Section 11 of the Angewandte treatise, “Generalized Selection Rules for Pericyclic Reactions.” Figure 22 shows one of these pages. Other pages were modified by Woodward who advanced the theory by incorporating “phantom orbitals” that did not appear in the final publication. But as shown in Section 6.d.9, Woodward may have composed the majority of this section.

FIGURE 22.

(A) Page 4 from a seven‐page draft section (numbered pp 5100–5106) entitled “Cycloadditions with Single Orbital Components” [209]. (B) These concepts found their way into Section 11 of the Angewandte treatise [13] on “Generalized Selection Rules for Pericyclic Reactions.” Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969); 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

What is most important here is that the generalized formalism, with its σ, π, and somewhat abstruse ω with their associated electron number notation did not appear in Woodward and Hoffmann's drafts until late in the game, i.e., in HD‐2. In developing the generalized selection rules, Woodward and Hoffmann were not just summarizing what they had wrought in 1965. They were learning and understanding in the course of writing these drafts. And they were thinking of new ways to teach their discoveries. And their audience noticed that.

7.3.4. Hoffmann Brought Literature References to Woodward

In addition to sending Woodward his 119‐page typewritten draft (CD‐2) in late 1967 or early 1968, Hoffmann also included

“20 pages of references from my notebooks, as well as a folder with crucial articles containing many more references. . . . . I suggest that you give these + 1966 Annual Reports Chem Soc. to a graduate student to compile a good bibliography” [87].

Some might consider Hoffmann's suggestion a bit cheeky. I do. Furthermore, imagine a 29‐year‐old chemical physicist providing literature references in organic chemistry to “the” Nobel laureate R. B. Woodward, and then suggesting that one of his (Woodward's) students do literature work on their behalf! As Albert Eschenmoser, Woodward's long‐term collaborator on the total synthesis of vitamin B₁₂, said of Woodward:

“He had all of organic chemistry at his disposal in his mind, in front of him. At a symposium, something would come up and RBW could give an example, out of the air somehow” [210].

As a second example of Hoffmann supplying literature references, we point to the rather detailed page reproduced in Figure 23. In his letter to Woodward dated January 2, 1968 [181], Hoffmann wrote with awe that he had just uncovered an antara–antara Cope rearrangement (Figure 24). The W‐H publications benefited from Hoffmann's knowledge and hard work in scouring the literature for examples of pericyclic reactions. Hoffmann remains appreciative of Woodward, saying:

FIGURE 23.

Page 2 from Hoffmann's section on “2 + 2 Cycloadditions in the Photochemistry of the Cyclohexadienones and Cyclohexenones” [211]. Hoffmann was very well prepared to write on this subject, in that he had performed eHT calculations on ketone photochemistry, some of which were cited in one of E. J. Corey's 1964 publications [212]. Noteworthy, Hoffmann's own submission on the topic had been rejected by JACS and never resubmitted (discussed in some detail in a previous publication in this series [205]).

FIGURE 24.

(A) An excerpt from Hoffmann's letter to Woodward dated January 2, 1968 [181]. (B) An excerpt from Mukai's 1967 publication [213] as seen by Hoffmann in 1968. Reprinted from T. Miyashi, M. Nitta, T. Mukau, Tetrahedron Letters 7 (1968): 3433–3437, https://doi.org/10.1016/S0040-4039(01)89859−2. Copyright (1968), with permission from Elsevier.

“I still needed the best organic chemist in the world for his intuition on what was and wasn’t right” [169].

7.3.5. Hoffmann Corrected Errors in the Structures

Hoffmann provided more than theoretical chemistry input and literature references to the W‐H collaboration. He brought substantive organic chemistry into their text and references as well. Consider his corrections in Figure 25 to the stereochemistry of the structures drawn in one of the manuscript's drafts by saying, “Some minor corrections to the section on ‘2 + 2 cycloadditions . . . .” [214] Note the complexity of the photochemical rearrangements that Hoffmann had drawn. These structures are as graphically elegant as if they were drawn by a true organic chemist. For Hoffmann's draft of the first page of [2 + 2] cycloadditions in the photochemistry of enones and dienones, see Figure S10.

FIGURE 25.

Very complex rearrangements are not atypical in pericyclic reactions, especially photochemical pericyclic reactions. (A) An excerpt from Hoffmann's undated letter ca. spring1968, on the University of Puerto Rico letterhead. Hoffmann was providing “minor corrections to the section on “2 + 2 cycloadditions . . . ” [214] (B) Parallel excerpt from Woodward and Hoffmann's Angewandte treatise [13]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.3.6. Hoffmann Predicted New Chemistry

Hoffmann also predicted new organic chemistry. As illustrated in Figure 26, he anticipated the possibility of synthesizing two isomeric 1,2‐disubstituted cyclooctatetraenes from the same source, namely a 5,6‐disubstituted bicyclo [4.2.0]octa‐1,3,5‐triene (compound 126, using Woodward and Hoffmann's nomenclature in Figure 26B). In this instance, ring opening of 126 is sterically restricted to either a six‐electron thermal W‐H allowed disrotatory motion of the 1,3‐cyclohexadiene fragment or a four‐electron thermal W‐H allowed disrotatory motion of the cyclobutene moiety. Two different cyclooctatetraene isomers would result, assuming only pericyclic reactions and no further isomerizations.

FIGURE 26.

(A) Excerpt from Hoffmann's draft [215] sent to Harvard in 1967 or early 1968. (B) Excerpt from Section 5.1 of Woodward and Hoffmann's Angewandte treatise [13], derived from (A). Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training in artificial intelligence technologies or similar technologies.

Charles R. Dawson and Ronald C. D. Breslow, Hoffmann's undergraduate organic chemistry teachers at Columbia College, would have been proud of him. I sense that Hoffmann is proud of himself for these ideas, too [163]. And while ultimately Hoffmann went on to study and publish in inorganic chemistry and solid‐state physics, he never forgot organic chemistry, his first love [206]. Actually, he never left organic chemistry. For example, near the end of his career, he published on cyclooctatetraene [216], on jailbreaking benzene dimers [217], on allyl isosteres [218], and on the Bergman reaction [219].

7.3.7. From Hoffmann's Laboratory Notebooks to the W‐H Treatise

It has been rewarding for me to find new concepts, text, chemical pictography, and conclusions in Hoffmann's laboratory notebooks that were incorporated into the W‐H publications. These occurrences ought not to be surprising. That is what science is about: making and recording observations and then publishing those results. Several examples of these discoveries from Hoffmann's laboratory notebooks have already appeared in this treatise, e.g., Figures 25 and 27. Figure 27 deals with W‐H forbidden double valence isomerizations of cyclooctatetraene ⇋ cubane. Figure 28 illustrates Hoffmann's derivation of the selection rule for group transfers and eliminations (Section 8 in their Angewandte treatise).

FIGURE 27.

(A) Excerpt from Hoffmann's Laboratory Notebook 16, page 93, late February 1965. Double [2 + 2] cycloadditions of cyclooctatetraene to cubane along with two correlation diagrams. See also Figure 41A. (B) Figure 16 from Woodward and Hoffmann's 1969 treatise (W‐H 11) [13, 15]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

FIGURE 28.

(A) Hoffmann's early research notes on group transfers and eliminations. These are reproduced from Hoffmann's Laboratory Notebook 17, page 131, one of his last pages before moving from Harvard to Cornell, mid‐June 1965. (B) From Section 8 in Woodward and Hoffmann's 1969 treatise (W‐H 11) [13, 15]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.3.8. Hoffmann's Additional Inserts to Later Drafts of the Angewandte Manuscript

Many multipage section drafts are present in Hoffmann's and Woodward's archives. These drafts are additions to the master draft that Hoffmann provided Woodward in the winter of 1965–1966 (Section 6.b.2). I shall present the following items as additional ‘building blocks’ of the final manuscript in an order that represents the sequence of topics in the final publication. The following examples were chosen for some combination of substantive text and graphic beauty.

The top of the first page of one of these sections, namely “Orbitals and Bonding” [221], is reproduced in Figure 29. Not included in Hoffmann's first draft, this section represents Hoffmann's tutorial on the fundamental aspects of MO theory such that the average organic chemist could understand some of the more complicated MO theoretical treatments that appear in the text. As shown in Figure 29, Hoffmann's draft was directly reproduced in the publication. Figure 30 contains a second excerpt from this section. Note Hoffmann's artistic graphic in Figure 30A and how well his text and graphics found their way into the final publication (W‐H 11), as shown in Figure 30B.

FIGURE 29.

(A) Page 1 of a nine‐page handwritten section by Hoffmann, ca. 1968 [221]. The section is entitled “Orbitals and Bonding,” just as it would appear in the Angewandte treatise [13] shown in (B). The red “II” at the top‐right in (A) is in Woodward's hand in HD‐1 which would ultimately appear, as shown in (B) in Section 2 in the Angewandte treatise. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

FIGURE 30.

(A) Excerpt from Hoffmann's draft section “Orbitals and Bonding” [221]. (B and C) From Section 2 in Woodward and Hoffmann's Angewandte treatise [13]. for a system described in (A). Some of the text in the draft was modified in the treatise. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

Figure 31 reproduces Hoffmann's “cover note” that accompanied Hoffmann's 18‐plus page handwritten section on electrocyclizations to be added to HD‐1. Surely Woodward would have smiled at Hoffmann's somewhat reserved humor. Combined with Hoffmann's entry into Woodward's autograph book (Figure 18), one can hardly dispute the necessity for some degree of levity in the midst of their very intense multiyear project. Figure 32A is an excerpt from page 15 of this new section, labeled Section “K” in red. Figure 32B shows that Hoffmann's text and graphic were reproduced in W‐H 11 [13, 15]. Figure 33 is the first page of Hoffmann's section entitled “Sequential Sigmatropic Shifts” [203] along with several excerpts from W‐H 11 showing their use in the treatise [13, 15].

FIGURE 31.

Handwritten cover page for Hoffmann's section on electrocyclizations that had at least 18 handwritten pages [215].

FIGURE 32.

(A) From Hoffmann's draft section K (note page K15, HD‐1) [215]. (B) From Section 5.1 in Woodward and Hoffmann's 1969 treatise [13]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

FIGURE 33.

(A) The first page of Hoffmann's section entitled “Sequential Sigmatropic Shifts” [203]. (B) and (C) Graphics from Woodward and Hoffmann's Section 7.1 in their Angewandte treatise [13]. Compare (C) with (A). Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

As an example in Hoffmann's section on sequential sigmatropic shifts, I share Figure 34 [222]. The graphs on this page are particularly delightful, in that they are “essentially a superposition of snapshots of all of the possible positions of labels attached to the methylene carbon atom as it migrates around the six‐membered ring” [13]. Some of Hoffmann's notebook pages are worthy of being framed and hung in a history of science art museum—and perhaps even an actual art museum.

FIGURE 34.

(A) An excerpt Hoffmann's draft insert for HD‐1 on sigmatropic shifts [222]. (B) From Section 7.2 of the Angewandte treatise [13]. Note the differences between the chemical pictographs in the draft and in the final publication. According to Woodward and Hoffmann, “the illustration is essentially a superposition of snapshots of all of the possible positions of labels attached to the methylene carbon atom as it migrates around the six‐membered ring” [13]. The reaction type in this figure seems a frill. But testifying to the care that Woodward and Hoffmann took even with window dressing is that they changed the presentation from the draft in (A) to the final artwork (B, right). Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

Following the publication of the five W‐H JACS communications in 1965 [2, 3, 4, 5, 6], many in the organic chemistry community began to focus on pericyclic reactions. As Hoffmann recalled:

“We had a problem, in that so many reactions and stereochemical outcomes were coming in, published it seemed in every issue of Tetrahedron Letters and JACS. We almost couldn’t keep up with the outcomes. Yes, it's a problem all of us would like to have, but at the least it impelled us to finish the Angewandte Chemie manuscript!” [107]

7.3.9. Hoffmann Seeking Woodward's Guidance (And Hoffmann Making a Prediction)

Even to this day, Hoffmann is effusive in his credit to Woodward [7, 223]. As one example of his reliance on Woodward during their collaboration, in mid‐1968, Hoffmann sought Woodward's opinion on the stereochemistry in the bicyclobutonium interconversions (Figure 35). Hoffmann wrote: “ . . . but I’m not very sure of myself on some” [193] of the steps.

FIGURE 35.

Excerpt from an undated letter from Hoffmann to Woodward, ca. mid‐1968 [193].

For another example of his reliance on Woodward, consider Hoffmann's April 30, 1968, note reproduced in Figure 36. Hoffmann asked a rhetorical question on ketene reactions with ethylene (which find their way into the final version of their paper) [224]. But the letter can also be interpreted as a collegial request for Woodward's input. Hoffmann was surely referring to Montaigne and Ghosez's 1968 publication in Angewandte Chemie [225].

FIGURE 36.

(A) Excerpt from Hoffmann's April 30, 1968, letter to Woodward [224]. (B) Evidence presented in March 1968 by Montaigne and Ghosez in 1968 that the addition of dichloroketene with olefins is a cis stereospecific cycloaddition. Reprinted with permission from: R. Montaigne, L. Ghosez, “Stereospecific cis Addition of Dichloroketene to cis‐ and trans‐Cyclooctenes,” Angewandte Chemie International Edition 7 (1969): 221, https://doi.org/10.1002/anie.196802211. Copyright 1968 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

These notes, flying back and forth between Ithaca and Cambridge between 1967 and 1969, justify my (JIS) feeling that the Woodward‐Hoffmann collaboration reached its apex, its full creative power during this time. The Woodward‐Hoffmann relationship now deserved to be called a true, intimate collaboration. Such connectivity was not present earlier in their joint research—and is not found in later joint publications.

7.4. Woodward's Contributions to the Drafts

In addition to the examples discussed in this section, please see (Figures S13 and S27) for the additional information.

7.4.1. Woodward as the “Secretary”

Someone had to organize, manage, and distribute the various inserts and massive drafts that were to lead to the Angewandte treatise [226]. That someone had to be responsible for the typing of the drafts, the organization and management of the documents, and the final submission. That person was Woodward, aided by his highly organized and professional secretary Dolores Dyer who had worked for him since the early 1950s. During the writing of the various Harvard drafts, Woodward kept track of what he and Hoffmann were to and did write. Several additional examples of Woodward's “secretarial” activities are reproduced in Figures 37, 38, and the Figures S25–S28. Figure 39 provides documentation of Woodward's “secretarial” work as it relates to renumbering the references, the chemical formulae, and the figures in the third Harvard draft (HD‐3). This was a tedious, time‐consuming task of great complexity entailing zero intellectual, conceptual, philosophical, or pleasurable value. Yet it seems that Woodward did not shy away from it. And Hoffmann did his share also (see Figure S24).

FIGURE 37.

Woodward's notes for tasks that Hoffmann had agreed to provide for the Angewandte draft manuscript [226, 227].

FIGURE 38.

Woodward's handwritten note to himself, ca. 1968, exhibiting his excitement and his anticipation of the inclusion of certain ideas into the Angewandte treatise [228].

FIGURE 39.

Woodward's table for renumbering references and formulae for Section 9 for the Angewandte treatise, likely in the preparation of HD‐3 [229].

As Hoffmann said:

“In my years with Woodward, I did not see any increase in compulsiveness or perfectionism. The perfectionism was much in evidence in the writing of our long paper” [230]. [Italics added]

7.4.2. The Table of Contents

Possibly one of Woodward's most important administrative contributions to the writing of the Angewandte treatise was his construction of the first TOC (Figure 15). A TOC is more than a pointer to the location of different topics within a document. A TOC also serves as an organizational scheme for the authors, aiding in the strategic and operational planning of and accountabilities within the writing. Hoffmann “almost never” [107] used TOC in drafting a manuscript.

7.4.3. The Introduction

Woodward and Hoffmann had already used, or rather cannibalized, Hoffmann's original draft Angewandte introduction for their Accounts publication (Section 5.b). They needed a new beginning for their Angewandte submission. It is also possible that Woodward saw Hoffmann's introduction as a bit negative (talking about poor theory). Woodward also correctly perceived what was new about what they were doing. The two of them were studying reactions, emphasizing high or low energy transition states (which go along with forbidden or allowed reactions). People had calculated potential energy surfaces for much simpler reactions (such as the hydrogen exchange reaction, H + D–D → H‐D + D), with approximate methods. They did not explore the quantum chemical reasons for the height of the barriers obtained, as Woodward and Hoffmann did for pericyclic reactions. And the reactions Hoffmann could and did study with eHT were substantially more complex than those studied earlier, in part because eHT could handle nonplanar molecules and in part because computer power had increased tremendously by the mid‐1960s.

In Figure 40A, one can study the text that Woodward wrote and then crossed out, especially the text that he wrote and crossed out and then wrote again. Even Woodward needed to revise his own writings! Figure 40B includes a dramatic moment in the Woodward‐Hoffmann collaboration. In spite of Hoffmann's massive input, indeed the great majority of his intellectual contributions to the W‐H rules as revealed in the previous publications in this series [7, 110, 111, 112], Woodward chose to place his own name first. Younger readers may not know: in the 1960s and 1970s and even in later years, the senior author was always listed first in the byline of a publication. Woodward was proclaiming that he was senior author of the Woodward‐Hoffmann masterpiece, even though an excellent case could be and has been made [7, 110, 111, 112] that he was the lesser of the two equals in the Woodward‐Hoffmann collaboration. Figure 40 reveals the integrity of text that flowed from (A) to (B) to (C).

FIGURE 40.

The “Introduction” in Woodward and Hoffmann's Angewandte treatise. (A) Woodward's handwritten draft for HD‐1 [231, 232]. (B) Typewritten draft of the “Introduction” prepared by Dolores Dyer [183]. The authors’ names were handwritten by Woodward. The text in footnote aaa at the bottom of (B) was written by Dyer. (C) From the Angewandte treatise [13]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.4.4. On MO Theory, Orbitals and Bonding, Correlation Diagrams

Woodward learned a lot of MO theory from Hoffmann, and even before Hoffmann, from his close friend and Harvard colleague William E. Moffitt (1925—1958) [233, 234]. Why from Moffitt? Because in the early 1950s, Moffitt explained the structure and reactivity of ferrocene—an abbreviated engagement of Woodward's [235, 236, 237]—using MO theory [238]. And in the late 1950s, in collaboration with Woodward, Moffitt had used MO theory to provide the underlying theory of the octant rule [239]. Figures 41 and 42 display Woodward's knowledge of and confidence in his knowledge of MO theory, a capability that was also evidenced in his autobiographical essay [11] on the W‐H rules published in 1968 (W‐H 9).

FIGURE 41.

(A) (Upper graphic) In Woodward's hand [220] and (lower graphic) from Figure 15 in the Angewandte treatise [13, 15]. (B) (Left) In Woodward's hand, formation of the π orbitals of benzene from the σ orbitals of prismane [220] which was (B) (Right) translated into Figure 27 in the Angewandte treatise [13]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969); 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

FIGURE 42.

(A) An excerpt from an undated insert [240] in Woodward's handwriting that discusses the pedagogical reasons used by Woodward and Hoffmann in developing their 1969 treatise [13]. (B) An excerpt from the 1969 treatise in which Woodward's words were used therein [13]. One sense from these and other chemical and orbital sketches in their joint work is that Woodward and Hoffmann both loved drawing orbitals and molecules in their full three‐dimensional complexity. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.4.5. On the Photochemical Transformation of 1,3‐Butadiene to Bicyclobutane

Both Woodward and Hoffmann were fascinated with the electrocyclization 1,3‐butadiene ⇋ bicyclobutane as were other chemists. Hoffmann corresponded with Jean‐Marie Lehn [241, 242, 243] and Dauben [244, 245, 246, 247] on this transformation. Interestingly, in 2023, Golfmann and Walker published a review on “Bicyclobutanes as Unusual Building Blocks for Complexity Generation in Organic Synthesis” [248]. Figure 43 contains a graphic illustrating Woodward's thinking about the stereochemistry of the 1,3‐butadiene ⇋ bicyclobutane reaction [249]. The stereochemistry of the cycloaddition of a cross conjugated 1,3‐butadiene ⇋ bicyclocyclobutane is harder to visualize and then analyze than the related electrocyclization of 1,3‐butadiene ⇋ cyclobutene.

FIGURE 43.

(A) Woodward's chemical pictographic noodling around the stereochemistry of the 1,3‐butadiene ⇋ bicyclobutane reaction [249]. (B) Graphic from Figure 12 in Woodward and Hoffmann's 1969 treatise [13, 15]. Note that Hoffmann's CD‐1 and CD‐2 have a section on the 1,3‐butadiene ⇋ bicyclobutane transformations, with a note in it: “Here we heartily recommend to the reader the company of a set of Prentice‐Hall molecular models” [79, 80]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.4.6. Sigmatropic Reactions

While easy to recognize from a structural atom‐connectivity perspective, the underlying theory of sigmatropic reactions is the most difficult pericyclic reaction to understand (ctually, some [i, j] sigmatropic reactions are hard to see as being a sigmatropic reaction). As discussed in a previous publication in this series [111]. Hoffmann used five different MO‐based tools on sigmatropic reactions before he was comfortable with his transition‐state‐based orbital symmetry explanation. One of these MO‐tools, and perhaps the most fundamentally‐based, involved the use of perturbation theory (PT). The 1965 JACS communication [4] on sigmatropic reactions was nearly devoid of mechanistic explanation. Hoffmann provided a rich though much abbreviated PT mechanism in the Angewandte treatise, as shown in the upper left‐hand graphic in Figure 44. Woodward simplified that graphic into a less decipherable representation (subsequent graphics in Figure 44).

FIGURE 44.

Perturbation theory explanation of the mechanism of sigmatropic reactions [4, 13, 111]. Clockwise from the Top‐Left: Page 606 from HD‐2 with its graphic crossed out. Handwriting by Woodward [250]. Pages 606a and 606b in Woodward's handwriting, likely for HD‐2 [250]. Lastly, Figures 31 and 32 from the 1969 Angewandte treatise [13]. See text and above citations in this caption for additional explanations. The last graphic (Lower left‐hand corner) is reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

These four graphics illustrate Woodward's influence on the presentation of underlying MO theory of orbital symmetry control in pericyclic reactions in this treatise. Despite the effort to explain these figures, in the end, two master expositors—Woodward and Hoffmann—did not succeed (in my opinion) in providing the community with a clear theoretical understanding of the sigmatropic reaction [111]. And now, I must apologize to the interested reader. The explanation of Figure 44 is, indeed, quite complex and beyond the scope of this publication. However, a full, pedagogically oriented explanation is provided in a previous open‐access publication in this series [111]. The quantum chemical explanation of sigmatropic reactions is a worthwhile topic to explore, especially for those who have little or no experience in qualitative perturbation theory.

7.4.7. On Ketenes in Original “Violations” Section

Woodward drafted many pages for the Angewandte manuscript. Perhaps the most controversial section of that treatise is entitled “Violations,” controversial because chemists over the years have tried to find violations of the Woodward‐Hoffmann rules [251, 252, 253, 254, 255, 256]. Woodward's original 18‐handwritten page draft of the “Violations” section [257] was reduced to one page in the Angewandte publication [13] and two and one‐half pages in the monograph [15]. Nonetheless, portions of the text from the original 18‐page draft found their way elsewhere into the main body of the treatise, including some of his extensive discussion of singlet carbenes (see Figure 45). Even Woodward's writings required many stages of drafting and even complete deletions [257] (I say “even Woodward's” because he was so well known for his eloquent writing). Why a multipage discussion of ketene chemistry would have been initially placed in the section on “Violations” is unclear—except perhaps that a facile, formal [2_s + 2_s] cycloaddition seemed to be a violation of the selection rules. In the end, following the idea that the reaction could not be a violation, but involved initiating and orienting an acid–base interaction, such a 2_s + 2_a addition helped Hoffmann and Woodward round out their understanding of organic reactivity.

FIGURE 45.

(A) An excerpt from page 3 of Woodward's original 18‐handwritten page draft of the section “Violations” that (B) appeared in Section 10.2 of the Angewandte treatise entitled “Cycloaddition Reactions of Ketenes” [257]. Reference 223 points to the observation by Montaigne and Ghosez in 1968 that dichloroketene undergoes stereospecific cis addition to cis‐ and trans‐cyclooctenes [225] (see Figure 36). Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.4.8. “Violations. There are none!”

Woodward's first drafts of this famous “Violations” section are reproduced in Figure 46 along with an excerpt from the Angewandte treatise [13].

FIGURE 46.

(A) Woodward's earliest outline of the “Violations” section [258]. (B) An excerpt from Woodward's second draft of this section, ca. 1968 [257]. (C) An excerpt from the 1969 treatise [13]. Note that the section number “XXIII” in red pencil at the top of (B) does not correspond to Section “12” in the actual publication. (C) is reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.4.9. The Generalized Selection Rule

Woodward contributed significantly to the formulation of the Generalized Selection Rule and drafting of the text describing the rule in the Angewandte treatise. He minored in mathematics as an undergraduate at MIT, and throughout his archived papers are many pages revealing a variety of mathematical derivations. Thus, deriving the mathematical formulae generalizing the qualitative aspects of the W‐H rules would have been a fun engagement for Woodward. Figure 47 reproduces several excerpts of his derivation of the generalized selection rules for the W‐H rules [259].

FIGURE 47.

Four pages [259, 260] from Woodward's handwritten notes of his derivation of the W‐H generalized selection rules, the results of which appeared in Section 11 of the Angewandte treatise [13].

Woodward took his thinking one step further to ultimately develop the “Generalized Selection Rules for Pericyclic Reactions” (Figure 48). According to Hoffmann:

FIGURE 48.

Exhibits related to Woodward and Hoffmann's “Generalized Selection Rules for Pericyclic Reactions” for the W‐H rules. (Top) Pages 7A and 8 from Woodward's handwritten draft of this section for HD‐2 [261]. (Bottom) Excerpts from Section 11 of Woodward and Hoffmann's 1969 Angewandte treatise [13]. This illustrates Woodward's primary role in the development of the Generalized Selection Rules for pericyclic reactions. The graphic and text at the bottom are reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

“These pages illustrate his very meticulous algebra. I think it is more likely that he came up with this himself, rather than checking a result of mine [262].

7.4.10. The “Conclusion” Section

Given what we know of Hoffmann today as a writer [263] and his deep interests in philosophy and almost all intellectual pursuits [264, 265, 266, 267, 268, 269, 270], it is hard to imagine that the “Introduction” (Section 6.d.3) and “Conclusion” [265] sections were entirely in the hand of Woodward (Figure 49A) [271]. Yet they most certainly were. Note the beauty of Woodward's words and the indistinguishability of his first draft (Figure 49A) from the published version (Figure 49B). Note the end mark that Woodward used at the bottom of Figure 49B. This small graphic element must have been written with no small amount of pleasure by Woodward. This symbol appears in other handwritten manuscripts in the Woodward papers at the Harvard Archives.

FIGURE 49.

(A and B) Woodward's handwritten drafts [271] of the “Conclusions” (Section XXII of HD‐1) for the Angewandte treatise. (C) Excerpt (likely) from the second Harvard draft (HD‐2) [272]. (D) Excerpt from Section 14 of Woodward and Hoffmann's 1969 treatise [13]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.4.11. Woodward's Words

Several examples of Woodward's rather dramatic [273, 274, 275, 276] use of language appear in Section 6.d.11 of the Angewandte treatise. A single‐sentence paragraph in Section 6.3 of that publication (“The [2 + 2 + 2] Cycloaddition Reaction” section) is so Woodwardian [273, 277] that I include it as a celebration of arguably the most flamboyant of language in the chemistry literature. It reads:

“Lest any might cavil at the discussion of processes which on casual consideration could appear to be so improbable of realization as only to merit summary dismissal, we may allude to the essentially strain‐free, as yet unknown, molecules ( 284 ) and ( 286 ), whose double bonds are so constrained by the frameworks in which they are embedded as to favor photochemical conversion to ( 285 ) and ( 287 ), by the all‐antara paths mentioned above” [13, 15]. [Text and structures 284 – 287 have been reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.]

It should be mentioned that the two cases (284) and (286), the subjects of this beautiful Woodwardian prose, appear prominently in one of the few unpublished manuscripts by Hoffmann (and perhaps never actually submitted manuscripts [206], though presented at the 1968 Karlsruhe meeting [278]). That manuscript is entitled ”Some Theoretically Interesting Molecules” [279]. This draft [279] is undated, but the last references in it are from 1966. Hoffmann thinks it's likely to have been composed, with amateur photographs of models of the molecules, in 1967 or 1968 [206].

Figure 50 reproduces an 84‐word single‐sentence paragraph of Woodward's [280] that further illustrates the Woodwardian writing style. It is a boastful sentence, both in terms of its language and what it asserts. Unfortunately for the lovers of Woodwardian prose [48, 51, 52, 170, 171, 281, 282], this text was not used in any of Woodward's publications—but it is memorialized herein. It must have been a source of dismay when Woodward himself ruled it out, as it is likely that Hoffmann would not have (or could not have) done so.

FIGURE 50.

An 84‐word single‐sentence paragraph by Woodward that encapsulates his view of the principle of conservation of orbital symmetry [280]. This text was not included in any W‐H publication. This dismissal was almost certainly performed by Woodward and not by Hoffmann, the more‐junior partner (in status and seniority, not in terms of intellectual contributions to the program).

7.4.12. Woodward's Influence on Hoffmann's Words

As shown in Figure 51, Woodward paid a lot of attention to and improved the English in the drafts. One ought not to think that “words” came easily for Woodward. He painstakingly searched for the right words, as most authors do. See Figure 52 for one such example.

FIGURE 51.

A page from HD‐1 modified by Woodward [283]. The originating text is almost certainly from Hoffmann.

FIGURE 52.

An illustration of Woodward searching for the most appropriate word for “essential” and “nonessential” [284].

7.4.13. References and Figures

Woodward added references also (Figure 53). Ironically, these references to chemical theory were added by Woodward, the organic chemist, not by Hoffmann, the chemical physicist. Woodward also drew by hand many of the more complicated chemical pictographs (Figures S21 and S22).

FIGURE 53.

Notes in Woodward's hand that cite several publications of Kenichi Fukui, some with Hiroshi Fujimoto, Fukui's one‐time graduate student and subsequently his colleague at Kyoto University (and in between, a Hoffmann postdoctoral fellow) [285, 286].

7.4.14. Woodward Seeking Hoffmann's Guidance

Several documents exist in which Woodward asked for advice (Figure 54) or requested text from Hoffmann (Figure 55) [288]. Ultimately, a decision was made—seemingly by Woodward—not to “use a little amplification” [287].

FIGURE 54.

An undated note from Woodward to Hoffmann with Hoffmann's reply below the line, ca. 1968 [287].

FIGURE 55.

An undated note from Woodward to himself that served as a placeholder for a future action, then as documentation for later decisions [288].

7.5. What is New in the Angewandte Treatise?

7.5.1. New Nomenclature

7.5.1.1. Preface

Woodward understood the power of words to communicate and sell his ideas and his aura . For example, Woodward understood the power of eponymy. He named several key intermediates in his total syntheses after his students who had synthesized those compounds, e.g., in his tetrodotoxin research, he coined these “trivial” names: “Singer acetate” [289] after Abraham Singer; “Gougoutas hydrochloride” [289] after Jack Zanos Gougoutas; and “Rajappa hydrochloride” [289] after Srivivasachari Rajappa [51].

What words would Woodward and Hoffmann use for their new orbital symmetry concepts?

In the sections that follow, I shall respond to this question as the historical documents allow. This section focuses on nomenclature that was provided for the first time in W‐H's 1969 Angewandte treatise [13]. I have previously discussed [110, 111, 112, 136] the new nomenclature provided by Woodward and Hoffmann in their 1965 JACS communications, e.g., electrocyclization, sigmatropic, suprafacial, and antarafacial.

Ironically, a SciFinder search for terms such as “pericyclic,” “cycloreversion,” or “cheletropic” do not lead to the Angewandte treatise. Indeed, most of the new nomenclature devised by Woodward and Hoffmann is not found in the abstract to the Angewandte treatise found in SciFinder. This is not surprising, given the brevity and understatement of that abstract. Perhaps never has so little been said about so much:

“A review is given with 262 references. Developments by the authors and others during the past 3 years are surveyed, and some new projections are made” [290].

This may also be an example of obliteration by incorporation [291, 292, 293, 294].

7.5.1.2. “Conservation of Orbital Symmetry”

The term “conservation of orbital symmetry” was first used on September 29, 1965, in Woodward's response [45] to the editor of Angewandte Chemie, Helmut Grünewald (Figure 3). This term appeared shortly thereafter in the subtitle of a news article in the December 6, 1965, issue Chemical & Engineering News (Figure 56) on Woodward's failed route toward the synthesis of vitamin B₁₂ [295]. The failure was due to his (mis)prediction of the stereochemistry of an electrocyclization in a key step. That synthetic failure ultimately led to the W‐H rules, not bad for a mistake made good [2, 57, 136] and a synthetic route canceled! A full discussion of Woodward, his postdoctoral student Subramania Ranganathan, and the failed vitamin B₁₂ synthesis is available [34, 56] (Figure S32).

FIGURE 56.

Excerpt from page 38 of the December 6, 1965, issue of Chemical & Engineering News by an anonymous journalist featuring the first public use of the term “principle of conservation of orbital symmetry” [295]. Republished with permission from Chemical & Engineering News. Copyright 1965 by the American Chemical Society.

In 1966, Woodward used this term as the title of his lecture at Sheffield University and as the title of his 1967 publication [11] which was based on that lecture (W‐H 9, discussed in the previous publication in this series [7]). Hoffmann and Woodward used that term as the title of their Accounts publication (W‐H 10), also discussed in the previous publication in this series [7], and in the inaugural issue of Accounts of Chemical Research [12]. By 1969, the term was well known in the community. It is no surprise that “Conservation of Orbital Symmetry” was used again as the title of their Angewandte Chemie treatise (W‐H 11) [13] and its monograph reprint the following year [15].

7.5.1.3. “Pericyclic” and “Annulotropic”

In their Angewandte treatise, Woodward and Hoffmann defined the term “pericyclic” as:

“reactions in which all first‐order changes in bonding relationships take place in concert on a closed curve” [13].

As important as Woodward and Hoffmann's new term “pericyclic” is, Woodward and Hoffmann's definition is somewhat vague. Hoffmann is uncertain as to what “first order change in bonding” actually means [296, 297]. Furthermore, according to this definition, some W‐H forbidden reactions or cyclic valence isomerizations are not pericyclic reactions because the reactions are nonconcerted.

The Oxford English Dictionary (OED) defines pericyclic as follows:

“Of a reaction: involving a concerted rearrangement of bonding in which all the bonds broken or formed in the reaction lie on a closed ring, whether or not a cyclic molecule is involved. Cf. Woodward–Hoffmann n” [298].

Their documented first use is its appearance in Angewandte Chemie in 1969.

The 2021 edition of IUPAC's “Glossary of Terms Used in Physical Organic Chemistry” defines “pericyclic” as a

“Chemical reaction in which concerted reorganization of bonding takes place throughout a cyclic array of continuously bonded atoms” [299].

Why would the IUPAC Subcommittee on Structural and Mechanistic Chemistry redefine a term, given that the term had already been defined by its inventors, both of whom are Nobel laureates, one for the discovery of the W‐H rules and its attendant nomenclature? Perhaps the answer to this question lies in the ambiguity of Woodward and Hoffmann's definition. If so, then perhaps the IUPAC Commission's action is warranted.

As shown in Figure 57, Woodward spent much effort in choosing the term for what was to become “pericyclic reactions.” At first, he focused on the prefix “peri” which means “around.” In Greek, “kyklos” means “cycle.” “Perikyklos” means “sphere.” Woodward also considered the terms “closed curve,” “concerted,” and “closed circuit.” For more details, see the caption for Figure 57. Apparently, Woodward asked Dodie Dyer to do some research on several of his shortlisted choices, the results of which are partially shown in Figure 58 [303].

FIGURE 57.

Four pages of handwritten notes by Woodward dealing with the search for a term that later would become “pericyclic” and for its definition. My reading of Woodward's text is as follows (from the top reading downward): (A) “perisynchronous, peribolic, peribolic [repeated], peritropic, periclistic (“That might have been a direction to pursue in namesmithing; “Kleistos’ in Greek means ‘closed’ or ‘shut’” [114]), synechis, continuous, clastic, periphery, periplastic, plassen” [300]. (B) “Loop, laqueus, ioniuntiunctio, perimetric, perimeter ?? is boundary of a closed planar figure, closed circuit, closed curve in space, changes in bonding relationship occur in a concerted fashion, through a closed‐linear transition state, claudene, to close, concerted bonding process through closed linear transition state, annulotropic reactions, cyclotropic reactions, electrotropic reactions, cyclosynchronomous or cyclosynchronous, peripheric” [301]. (C) Annuloelectric, concerted cyclic, perimetric reactions, cooperative changes in bonding relationships take place on a single closed curve, closed circuit circuities” [302]. (D) Woodward's notes read, “annulotropic reactions ‐‐‐ take place through a transition state which has a continuous linear circuit of coupled electrons” [302].

FIGURE 58.

Some of Woodward's ideas for what became “pericyclic” were researched by his secretary Dodie Dyer. These included “periplastic” and “periplus” [303].

Ironically, the term “pericyclic” appeared first in several publications not authored by Woodward and Hoffmann before it appeared in their Angewandte treatise (submitted January 7, 1969; published in November 1969). As best as I know, the first use of “pericyclic” as intended by Woodward and Hoffmann appeared in the publication by Ian Fleming and his student M. J. Bishop's entitled “Pericyclic reactions I. An examination of some potentially pericyclic reactions in the mass spectrometer” [304]. That article was submitted to the Journal of the Chemical Society C: Organic on January 20, 1969, and appeared in the journal's 13^th (of 20) issue, ca. June 1969. “A Search for Some Pericyclic Reactions” was the title of Bishop's Ph.D. dissertation published in 1969 [305]. It is not unreasonable that Fleming would know of and likely had received a copy of Woodward and Hoffmann's Angewandte submission in January 1969.

As a postdoctoral student of Woodward's in the early 1960s, Fleming was present during the early days of Woodward's struggles with the no‐mechanism problem, and Woodward sought Fleming's help [34, 306] on the mechanism of his five mysterious reactions, all eventually solved by the W‐H rules. But there is another way that could have led Fleming to learn of the term “pericyclic.” Fleming, a professor of chemistry at the University of Cambridge (England), certainly attended the symposium “Orbital Symmetry Correlations in Organic Chemistry” held in Cambridge on January 7—9, 1969 [307]. Woodward was one of the speakers, as was Hoffmann. One of them could well have used the term “pericyclic” at that symposium.

Lionel Salem also used the term “pericyclic” in his October 1969 article “Orbital Interactions and Reaction Paths” [308] in Chemistry in Britain. This was just a month before the appearance of the Woodward and Hoffmann treatise in the November 1969 issue of Angewandte Chemie [13]. Salem was also consulted by Woodward on this topic before Hoffmann entered the scene [34].

There were actually many other appearances of “pericyclic” in the early scientific literature. A SciFinder search reveals the terms “pericycle” and “pericyclic” in more than 75 publications from 1897 [309] into 1969 [310]. According to the OED:

Pericycle: “Originally: a layer of non‐vascular tissue between the vascular cylinder and the cortex. Later: spec. the outermost, non‐vascular layer of the vascular cylinder of a root, immediately below the endodermis” [311].

There are two early, non‐W‐H uses of “pericyclic” in the chemistry literature via SciFinder. The SciFinder abstract constructed by Chemical Abstracts reported that “pericyclization of the unstable hydroxymethylene derivative [1] with 93% H₂SO₄ gave [2]” [312]. However, the actual publication by D. H. Read reported that 1 “underwent peri‐cyclisation to give [2]” [313]. In this sense, “peri” typically refers to positions 1 and 8 in naphthalenes and reactions involving those carbon atoms. The conversion of 1 to 2 as a peri‐cyclization is a stretch of this general usage. The second reference to “pericyclizations” is in an article in a rather obscure Russian journal from 1968 [314] which, of course, would require a translation to arrive at “pericyclizations.”

7.5.1.4. “Cheletropic”

Hoffmann first discussed cheletropic reactions in his 1966 publication in the Transactions of the New York Academy of Sciences [7, 9]. Examples of cheletropic reactions appear in Woodward and Hoffmann's Accounts article [12]. Nonetheless, the term “cheletropic” was defined in their Angewandte treatise as,

“those processes in which two σ bonds which terminate at a single atom are made, or broken, in concert” [15, 223].

Again, the IUPAC recommended definition of “cheletropic” is not a repeat of Woodward and Hoffmann's. Charles L. Perrin et al. proposed:

“Cycloaddition across the terminal atoms of a π system with formation of two new σ bonds to a single atom of a monocentric reagent. There is formal loss of one π bond in the substrate and an increase in the coordination number of the relevant atom of the reagent” [299].

Two pages of Woodward's notes relating to cheletropic reactions were found in his archives (Figure 59). Woodward was fascinated with the metaphor of the claws of a crab. In Greek, the claws of a crab are sometimes referred to as “χηλή” (chēlé or chēlā), which translates in English to “claw” or “pincer” or alternatively “hoof” of a mammal. One of Woodward's trial words was, indeed, “cheletropic” (Figure 59B). Interestingly, while most of Woodward and Hoffmann's novel nomenclature appears in the OED, “cheletropic” does not (as of August 25, 2025).

FIGURE 59.

Two pages of handwritten notes by Woodward [315] dealing with his search for a term that later would become “cheletropic.” My reading of Woodward's text is as follows (from the top reading downward): (A) “two bonds to (same) (single) atom broken or made at same time; bi; cataclastic; cheno; two headed; ambiguous; two choices; simultaneous; sigmatropic; ambi(tropic)(turning)(chady); ambi(+ paths; ambivestic; amphi” [315]. (B) “crab.; claw of a crab = brachium; tropic; chelat [with arrow pointed to a Na atom bonded to a 2,4‐pentandiol; klasis—a breaking; bracchitropic; cheletropic; bracchitropic” [315].

Again, Woodward and Hoffmann were not the first to use the word “cheletropic” in the chemical literature. That honor goes to Ronald Grigg et al. who, just several weeks earlier, used “cheletropic” to describe the “thermal elimination reactions of nitrogen and sulphur heterocycles” [307]. Ronald Grigg et al. cited Woodward's lecture at the Cambridge symposium on orbital symmetry [307] cited immediately above (Section 6.e.1.iii). Grigg et al.'s publication was submitted on August 12, 1969, some 7.5 months after Woodward and Hoffmann's Angewandte submission, yet was published ca. October 1, 1969, a month earlier than the Angewandte treatise. This is another example of “later submission and earlier in print” [316].

7.5.1.5. “Cycloreversion”

The term “cycloreversion” first appeared in the text of the Angewandte treatise in the title of Section 6, “Theory of Cycloadditions and Cycloreversions.” Its appearance was without definition or fanfare. That being said, in the 2022 edition of IUPAC's Gold Book, the Glossary of Terms Used in Physical Organic Chemistry, the following definition appears:

“cycloelimination. Reverse of cycloaddition. The term is preferred to the synonyms ‘cycloreversion’, ‘retro‐addition’, and ‘retrocycloaddition’” [299].

The term “cycloreversion” first appears in SciFinder in 1970 and does so in four abstracts [317, 318, 319, 320]. Two of these abstracts [318, 319] are from the laboratory of Jerome A. Berson, another Woodward student who made many important contributions to the chemistry of pericyclic reactions [321, 322].

7.5.1.6. “Concerted”

As Hoffmann said:

“We were brainstorming for words, and it is boring to repeat ‘concerted’ and ‘cyclic’; we were looking for roughly the same idea and words that sound good. Don’t get too hung up on words, the writing process is not necessarily rational” [107, 323].

7.5.1.7. Words That Were Not Adopted by the Community

One of Woodward's notes is particularly interesting. It reads:

“Stand back

Now we have completed our general theoretical presentation (have we?)

Is this the place for unified treatment of ‘electrotropic reactions’” [324]. (See Figure 60A.)

FIGURE 60.

(A) Woodward's note using “electrotropic reactions” [324] instead of the eventual “pericyclic reactions” [13, 15]. (B) Five other words Woodward invented [325] that did not find their use in the orbital symmetry world.

As we well know today, “electrotropic” was never presented to the community.

7.5.1.8. W‐H's “Device of Denominating the Type of Orbital Involved in a Concerted Reaction”

I now point to a second type of nomenclature that goes beyond new words for new chemical concepts, that being Woodward and Hoffmann's novel and

“useful device of denominating the type of orbital involved in a concerted reaction by a subscript preceding the number of electrons in the relevant orbital. Thus _σ2_{a π}4_s etc.” [13]

In this nomenclature, the Greek and English letters are subscripts. Woodward's derivation of this nomenclature is illustrated in Figure 61.

FIGURE 61.

(Top) Woodward's handwritten insert [326] on a new nomenclature, one example being [_π2_s+_σ2_a] for the W‐H allowed thermal conrotatory ring opening of cyclobutene to 1,3‐butadiene. This represents a cis addition of the σ‐bond to the π‐bond. (Bottom) Excerpts from the 1969 Angewandte treatise showing a [_π2_s + _σ2_a] cyclization [13]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence or similar technologies.

7.5.2. New chemistry

7.5.2.1. Preface

The experimental data of almost all the chemical reactions explained by the W‐H rules and discussed by them in their Angewandte treatise came from laboratories other than Woodward's (or Hoffmann's). Do we need further support for the value of the scientific literature that is widely accessible and with appropriate citations? If so, consider the Woodward and Hoffmann publications (jointly and singly authored). Economic factors may influence certain specific instances, such as patents and industrial secrets, but to create new ideas and to test wild hypotheses, one needs just what chemistry had in 1969.

Perhaps the singular most consequential set of experiments that required the W‐H rules for their mechanistic explanation were actually performed in Woodward's own laboratory in 1963 or very early in 1964. These results were not discussed in the Angewandte treatise! [13] They were obtained by Ranganathan in Woodward's early vitamin B₁₂ synthetic routes [11, 20, 21, 56], performed during Ranganathan's postdoctoral research with Woodward at Harvard in 1963—1964. These are cited in Figure 56 and described more fully in the (Figure S8) as well as in the just‐cited literature references. Ranganathan later conducted research at the Woodward Research Institute in Basel, Switzerland, and ultimately taught and conducted research at the Indian Institute of Technology, Kanpur [61, 62, 327].

7.5.2.2. Following the Molecular Orbitals. Pedagogy in MO Theory

In 1967 and 1968, when they were drafting their Angewandte treatise [13], Woodward and Hoffmann understood that organic chemists had little, if any, knowledge of MO theory and certainly little, if any, experience in using correlation diagrams or qualitative perturbation theory—two tools extensively used by Hoffmann in his analyses of pericyclic reactions. They correctly concluded that their treatise must include some of the basics of MO theory using pedagogically relevant examples. This was Hoffmann's task, though Woodward had his hand in some of this pedagogy. And it was accomplished reasonably well—“reasonably well” given that theirs was a journal article, not a textbook on MO theory (I quickly add, neither of the then‐new textbooks in MO theory for organic chemists—John D. Roberts’ volume entitled Notes on Molecular Orbital Theory [328] and Andrew Streitwieser's Molecular Orbital Theory for Organic Chemists [329], both published in 1961—taught correlation diagrams or qualitative perturbation theory).

Organic chemists were also unfamiliar with many of the key participants in the development of theory that would soon be applied by organic chemists to organic chemistry, including Hoffmann himself and others, such as Longuet‐Higgins, Luitzen Oosterhoff, and Fukui. As Hoffmann commented:

“Woodward pushed me to explain things, but to him, not to the eventual readers of our papers. The writing of a full paper brought a more general audience into view, and it made sense that I would write those sections” [68, 107].

And Hoffmann did.

7.5.2.3. All Pericyclic Reactions Are Cycloadditions. Trans‐addition of 1,3‐Butadiene to Cyclobutene

During his research that uncovered the mechanisms of cycloadditions [3, 110] and sigmatropic reactions [4, 111], Hoffmann began to realize that there were mechanistic connections between the various types of reactions he was studying. Eventually, he and Woodward discovered that all pericyclic reactions can be identified and described as cycloadditions [13]. Even an electrocyclization reaction can be viewed as an internal cycloaddition. That is, the conversion of 1,3‐butadiene to cyclobutene can be analyzed as a [ _π 2+ _π 2] cycloaddition; and the cycloreversion of cyclobutene to 1,3‐butadiene can be viewed as a [ _σ 2+ _π 2] process (Figure 62).

FIGURE 62.

A thermal, W‐H allowed conrotatory four‐electron electrocyclization also shown as a [_π2_a+_σ2_s] cycloaddition. Reproduced from the Angewandte treatise [13, 15]. See also Figure 63. The revolutionary nature of the W‐H advancements, and how easily they were adopted, is reflected by how mysterious this representation might have looked to a physical organic chemistry student (or even chemistry faculty member) in 1965 and how obvious and intuitive it all was just a few years later. The graphics in this figure are reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

But this leads to a curious nomenclature situation. If electrocyclizations, cycloadditions, and sigmatropic reactions are all cycloadditions as pointed out by Woodward and Hoffmann in their 1969 treatise, what will we name what are formally and more narrowly called “cycloadditions” to avoid ambiguity? Or the converse, if we reserve the term “cycloaddition” for [2 + 2], [4 + 2], etc. reactions, what do we call all of these reactions? Perhaps we call them “pericyclic reactions.” But according to Woodward and Hoffmann's definition of “pericyclic” (see Section 6.3.1.iii), only those reactions that are in accord with the Woodward‐Hoffmann selection rules are pericyclic reactions. That is, a four‐electron disrotatory electrocyclization is not only W‐H forbidden but also nonpericyclic. A reviewer concluded, “I think the ambiguity has largely resolved itself.” In this instance, the resolution has been “benign inattentiveness” or perhaps “inattentional blindness,” where an individual fails to perceive an unexpected, yet visible, stimulus because their attention is focused on something else.”

Lastly, I point out that Figure 62 refers to a trans addition of a single bond to a double bond, in this instance, in a cyclobutene (see also Figure 63). As a historical note, I offer the following: prior to the Woodward‐Hoffmann rules, in 1962 or 1963, Woodward had commented at a Woodward Thursday night group meeting that Vogel's [330] and Criegee's [331] ring opening of a highly substituted cyclobutene to the E, Z‐1,3‐butadiene occurred in a trans fashion [20, 21]. Could it be that Woodward's realization in itself constituted a piece of the involuted road to understanding pericyclic reactions? Surely so. But it takes a master's reasoning to see the type of intramolecular trans addition of a σ bond to a π bond.

FIGURE 63.

Trans addition to a cyclobutene via a conrotatory motion, as indicated by the red arrows (both motions are clockwise when viewed toward the cyclobutene molecule). In this graphic, the HOMO of the σ bond is interacting (mixing) with the LUMO of the π bond, a stabilizing interaction. These were first termed “trans additions” in the 1968 Accounts essay [12] and then “antarafacial cycloadditions” in the Angewandte treatise [13]. See also Figure 62.

In some sense, the self‐consistency of the stereochemical thinking here—of showing chemists that one could think of electrocyclic reactions in terms of trans or antarafacial cycloadditions—somehow added a lot of realism to the “antarafacial idea.” Antarafacial additions is a concept that was not initially obvious—not to Hoffmann, not to Woodward, and not to those studying pericyclic reactions.

7.5.2.4. A Common Mechanism and Its Consequence, the Generalized Selection Rules

Woodward and Hoffmann's “Generalized Selection Rule for Pericyclic Reactions” is:

“A ground‐state pericyclic change is symmetry‐allowed when the total number of (4q + 2)_s and (4r)_a components is odd” [13].

Note: q and r are integers.

What a mouthful! By inference, for that number of electrons, the photochemical reactions are symmetry‐forbidden. And “symmetry” refers to “molecular orbital symmetry.”

Of the recognition of the common mechanism of W‐H allowed pericyclic reactions and the generalized selection rule, Hoffmann said:

“Why did we not see it all as one? Likely because as we used different molecular orbital methods for the three classes of reactions” [332].

and

“Boy, am I glad we did. If we had not, neither Woodward nor I, nor any reader, would have really understood the real workings of perturbation theory, of orbital interactions. The intricate path we followed taught us so much more” [107].

and

“But the irony is no one really uses the mathematical formalism of the generalization but rather the concept and the individual selection rules” [333].

Actually, I ask—with all seriousness—what are the “Woodward‐Hoffmann rules”? One answer is, the generalized selection rules. But as Hoffmann stated above [333], few chemists use the generalized selection rules. Another answer, the W‐H rules are the selection rules proposed by Woodward and Hoffmann in their first three JACS communications of 1965 [2, 3, 4]. Additional selection rules for cheletropic reactions and group transfers and eliminations were provided in their 1969 Angewandte treatise [13]. As an anonymous reviewer of this publication remarked:

“Most introductory courses teach, and most students learn far more easily the idea of aromatic transition states than Woodward and Hoffmann's generalized selection rules.”

7.5.2.5. Ketene Cycloadditions

One of Woodward's earliest chemical passions involved the reactivity of ketenes. His October 28, 1941, organic symposium at Harvard was on the “Dimerization of Unsaturated Carbonyl Compounds” [334]. Woodward's first publication on the topic came in 1950, “The Structure of the Acidic Dimer of Methylketene” [335]. In 1962, he published on “Die Cycloaddition von Äthoyacetylene an Diphenylketen” [336]. And the Woodward‐Hoffmann 1965 publication on cycloadditions stated briefly that

“those cycloadditions which do occur in cases prohibited by our selection rules for concerted reactions must proceed through multistep mechanism [an example of circular reasoning] (e.g. formation of cyclobutene derivates by dimerization of allenes and ketenes . . .)” [3]

Critical was the discovery that an antarafacial addition to a double could occur via a perpendicular orientation of one of the double bonds (Figure 64), Woodward and Hoffmann developed a substantial insight into the reaction of ketenes in their Angewandte treatise [13]:

FIGURE 64.

An illustration of [_π2_s+ _π2_a] cycloadditions, adapted from structure 452 in Woodward and Hoffmann's Angewandte treatise. In order to achieve this mode of cycloaddition, one of the ethylenes must be placed 90° relative to its reaction partner and rotated 90°, i.e., the two p‐orbitals are pointing toward the reader in the more remote ethylene. These graphics show the pair of HOMO–LUMO interactions, which are stabilizing, i.e., a W‐H allowed reaction. That ketenes undergo [_π2_s+ _π2_a] cycloadditions rapidly may be a consequence of the presence of the C = O bond which is sterically less demanding than substituents on the sp² carbon atom in cases of R¹R²C=CR³R⁴. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1996): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

“Section 10.2. Cycloaddition Reactions of Ketenes . . . The combination of ketenes with olefins to give cyclobutanes [actually, cyclobutanones] has long been known. In recent years, the reaction has been subjected to very searching scrutiny, and the evidence is now conclusive that it is concerted. . . . A concerted combination of ethylenes [with ketenes] to give a cyclobutane must follow the [_π2_s + _π2_a] path, and in the case at hand the facts demonstrate beyond question that the olefinic reactant participates in a suprafacial way” [13].

The important point here is the detailed consideration of orbital overlap requirements for making a bonding arrangement, i.e., an allowed reaction. Such considerations lead to the consideration as to which regions of a potential energy surface (PES) are of low energy and which are of high energy. This kind of insight—how orbital symmetry requirements shape potential energy surfaces—would survive in its insight when a shift eventually occurs (it is in process) to trajectories and dynamics in organic reactions [256].

7.5.2.6. Nonlinear Reaction Trajectories

On January 21, 1966, Hoffmann wrote an important two‐paragraph letter to Woodward with an equally important attachment [77] (Figure S33). In the first paragraph of that letter, Hoffmann reported on the stereospecific thermal decomposition of cis−1,2‐diphenylethylene episulfone to cis‐stilbene [337] (Equation (1) in Scheme 1) which, when analyzed by the correlation diagram model, “has a [MO energy] level crossing in it” [77]. This MO energy level crossing is indicative of a W‐H forbidden reaction. Hoffmann then wrote: “I don’t know what to make of this . . . [perhaps] a three atom diradical which can retain configuration” [77]. Three years later, in the Angewandte treatise [13], Woodward and Hoffmann suggested “that the elimination follows the concerted symmetry‐allowed nonlinear cheletropic path [emphasis added] [Equation 1, Scheme 1]” [13]. By a “least motion path,” Hoffmann was referring to a least motion trajectory that maintains C_2v symmetry; the nonleast motion pathway is one in which the SO₂ group bends away from the perpendicular (Scheme 1).

FIGURE 67.

The thermal and photochemical stereospecific pericyclic reactions in which the latter (Equation (6)) has the opposite stereochemical relationship to the thermal reactions (equations (3)–(6). NP = not produced in the indicated reaction. Equation (3) chemistry from Emanuel Vogel [330, 372], equations (4) (2013); (6) from Rudolf Criegee [331, 373, 374, 375, 376], and Equation (6) from Rangaswamy Srinivasan [377].

SCHEME 1.

Cheletropic reaction of cis−1,2‐diphenylethylene episulfone showing both a linear and a nonlinear trajectory in the expulsion of sulfur dioxide.

Today, one would probe nonlinear trajectories by reaction dynamics calculations, such as those developed by Barry Carpenter [338, 339, 340], among others [341, 342]. In 1969, Hoffmann had no way of deciding whether the sulfur of the SO₂ would move away along a line, but the oxygens would bend. Or whether the whole SO₂ molecule would move along a nonlinear trajectory, the sulfur atom not at equal distance from the carbons along the motion, the oxygens bending at the same time. However, in research performed outside of the W‐H umbrella and published in 1968 (submitted on May 31, 1967), Hoffmann performed eHT calculations on the addition of methylene to ethylene [70]. He and his coworkers discovered that the symmetrical approach was symmetry forbidden and that the lower energy reaction path is asymmetrical. This publication was cited in the Angewandte treatise as an example of a “nonlinear cheletropic class” [13]. (Think a little about the trouble one gets into when one tries to define these things too firmly. What if it's off 0.001 Å from linear? Is that nonlinear? Woodward and Hoffmann were describing a trajectory and doing so approximately).

7.5.3. Some Odd Topics

Woodward and Hoffmann had their own peculiarities and idiosyncrasies that led to several topics within their Angewandte treatise that seem odd in retrospect. “Odd” in that they ultimately had much less impact on the chemistry community than the rest of the treatise. “Odd” in that they don’t really seem to belong. These include 5% of the text on sequential sigmatropic shifts with three rather exquisite graphical representations. Hoffmann calls this:

“the rococo face of the long paper. The boys are having fun” [163].

In contrast, and arguably in a rather unbalanced and unfortunate pedagogical strategy, the entire explanation of the mechanism of sigmatropic reactions took less than half that space and was rather poorly described [111].

7.6. What is Not in the Angewandte Chemie Treatise

7.6.1. The “Catalog of Concerted Reactions”

Hoffmann's “Catalog of Concerted Reactions” (W‐H 8) was in large part produced by Hoffmann and two undergraduate summer interns for two reasons. First, Hoffmann was trying to sketch the boundaries of pericyclic reactions. And second, it was an easy entrée into a response to Angewandte's invitation [35] to publish a “more elaborate review” than their first JACS communication of 1965 [2]. Hoffmann's “Catalog” (Figure 65) [343] made it through the two Cornell drafts and into the first typewritten manuscript at Harvard (HD‐1) but disappeared thereafter. It simply did not have the scientific content density of the rest of the text. In 2025, Hoffmann reconsidered:

FIGURE 65.

Section XXI of the first Harvard typewritten draft (HD‐1) of the Angewandte treatise [343].

“It might have served its purpose as an enumeration on the way to electronic conclusions. I don’t regret omitting it, but it could also have gone in” [107].

7.6.2. Nomenclature

Woodward and Hoffmann recognized that there would be instances in which a compound could undergo two or more allowed reactions. But Woodward and Hoffmann did not offer a term to differentiate between these situations. See Figure 66 for an example from Woodward's notes [301]. In the early 1970s, Kendall N. Houk, a former graduate student of Woodward, proposed the terms “perispecific” [345] and “periselective” [346, 347] for just these situations. In due course, other variations of valence isomerizations and related reactions led to terms such as “pseudopericyclic” [348] and “coarctate” reactions [349, 350].

FIGURE 66.

An example of periselective reactions from Woodward's notes, ca. 1968 [301]. Woodward was in part citing Karl F. Bangert and Virgil Boekelheide's March 1964 publication in JACS [344].

7.6.3. Allowedness via Transition State Aromaticity; Hückel Versus Möbius Transition States

In Section 5.f, I mentioned that Woodward and Hoffmann failed to discuss or even cite in their 1968 Accounts essay [12] either Zimmerman's Hückel/Möbius transition state concepts [118, 119, 120, 351] or Dewar's aromaticity/antiaromaticity transition state analyses [121, 123] for pericyclic reactions. It could be argued that those omissions were not specific to Zimmerman's and Dewar's literature, in that very few literature references to any subject were cited in that Accounts article. But beyond the credit issue discussed in Section 6.f.10, the concept of transition state stabilization could have been used to explain the difference between allowed and forbidden reactions as proposed by Zimmerman [118, 119] and Dewar in 1966 [121]. The possible reasons and consequences of this omission are discussed in detail in a forthcoming publication in this series [352].

7.6.4. Whether Many of the Stated Reactions Are Actually Concerted or Not

The concertedness of many of the reactions reported in the Angewandte Chemie article had not been established at that time. Woodward and Hoffmann did point that out. However, in reading the publication, it is easy to overlook that criterion of validity. Also, as we know today [253] but did not know in the 1960s, it is possible that some reactions that do not follow the W‐H rules (i.e., are forbidden reactions) can still be concerted. As Huisgen and Reiner Sustmann said in 2011 with regard to a presumed pericyclic reaction:

“The transition structures (TSs) of these processes look awkward to a varying degree. Woodward and Hoffmann mentioned the vinylcyclopropane rearrangement only in passing; they did not refer to the question whether or not the rearrangement would in reality be concerted. However, Woodward's authority may have played a role, when subsequently the orbital‐symmetry‐controlled, concerted courses dominated the discussion” [353].

and in Kenneth L. Servis's contemporaneous review:

“That the concertedness of many of the cited examples has not been established is not always clearly brought out” [354].

“Good” [206], observed Hoffmann.

Recent research has shown that even forbidden pericyclic reactions can have PESs in which a diradical mechanism is concerted [253]. Furthermore, a classical example of an allowed sigmatropic reaction reported by Berson and Nelson in the late 1960s [321, 322] was much later shown to be only coincidentally a concerted reaction; rather, the reaction's stereochemistry is consistent with an allowed concerted sigmatropic rearrangement but the stereochemistry is determined not by orbital symmetry energetics, but by reaction dynamics [339, 355, 356, 357, 358].

7.6.5. What to Do ‘If There is No Relevant Symmetry Element’ in the Reaction

It is obvious, but rarely discussed, that all of Woodward and Hoffmann's theoretical treatments embodied the barebones hydrocarbon skeletons of their substrates, such as cyclobutene ⇋ 1,3‐butadiene for electrocyclizations [2]; ethylene dimerization ⇋ cyclobutene and ethylene + 1,3‐butadiene for [2 + 2] and [4 + 2] cycloadditions [3], respectively; R₂C=CH‐(CH=CH) _n ‐CHR’₂ for sigmatropic reactions [4], and so forth. Woodward and Hoffmann mentioned that substituents with minor electronic effects will not alter the controlling influence of orbital symmetry considerations in such pericyclic reactions [13]. As Hoffmann summarized:

“The symmetry ideas apply only when there is symmetry, but symmetry is easily broken. A chemist knows that minor changes do not make much of a difference. There is a bigger worry about highly polarizing substituents like cyano or amino groups. But when you put even a methyl group, almost every correlation diagram for a forbidden reaction becomes allowed, all the forbidden reactions become formally allowed. But they don’t really become allowed. Strict symmetry is not behind making a reaction allowed or forbidden, it is the approximate symmetry, meaning the evolution of bonding orbitals transforming into bonding orbitals of product that is important” [107, 359].

However, Woodward and Hoffmann, almost in passing do mention that some pericyclic reactions do not have the requisite symmetry element to be analyzed for their W‐H allowedness or forbiddenness. For those cases, the allowedness or forbiddenness was determined by extrapolation and analogy of cases that did possess the requisite symmetry elements.

7.6.6. Sigmatropic Reactions: Following the Orbitals

Nearly 40 years after the publication of Woodward and Hoffmann's articles, Hiroyuki Nohira and, in later publications with his son Toshiyuki Nohira, described what they called “dynamic correlation diagrams for sigmatropic reactions based on orbital phase conservation theory” [360, 361, 362, 363]. This method is much simpler than its name implies, though their publications can be difficult to understand. The technique follows the MOs of several sigmatropic reactions in a multitude of steps from reactant to product. While this approach does not result in any predictions different from or beyond what was predicted by Woodward and Hoffmann's generalized selection rules, nonetheless Nohira and Nohira produced another and fruitful way of looking at and understanding pericyclic reactions. Furthermore, Nohira and Nohira's tool could have been implemented in the 1960s, since it did not depend on any research findings or theoretical advance that came about only years later. (In 1972, Zimmerman published a related article entitled “Following: the Molecular Orbital Counterpart of Electron Pushing” [364].)

7.6.7. Concerted S _N 2 Reactions

Woodward frequently asked Hoffmann about S _N 2 reactions and pericyclic reactions [53]. Hoffmann always responded that S _N 2 reactions were not pericyclic reactions and thus lay outside the scope of their selection rules (and did not constitute violations of them) [365, 366, 367, 368]. According to Hoffmann:

“Woodward tried to get me to say there was an orbital symmetry reason for the S _N 2 reaction going with inversion, like a [2_s + 2_a] cycloaddition. But I resisted, because the cycle was not closed (the leaving group did not communicate with the entering group, so to speak), so that quantum mechanical phase information could not decide this was a high or low energy process” [368].

But Woodward was persistent. In Woodward's letter to Salem on June 24, 1969, after apologizing for his delay in reading Salem's recent publication on MO theory and orbital symmetry and other concerted reactions, Woodward wrote that events

“. . . have made it impossible for me to find the time to give your Chemistry in Britain manuscript [308] the attention I know it deserves. When I find that time, I shall be most interested to read your views on the S _N 2 reaction, to which I have devoted a good deal of thought myself. I do know that one must be careful not to prove too much in that area” [369].

7.6.8. On Pedagogy

The Angewandte treatise [13] is not a textbook, even though its length and content may well suggest that that is one of its purposes and has been one of its uses. As noted earlier, Hoffmann joked to Woodward that the length of the manuscript was like that of a textbook. Some of its pedagogy is especially concise, and disadvantageously so, such as its theoretical treatment of sigmatropic reaction [111]. Other sources need to be consulted, e.g., for sigmatropic reactions and secondary orbital [111] interactions, see previous publications in this series [111, 112]. In the Angewandte treatise, many of the reactions are treated as if their mechanism had been conclusively established as being concerted. Very relevant, recent research results—published since the first W‐H articles appeared, indeed, research conducted to test the W‐H rules—were omitted. On the other hand, perhaps the most demanding test of the W‐H rules performed in the 1960s, that being the stereochemistry of Berson and Nelson's sigmatropic reaction [321, 322] (Equation (2)) reported in 1967, was discussed.

7.6.9. Photochemistry

Oosterhoff was the first to propose orbital symmetry as the mechanism for electrocyclizations [370, 371]. One reason that neither he nor his Dutch colleague Egbert Havinga [370] ran with that idea and expanded upon it is simple—and sad. Oosterhoff did not believe that it was generally valid [371]. He was primarily uncertain as to the application of his orbital symmetry idea to photochemical reactions. And Havinga was more occupied with firming up his chemistry, i.e., cleaning up errors in his structure assignments and their chemical connectivity which, in fact, he had wrong in 1961 [370, 371], than he was in following up on an explanation neither supported nor encouraged by his theoretician‐colleague Oosterhoff, who had first discovered the idea in December 1960 [371].

In fact, the weakest aspect of Woodward and Hoffmann's theory is its treatment of photochemical reactions. This did not prevent them from deriving selection rules for photochemical reactions, either in the very beginning [2, 3, 4] or in their last publications [13, 17, 18, 19]. And indeed, the experimental results do show alternations in stereochemistry with their thermal counterparts. For the most sensitive test of a photochemical/thermal pair of reactions, see Figure 67. Compare the stereochemical relationships in the three thermal reactions (equations (3) (2013) ; (5)) with that of the photochemical reaction (Equation (6)). There is a clear alternation in stereochemistry as predicted by the Woodward‐Hoffmann rules. Compare, for example, the 9 → 10 relationship (Equation (5)) with the 11 → 12 relationship (Equation (6)).

Oosterhoff's concern—and Hoffmann's as well, probably shared by Woodward—was the complexity of additional electronic states and the interconversion between the states. In what state does the pericyclic photochemical reaction occur? Might the initial conformation, which is the conformation reached in the first excited state due to the Franck‐Condon principle, bias the stereochemistry of the reaction? Even in the 1960s and 1970s, chemists such as William G. Dauben [90, 378], Havinga [379, 380], and John E. Baldwin [381] proposed ground state conformational control of photochemical reactions.

Today we know that certain features in the potential energy surfaces of excited states, often characterized as conical intersections [253, 382, 383, 384, 385, 386], dictate photochemical reactions. These seem to be often related to potential energy wells in an excited state that reside just above potential energy barriers in the ground state. To the extent that this is true, Oosterhoff and W. Th. A. M. van der Lugt found theoretical evidence for this in the late 1960s [387, 388]. That Woodward and Hoffmann spoke little of photochemical reactions in their Angewandte treatise [13] is further evidenced by their near omission of a reference to this seminal van der Lugt‐Oosterhoff reference (see the insert at the bottom left of Figure 71D(top) in Section 6.f.10.ii). Still, Woodward and Hoffmann's reluctance to discuss photochemical reactions in any detail in their publications was appropriate in 1965, certainly less so some short years later.

FIGURE 71.

Four drafts in sequence of development of the section on “Other Theoretical Work for the Angewandte treatise [13]. (A) Excerpt from Hoffmann's second draft (CD‐2) that was sent to Woodward ca. late 1966 or early 1967 [80]. (B) Woodward's revision for HD‐1 [389]. (C) Woodward's first draft of this section for HD‐2 [389]. (D, Top) Excerpts from Woodward and Hoffmann's third draft of the Angewandte treatise (HD‐2) [390], heavily annotated by Woodward. (E) Excerpt from the penultimate section of the Angewandte treatise [13]. Reprinted with permission from: R. B. Woodward and Roald Hoffmann, “The Conservation of Orbital Symmetry,” Angewandte Chemie International Edition 8 (1969): 781–853. Copyright 1969 Wiley‐VCH GmbH or related companies. All rights reserved, including rights for text and data mining and training of artificial intelligence technologies or similar technologies.

7.6.10. Credit Attributions

7.6.10.1. Authorship Order and Credit

As discussed in Section 5.c, in the decades before and after the 1960s, the senior author of a chemistry publication was listed first in the publication's byline. As discussed in Sections 4, 5.b and 6.b, Hoffmann provided a rather extensive typewritten draft to Woodward in late 1966 or early 1967 (CD‐2). We know that Hoffmann provided most of the detailed chemistry therein, most of the literature references, and most if not all, of the complex theoretical treatments that appear in the Angewandte treatise [13].

But, as I have shown in the previous pages, Woodward contributed in a major way to the final form and content of this treatise. He did so not only in improving Hoffmann's draft but also in writing new sections and in generating entirely new ways of seeing some subjects. Woodward also functioned as the manuscript's administrator, keeping track of all the numbering schemes, references, and many production requirements.

Figure 68 attests that Hoffmann placed his name first in the byline of the draft that he sent Woodward (CD‐2). We also know that Woodward counteracted that credit line by inserting his name first in a subsequent draft (likely HD‐1 but possibly HD‐2) (Figure 40B). And we know that in the final publication—both the 1969 Angewandte treatise [13] (Figure 40C) and the 1970 monograph [15]—Woodward's name appears first, Hoffmann's second. Likely Woodward put his name first on both the Angewandte Chemie submission [13, 14] and the 1970 monograph [15], and that was that. Fifty years later, Hoffmann does not remember discussing author order with Woodward [391].

FIGURE 68.

Cover page of Hoffmann's typewritten second draft of the Angewandte Chemie manuscript, ca. late 1966 to early 1967 (CD‐2). Note that in this draft, Hoffmann's name appears first, while Woodward's name is first in their 1969 Angewandte treatise. Not shown are the staple marks on the left edge of this page and the stapled version of this manuscript minus the cover page found in Hoffmann's files [391]. These staple marks are consistent with a draft of this manuscript from which the cover page had been dislodged.

There is evidence that Woodward considered the no‐mechanism problem and the orbital symmetry solution to be his territory [34, 49, 50, 57, 273] and that “possession” meant that he was the senior author. Furthermore, I know from many discussions with Hoffmann that he has always felt well‐treated by Woodward [53]. Certainly Hoffmann did not dispute this author order with Woodward. Doing so might have had negative consequences, and the reader will recall that Hoffmann is conflict‐adverse [78]. Likely the idea never came to Hoffmann's conscious mind.

The moniker became the Woodward‐Hoffmann rules rather than the Hoffmann‐Woodward rules. But even if the order of the names had been reversed in the Angewandte Chemie publication, the Matthew effect [293, 392, 393, 394, 395] would have likely influenced the eventual community‐used name to be the “Woodward‐Hoffmann rules.” Ironically, when most people think of the “Woodward‐Hoffmann rules,” they do not imagine the generalized selection rules. They sense the amorphous understanding of the subject, an understanding that incorporates the Zimmerman/Dewar formalisms that Woodward and Hoffmann essentially ignored.

7.6.10.2. Credit to Others

There are some credit attributions to others in the Angewandte treatise, but not many. And these largely appear in two indirect ways: in the numerous literature citations in the treatise and in the brief section entitled “Other Theoretical Work” discussed shortly herein. In his second (typewritten) draft that was sent to Woodward (CD‐2), Hoffmann included an acknowledgment section (Figure 69) [80]. That section did not appear in any further draft nor in the final version. Nor was Hoffmann's Junior Fellowship from Harvard's Society of Fellows acknowledged, perhaps because Hoffmann was long gone from Harvard when he prepared the first draft of the Angewandte publication. Still, it was during that fellowship (1962–1965) that most of the Woodward‐Hoffmann seminal discoveries were made.

FIGURE 69.

Page 119, Hoffmann's acknowledgment page, in CD‐1, ca. early winter of 1967. Note the name E. J. Corey is among those listed. No acknowledgement was included in the final manuscript nor in any of the jointly authored W‐H publications other than a statement of financial support in their article published in 1970 in Science [17].

Hoffmann's “Acknowledgment” draft (Figure 69) is notable because of the inclusion of E. J. Corey, given Corey's claim first made public in 2004 that Woodward had plagiarized his (Corey's) orbital symmetry explanation for electrocyclizations [24, 25]. I inquired of Hoffmann, and he responded:

“I think that in writing this section in 1966 or 1967, I was ‘aggregating’ all the people to thank, over the long period of creation of these ideas. And I remembered Corey's encouragement from those easy days in 1962 – 1964. The people are not in alphabetical order, nor in order of importance to the effort. I think they are just in the order I remembered them when I was writing a section of general thanks for discussions and communication of results” [180].

Corey was not unmentioned in the Angewandte treatise. His research is referenced three times in the text. But Corey was not acknowledged for any private discussions with either Woodward or Hoffmann. And perhaps Corey's most relevant chemistry, which we will identify as a “smoking gun” in the subsequent credit dispute [30, 32], is his total synthesis of dihydrocostunolide in 1963 with Alfred G. Hortmann (Figure 70) [396, 397, 398]. That total synthesis was omitted entirely from Woodward and Hoffmann's publications—until mentioned by Hoffmann in his 2004 rebuttal [223] to Corey's claim of plagiarism against Woodward [24, 25]. In the world of credit‐symmetry, Corey and Hortmann did not cite Woodward and Hoffmann in their dihydrocostunolide full publication [398], submitted 7 months after W‐H's electrocyclization article was published in January 1965 [2] (Corey and Hortmann's synthesis of dihydrocostunolide from santonin in 1963 [396] is a marvelous, early example of retrosynthetic analysis, which Corey had just publicly revealed [399, 400]).

FIGURE 70.

Pericyclic reactions reported by Corey and Hortmann in the early 1960s [396, 397, 398] was used in the total synthesis of dihydrocostunolide. The stereochemistry observed in the thermal versus photochemical transformation is evident (and the assignments of such have recently been confirmed by theoretical modeling of the NMR spectroscopic data [56]).

In that the Angewandte treatise extensively cites the literature, this is a more general route to the acknowledgement of the contributions of others. In that sense, there are several instances where the Angewandte treatise points to prior research, such as, in Woodward and Hoffmann's words:

“In some of our work we have utilized extended Hückel calculations to confirm qualitative conclusions. The virtue of these approximate calculations lies in the fact that they may be applied to σ as well as π systems. The method was originally developed in collaboration with Lipscomb and owes much to previous work of Longuet‐Higgins on boron hydrides” [13]. [emphasis in the original]

And then, there is the penultimate section in the Angewandte treatise, “Other Theoretical Work.” This section was first drafted by Hoffmann in CD‐1, and multiple drafts led to the corresponding Section 13 by Hoffmann in the Angewandte treatise [13] (Figure 71). This concise section includes citations to many contributors and their key publications to the no‐mechanism problem. Not cited was Corey, who asserted his own claimed (and unpublished until 2004 [24, 25, 401]) independent discovery of the frontier molecular orbital mechanism of electrocyclizations [402].

Finally, Woodward was not against providing acknowledgments. According to Lord (Alexander) Todd, his biographer:

“The one thing that [Woodward] would not forgive was what he regarded as scientific dishonesty – the endeavour to claim credit for sloppy work or for contributions properly attributable to other than the claimants” [49].

With regard to the orbital symmetry story, I can relate one instance in which Woodward tried, though unsuccessfully, to praise a specific individual. On December 8, 1968, he wrote [403] to editor of Angewandte Grünewald that Hoffmann and he wanted to add the following statement to the last page of the German edition of the monograph reproduction of the Angewandte treatise [16]:

“The authors take much pleasure in expressing their warm appreciation of the skill, penetration and congeniality with which Dr. H. Grünewald has worked with us to provide a German version of this memoir which properly conveys both the sense and spirit of the English original” [403].

Shortly thereafter, Grünewald informed Woodward that his request was received after the monograph had been shipped to the printers. Grünewald wrote:

“The only thing I can do therefore is to thank you most warmly for your appreciation of the translation work I have done (and which gave me very great pleasure indeed) and to hope that some future occasion will give me the opportunity to offer the same (or even better) services again” [404].

Ultimately there is only one important fact: The Angewandte treatise was devoid of an acknowledgment section.

7.7. Production and Distribution of the Treatise

The influence of Woodward and Hoffmann extended into areas typically within the domain of the publisher and the production staff. Housed in the Woodward papers at the Harvard Archives and in Hoffmann's papers at Cornell is an extensive collection of letters between the authors Woodward and Hoffmann and the journal and monograph publishers, Angewandte Chemie, Verlag Chemie, and Academic Press. And I mean “extensive!” Documents that deal exclusively with production of graphics and formatting and marketing will not be discussed herein. However, Figures S16–S20 and S23 contain numerous examples of (mostly Woodward's) details regarding graphics, color choices and specifications, and corrections of these in the galley proof stages.

7.7.1. Graphics

Woodward was compulsive regarding the text and chemical pictography in his publications [48]. As shown in Figure 72, he actually drew the lemniscate for the p‐orbitals that appeared in the Angewandte treatise [13] and autographed the page for posterity, “Lemniscate of Woodward!” [301] He provided Equation (7) for this lemniscate:

FIGURE 72.

Woodward's drawing of a p‐orbital, ca. 1968. At the bottom right, he wrote “Lemniscate of Woodward!” [301] The mathematical expression for this lemniscate was written at the bottom left (Equation (7)). The text for these two important items is enlarged for the convenience of the reader.

(7)

7.7.2. Color

Woodward's attention to detail seems unlimited. He even specifically dictated the exact color scheme for the phases of the orbitals:

“All plus orbitals should appear as blue, NOVA vitblau 4 F 584 DIN 16 538 1.E.

“All minus orbitals should appear as green, NOVA vitgrün 5 J 52” [405].

These colors are shown in several places in this publication, e.g., Figure 16 Right, Figure 21D, Figure 30B, Figure 41B, and Figure 64.

Of course, Woodward was well‐known for his love of the color blue [48, 49, 273, 406, 407]. As his daughter, Crystal, wrote of Woodward:

“Woodward's aesthetic feeling did not figure only in his work. His legendary preference for the color blue extended to his blue car and the blue suit and blue tie he wore every day. He cared enough to see to the tasteful decoration of his office and, after his second separation and divorce, of his apartment. In all of this there was a distinct refinement and elegance” [408].

But where did the green come from? Scott Denmark may have the answer. After reading an advanced draft of this manuscript, Denmark offered the following explanation for these colors:

“I have a strong recollection that his selection was inspired by the colors of the trams in Zürich (blue) and Basel (green)” [409].

Zürich was the home of several of Woodward's dearest friends (at the ETH Zürich). Basel was the home of the Woodward Research Institute [62, 327] and of the Swiss pharmaceutical giant CIBA for whom Woodward consulted [336, 410]. Thus, the connection between the colors of the trams in Zürich and in Basel and the colors of the orbitals in the Angewandte treatise is reasonable and fascinating.

7.7.3. On Woodward's Distribution of the Submitted Manuscript

Woodward was surely pleased with the completion of the Angewandte manuscript and relieved with its submission (as was Hoffmann). Well, the work was not yet completed. Woodward's archives and Hoffmann's files contain pages and pages of galley and page proofs with remarkable changes in many details (see Section 5). The manuscript was received in Germany in early January 1969. The publication appeared in the November 1969 issue of Angewandte. Do count the days from when the most technologically advanced journal operation in Europe received Woodward and Hoffmann's 300‐page manuscript, on which it starts work immediately, to the time the article is published. This extensive time period indicates the extraordinary care that went into the completion of the publication.

Woodward ordered a large number of reprints to distribute, too few for the need. He was eager to distribute the manuscript immediately upon its availability. Indeed, within the Woodward papers at the Harvard archives is a note on one of Woodward's blue note pages (Figure 73) that was written by Dodie Dyer. Woodward ordered 15 copies, but Woodward's note documents Dyer's distribution of 12 copies of the manuscript to Woodward's carefully‐selected chosen colleagues: Grünewald (2), one circulating copy to Woodward's research group, one for Dyer's files, one for Woodward's files, one for Hoffmann, and one for each of the following: Paul D. Bartlett, Frank H. Westheimer, Doering, Huisgen, and W. T. A. M. van der Lugt (Oosterhoff's student with whom he published the valence bond theoretical analysis for the Woodward‐Hoffmann rules [387, 411].) Last on the list was “EBW,” namely E. Bright Wilson. That totals 12 copies. Who were to be among the fortunate to receive this preprint? It is curious that Woodward did not send a manuscript to Oosterhoff or to Havinga who published Oosterhoff's orbital symmetry explanation [370, 412].)

FIGURE 73.

Dolores Dyer's list [303] of individuals to receive Woodward and Hoffmann's December 30, 1968, submission of “The Conservation of Orbital Symmetry” that appeared in Angewandte Chemie in November 1969 [13, 14] and in monograph form in 1970 [15]. See the text for the recipients’ names. The first two copies were sent to Grünewald as the Angewandte Chemie submission. It is interesting that Dyer listed Paul D. Bartlett before Hoffmann! Also worthy of note, Dyer specified that 15 copies had been “Xeroxed”—Xerox being the primary supplier of commercial copy machines in the 1960s. But she listed only 12 destinations for those 15 copies. Who received the other three? Of the organic chemists at Harvard at the time, several were not on the list, including E. J. Corey, Louis F. Fieser, and two untenured organic chemists, David Dolphin (an instructor; though the “DD” could refer to Dolphin, its position on the list strongly suggests that the “DD” refers to Dyer) and James D. White (assistant professor).

Notably, a copy was not sent to the other full professors of organic chemistry at Harvard, namely Corey, nor to William N. Lipscomb (in whose group Hoffmann had crafted extended Hückel theory and the eHT software used in deriving theoretical support for the W‐H rules), nor to Fukui (who also published [192] on orbital symmetry control before Woodward and Hoffmann). Of course, the list in Figure 73 is the first batch. We do know that Woodward also provided complimentary copies of the 1970 monograph [15, 16] to friends and associates.

7.8. Major Accomplishments of the 1969 Treatise

In their Angewandte Chemie treatise, Hoffmann wrote the first substantive draft, then supplied many new sections and literature citations. Woodward's major contributions were the introduction and violations and conclusion sections. Nonetheless, Hoffmann visited Cambridge a number of times to facilitate their collaboration on the writing of this major tract. And when push came to shove, as Woodward said, he “wr[o]te the paper, in my blood, while I have let all of my other responsibilities lapse completely during the past several months” [166].

The Angewandte Chemie treatise is (or has):

• •A collection of theoretical and physical organic chemistry pedagogical expositions, a review, perspective, and position paper all in one.
• •A brief introduction to qualitative perturbation theory, three‐center bonding, and frontier molecular orbital theory.
• •A classic in organic chemistry, produced with Woodward's compulsive attention to text, graphic, and production details and Hoffmann's (unique, for the 1960s) bridge between experimental chemistry and (molecular orbital) theory.
• •Examples and explanations of assumptions made in simple MO theory and group theory and symmetry considerations, e.g., the nonrole of trivial symmetry‐breaking elements.
• •The promulgation that electrocyclizations, cycloadditions, sigmatropic reactions, cheletropic reactions, and group transfers and eliminations are all of one reaction type—pericyclic reactions—and that they all share a common mechanism based on orbital symmetry control.
• •An extrapolation of the underlying mechanism from “orbital symmetry control” to the recognition that in a W‐H forbidden reaction, in the transition state, one of the originally bonding MOs is no longer bonding, i.e., that MO is at a considerably higher energy.
• •The proposal of a generalized selection rule detailing those valence isomerizations that predict which reactions are Woodward‐Hoffmann allowed and those that are Woodward‐Hoffmann forbidden, i.e., high energy reactions. In a W‐H allowed reaction, every MO bonding level of reactants correlates with a MO bonding level in the product.
• •A list of novel concepts and new nomenclature including definitions.
• •An encyclopedic‐type presentation of all the classes of pericyclic reactions with numerous examples and literature citations.
• •The presentation and mechanistic explanations of chemistry that had not previously been considered part of the no‐mechanism puzzle.
• •And as Hoffmann explained:

“We did provide the motivation for looking at orbitals, especially frontier orbitals and symmetry. The achievement in many ways was pedagogical. The Angewandte treatise is explained just enough, for the intelligent organic chemist to read it; it was at the right level. They could understand it” [53].

7.9. Woodward's Pride in The Conservation of Orbital Symmetry

In February 1971, Woodward received a handwritten letter from an Israeli student who had read the monograph and wanted to buy one for his own library but could not locate one in Israel. He sought Woodward's help. On February 29, 1971, Woodward sent a handwritten message to Thomas Lanigan of Academic Press, the American Publisher of the monograph:

“Is the book really not available in Israel? I’ll appreciate it if you would arrange to send him one. If he doesn’t pay for it, I will” [413].

On June 7, 1978, while at The University of Cambridge, Hoffmann wrote a short note to Woodward:

“When I looked up our book in the library here, to check precisely what we had in it, I noticed that you had signed it. Do you do this regularly as you go around the world?” [414]

By 1978, Hoffmann could be playful with Woodward, who himself could be quite frisky, if not mischievous.

Woodward's sense of accomplishment could hardly have been better savored, save the receipt of the 1965 Nobel Prize in Chemistry, than praise from chemistry's elite, especially those like Nobel laureates Sir Robert Robinson and Lord (George) Porter, each of whom was truly “a chemist's chemist.” Woodward must surely have valued receipt of a note from Robinson dated May 6, 1970, that requested a reprint of the Angewandte treatise and included praise from George Porter (Figure 74).

FIGURE 74.

Excerpt from a note to Woodward from Sir Robert Robinson, May 6, 1970, London, England [415].

8. W‐H 11b: The Wiley Monograph (the Reprint of the Angewandte Chemie Publication (W‐H 11a, Section 6)

The monograph The Conservation of Orbital Symmetry (W‐H 11b) is the English reprint of the Angewandte Chemie treatise (W‐H 11a; Section 6) [16]. A second monograph Erhaltung der Orbitalsymmetrie [16] was also published (also considered W‐H 11b), this being the German language reprint of the Angewandte Chemie treatise [14]. A short report on W‐H 11b is found in Figure S14, and a draft of and the final marketing brochure for the monograph is also found in the Figures S15, S29, and S30.

9. W‐H 12: Science

Just about the time that Woodward and Hoffmann submitted their Angewandte treatise on December 30, 1968, Hoffmann began writing an essay on the same topic for Science. It is a wonder that he did so. It could be imagined that Woodward and Hoffmann had said all they wanted to say on the topic. But Science was (and is) an important journal, and Woodward and Hoffmann had received an invitation. Philip H. Abelson had assured Hoffmann that his anticipated submission date of July 1, 1969, “is quite satisfactory” [416] (Abelson was the editor of Science from 1962–1984).

As was his style then, Hoffmann began writing in longhand (Figure 75). While both Woodward's and Hoffmann's names appear as coauthors of this publication, it was a collaborative work only in‐so‐far as it was based on their past collaboration. Hoffmann's way of writing about the same story from a different direction matched the theme of Woodward and Hoffmann's 1970 Science article [17] itself: nonleast motion reaction pathways. For those Science readers of 1970 unfamiliar with the Woodward‐Hoffmann rules, the Science essay was nicely explanatory. It was a shortened version of the Angewandte treatise. For those who were orbital symmetry fans by 1970, the Science essay's final paragraph was a useful condensation of what was really important therein, and I provide further condensation:

FIGURE 75.

First two paragraphs of Woodward and Hoffmann's 1970 publication in Science. (A) Hoffmann's handwritten first draft [417]. (B) From the publication itself [17]. Reprinted with permission from: R. Hoffmann, R. B. Woodward, Science 167 (1970): 825–831. Copyright 1970 The American Association for the Advancement of Science.

“One should not conclude from our work that least‐motion transition states are unattainable. These should in fact be observed more often than the non‐least‐motion ones. . . . The symmetry‐allowed nonleast‐motion pathways are almost by definition discriminated against by steric factors – they will thus often not manage to be competitive with nonconcerted processes. The point of our work is not that least‐motion processes are to be ignored. Rather, we urge a refocusing on the primary quantum mechanical electronic nature of all chemical reactions. The preservation of the bonding character of all electrons in a reaction is the primary feature of any chemical change. This tendency to maintain bonding will direct nuclear motions which may or may not be least‐motion ones” [17].

In an undated, handwritten note to Woodward (Figure 76), Hoffmann communicated that he had “sent the paper in to [Philip H.] Abelson,” Hoffmann's note suggests that Woodward had yet to see the manuscript. Hoffmann wrote:

FIGURE 76.

Hoffmann's undated handwritten note to Woodward, informing him that he has submitted their manuscript to Science [418]. Hoffmann's last sentence sounds like a rebellion against Woodward's writing style. Hoffmann's forcefulness might have edged toward impertinence.

“I would appreciate some comments from you (if they’re not stylistic, that is . . .)” [418]

Note that even in 1969, 5 years after their collaboration began [57, 136], Hoffmann still addressed Woodward as “RBW” [418] and not “Bob.” And by this time, Hoffmann had been at Cornell for 4 years and had been promoted to full professor.

The content of the Science essay was all contained in the previous W‐H publications. A casual observer might ask: “Why was this publication written in the first place?” I shall refer to Hoffmann's own contemporaneous judgement of this Science publication to provide a partial answer to this question.

The story is as follows. In early June 1973, Joseph Bunnett—the same Bunnett who, as editor of Accounts of Chemical Research, solicited what was to become W‐H 10 (see Section 5)—wrote to Woodward and Hoffmann about a project Howard Simmons and he were initiating. Bunnett wrote:

“Howard Simmons and I are organizing a collective volume to be entitled ‘Collected Accounts of Orbital Symmetry.’ This volume wil1 comprise articles reprinted from Accounts of Chemical Research, JACS, and other journals. It will be published by the American Chemical Society. . . . Our purpose in writing is not only to provide information, but also to request your approval of including your papers in the volume as detailed above [419].

On June 6, 1973, Hoffmann responded:

“I have spoken with Prof. Woodward about this project, and I can say for both of us that we are pleased that you are undertaking this project. We have complete confidence in your judgment in selecting the articles to be reproduced . . . . We would suggest that you consider replacing the Science 167, 825 (1970) review by something we’ve written more recently that we believe to be pedagogically effective. This is an article that has appeared In German in Chemie in unserer Zeit, 6, 167 (1972) and in Swedish in Kemisk Tidskrift, 85, No. 3, p. 28 (1973)” [420].

Ultimately, Bunnett and Simmons reprinted both the Science article [17] and the Chemie in unserer Zeit review [18]. I shall now discuss that latter review.

10. W‐H 13a and W‐H 13b: Chemie in Unserer Zeit and Kemisk Tidskrift

10.1. Invitation and Publication Details

On February 9, 1972, Per‐Olof Kinell of the University of Umeå invited Woodward to write a review article about orbital symmetry for their

“small chemical journal, Kemisk Tidskrift, which as one of its aims has to inform chemists about recent developments in various new areas. . . . The general level of articles in the journal varies to some extent depending, among other things, on the character of the subject. In this case, I think that a level corresponding to the chemical background of an undergraduate student would be appropriate” [421].

It would not be an understatement that such an invitation was not met with great enthusiasm by Woodward. But it did elicit a relatively rapid (for Woodward) and gentlemanly reply on March 14, 1972:

“It was most kind of you to invite me . . . Unfortunately, my many other responsibilities leave me no time for the preparation of such an article, and I have no choice but to decline, with regret.

It is just possible that Professor Hoffmann might be able to do something along the lines you have in mind. . . . ” [422]

After receiving a similar invitation dated June 9, 1972, from Kinell, Hoffmann responded on June 14, 1972:

“Thank you for your letter of June 9. Normally I would react negatively to a request such as yours, not because I am uninterested in contributing to the education of younger and older chemists but because there is so much that has been written about the subject of orbital symmetry. And repetition is boring – the reader reacts negatively to still another review of the same fashionable area of chemistry.

However, we have been so well received here in Sweden [Hoffmann's wife Eva is Swedish and they often summered there] that perhaps the writing of such an article would be a way of reciprocating the hospitality of all our colleagues. So, I would be willing to write something at a reasonably elementary level for Kemisk Tidskrift . . . ” [423]

On August 7, 1972, Kurt Nyberg, Chief Editor of Kemisk Tidskrift (as his letter reads) responded to Hoffmann, thanking him for his manuscript and informing him that they will begin translating the text into Swedish shortly. Nyberg also informed Hoffmann that they have “no objection to your publishing this article in ‘Chemie in unser Zeit.’” This duplicate publication will be discussed shortly.

On July 28, 1972, Hoffmann sent to Nyberg a package containing

“a red folder with the original of the manuscript I’ve promised for Kemisk Tidskrift, and a blue folder with a book and a note to the person who will do the drawings. . . .” [424]

In his cover letter, Hoffmann discussed in great detail various production details regarding the artwork and the translation of his manuscript into Swedish. He added:

“I have been asked by a Verlag Chemie publication for secondary school students “Chemie in Unser Zeit’ to write an article on orbital symmetry. Would you agree to republication of this article at approximately the same time that it would appear in ‘Kemisk Tidskrift’? They would do a German translation and draw their own figures. The audiences hardly overlap and so, I would like to go ahead with the project. But I await your permission, since there may be copyright difficulties. Please let me know your opinion on the matter” [424].

On August 7, 1972, Nyberg confirmed safe receipt of the submission and provided full assurances regarding Hoffmann's production requests. He then added:

“We have no objection to your publishing this article in “Chemie in unser Zeit”. As you write yourself, the audiences are totally different. The German journal might even borrow our illustrations . . . ” [425]

On October 2, 1972, Hoffmann wrote to Woodward, this time addressing him as “Bob”:

“I’m afraid that I agreed to write an article on orbital symmetry control for ‘Kemisk Tidskrift.’ I am attaching a typed copy of the English text. The paper will be published In Swedish. . . . I would appreciate it if you would read the text. Not being able to resist the opportunity of teaching some molecular orbital theory I took the correlation diagram approach. And I tried to discuss nonconcerted reactions as well. I don’t know how well this comes off.

Would you like your name on this article? I would be pleased to add it, providing we don’t have to revise too much of my inadequate English in order to bring it up to true Woodwardian prose.

It may also happen that ‘Chemie in unserer Zeit,’ a Verlag Chemie publication for secondary schools, will republish a German version of this article” [426].

Woodward immediately agreed to be a coauthor [427]. Also on October 2, 1972, Hoffmann wrote to Nyberg:

“Along with this letter I am returning to you one copy of the English version and one copy of the Swedish translation of the article. . . . I presume the next time I see this will be in proof form of the Swedish version” [428].

The timing of these two letters is worthy of note. On this same day of October 2, 1972, Hoffmann wrote to Woodward, asking him if he’d like to be a coauthor and inquiring of Woodward as to the changes he recommends; and Hoffmann also sent his revised manuscript in English and in Swedish to the publisher. That Hoffmann anticipated receiving no substantive alterations from Woodward seems likely, with one possible change: adding Woodward as a coauthor. In any event, within Hoffmann's papers is a corrected, typewritten manuscript with handwritten corrections and inserts. The first page is shown in Figure 77. Woodward must have said “yes,” he wanted to be a coauthor. It might be noted that Hoffmann was (and continues to be) remarkably efficient—he gets those letter and drafts done quickly and without requiring reminders.

FIGURE 77.

First paragraph(s) of the last joint Woodward‐Hoffmann publications (W‐H 13a and W‐H 13b). (A) From Hoffmann's handwritten draft [429]. (B) From Hoffmann's typewritten manuscript [430]. Note that Hoffmann added by hand “and R. B. Woodward” to the authors’ byline. (C) Hoffmann's English text in (B) translated into German, from Woodward and Hoffmann's Chemie in unserer Zeit publication (W‐H 13a) [18]. Reprinted with permission from: R. Hoffmann, R. B. Woodward, Chemie in unserer Zeit. 6 (1972): 167–174. Copyright 1972 John Wiley and Sons. (D) Hoffmann's English text in (B) translated into Swedish, from Woodward and Hoffmann's Kemisk Tidskrift 85 (1973): 28–30, 33−34, 37 (W‐H 13b) [19]. Reprinted with permission from the copyright holder (R. Hoffmann).

There is little remaining documentation regarding the Chemie in unserer Zeit publication. There is one letter from Hoffmann to Woodward dated November 10, but without a year (Figure 78) [431]. From the dates and substance of the correspondence regarding the Kemisk Tidskrift publication, i.e., seemingly obtaining permission from Kemisk Tidskrift to publish the same article in Chemie in unserer Zeit, the year for Hoffmann's letter to Woodward in Figure 78 would be 1972. But the Chemie in unserer Zeit publication appeared in 1972 and the Kemisk Tidskrift appeared in 1973. The only explanation is that article processing for Chemie in unserer Zeit was much faster than for Kemisk Tidskrift.

FIGURE 78.

Hoffmann's letter to Woodward regarding the galley proof of their article for Chemie in unserer Zeit, November 10, (likely) 1972, Ithaca, NY [431].

Hoffmann's letter to Woodward was accompanied by the galley proofs of their Chemie in unserer Zeit article. This letter is particularly noteworthy for its last sentence:

“In case you want any changes made, they can be communicated directly to [the publisher]” [431].

Hoffmann clearly did not anticipate any action by Woodward. Nor was Hoffmann encouraging any.

10.2. Substance of Chemie in Unserer Zeit (W‐H 13a) and Kemisk Tidskrift (W‐H 13b)

When he agreed to produce another essay on the Woodward‐Hoffmann rules in 1972, Hoffmann was faced with an interesting conundrum. He had already written and published two concise reviews on the topic, one in 1966 in the Transactions of the N.Y. Academy of Sciences (W‐H 7) [9], and a second in 1968 in the Accounts of Chemical Research (W‐H 10) [12]. And of course, in 1969 and 1970, Woodward and Hoffmann had published their major treatise in Angewandte Chemie (W‐H 11a) and its replicated monograph (W‐H 11b) [13, 14, 15, 16]. And then, Woodward and Hoffmann had published their essay in Science in 1970 [17]. What more was Hoffmann to write on the same topic? There was an added challenge for the new article (W‐H 13a and W‐H 13b): according to the publisher of this succeeding essay, the academic level was to

“correspon[d] to the chemical background of an undergraduate student would be appropriate” [421].

Hoffmann, of course, drafted the parent manuscript in English (see, for example, Figure 77A,B). In due course, these were translated into German (Figure 77C) and into Swedish (Figure 77D) by the two publishers—even though Hoffmann was fluent in both languages.

An examination of Hoffmann's solution—the two publications that resulted from this experience (W‐H 13a [18] and W‐H 13b [19])—reveals Hoffmann's modifications:

• •An extensive use of correlation diagrams, beyond that found in previous W‐H publications.
• •A much‐expanded discussion of group transfers and eliminations.
• •The elimination of the theoretical underpinning of electrocyclizations, cycloadditions, and sigmatropic reactions.
• •For the first time, the term “the Woodward‐Hoffmann rules” was used in a Woodward‐Hoffmann publication. This was thus an acknowledgment that their “generalized selection rules” had entered into “eponymous territory,” an honor that continues to this day.

Hoffmann was proud of these last Woodward‐Hoffmann publications. As discussed in Section 8, in his recommendation to Bunnett as to which of the W‐H publications ought to appear in his and Howard E. Simmons's anthology, Hoffmann wrote:

“We suggest that you consider replacing the Science 167, 825 (1970) review [W‐H 12] by something we’ve written more recently that we believe to be pedagogically effective” [420].

Hoffmann was referring to these two articles. As mentioned above, Bunnett and Simmons reprinted the Science review [17] and the Chemie in unserer Zeit essay [18].

10.3. Conclusions

The effect of these two publications on organic chemists of the time is likely to have been close to nil. Perhaps they influenced some undergraduate and graduate students in Sweden whose English was limited and thus were unlikely to have benefited from the English‐ or German‐language publications of Woodward and Hoffmann. Perhaps. Since Hoffmann wrote the manuscript in English, even though he was fluent in both Swedish and German, the publishers undertook the serious chore of translation into Swedish and German. The hard work by those involved—Hoffmann, as author; the two publishers, as translators—suggests that we, today, ought not to judge those individuals—or at least, not judge them too harshly. And certainly not judge them using the standards of today.

10.4. Coda

In early October 1972, in the midst of writing these last two publications (the Science and Chemie in userer Zeit and Kemisk Tidskrift), Hoffmann received an invitation from Howard E. Zimmerman to present a lecture at a proposed “Symposium on Orbital Control of Organic Reactions.” Hoffmann politely declined:

“The subject is as fascinating as ever. New experimental examples keep coming in and novel theoretical ramifications have kept myself and others busy. But frankly speaking I’m a little tired of lecturing about orbital symmetry. I would suggest that you ask in my place Woodward – now that the B₁₂ synthesis is done he probably has little to keep him busy ” [432].

No doubt that writing the two manuscripts cemented Hoffmann's desire to move on to new chemistry. While he would continue to utilize the concepts and MO tools developed during the orbital symmetry research, Hoffmann would not write again on this specific topic until propelled to do so in 2004 [223]. Hoffmann's resurgence in the orbital symmetry story was motivated by E. J. Corey's claim of plagiarism against Woodward [24, 25], a topic that shocked the chemistry community when first divulged in Corey's 2004 Priestley Medal address [433, 434, 435]. All of this will be discussed in great detail in the following publications in this series [30, 31, 32, 33]. I remark that Hoffmann has chosen when to write or not write about the Woodward‐Hoffmann rules in what might seem inconsistent. He answered positively the secondary invitations I have just discussed. But he did not discuss the subject where some might have expected it: not in his never‐published 1973 Cope Award nor in his 1981 Nobel Prize lecture [436, 437].

11. W‐H‐14: R. B. Woodward's The Arthur C. Cope Award Lecture

The third publication that fits into what I term Woodward and Hoffmann's “Looking Back” articles (Table 2) is Woodward's Cope Award lecture. In 1973, Woodward and Hoffmann shared the inaugural Arthur C. Cope Award, the only instance in which the Cope was jointly awarded. The presentations were made on August 28, 1973, at the 166th National Meeting of the American Chemical Society, in Chicago, IL. Both Woodward and Hoffmann gave lectures at the meeting. Woodward's handwritten notes [21] of his lecture have been preserved at the Harvard Archives, and these have been transcribed and printed in Otto Theodor Benfey and Peter J. T. Morris's monograph Robert Burns Woodward, Architect and Artist in the World of Molecules [438].

The content of Woodward's Cope Award address is a prequel to the events that began on or about May 5, 1964, the presumptive date of the initiation of the Woodward‐Hoffmann collaboration [136]. Woodward told the story of his earliest encounters with professional‐level chemistry including his own independent discovery at age 10 of what today we know of as the Diels‐Alder reaction (Figures 79 and 80). As Woodward said in his Cope Award address:

FIGURE 79.

An excerpt from one of Woodward's handwritten sections for one of the latter drafts of the Angewandte treatise [226]. The page number “1013”is somewhat puzzling. As shown in Table 4, many pages in the section on “Cycloadditions”) of HD‐2 were numbered in the 1000s. This graphic did not appear in the Angewandte treatise. However, it was presented minus the arrow as Figure 5 in Woodward's Cope Award address (as in Figure 80A) [20, 21].

FIGURE 80.

(A) Graphic as used by Woodward in his Cope Award address (see Figure 79) [20, 21].^. If the red bonds in equation (B) are momentarily disregarded as in a gedanken experiment, the Diels‐Alder reaction in equation (C) is generated. Reinserting the disregarded (red) bonds from (B) into (C) brings us back to (A). Adapted from: J. I. Seeman, “Five Decades of Dispute. A Triptych: Why Woodward?,” Chem. Rec. 22 (1922): 1–56, doi.org/10.1002/tcr.202200150.

“I recognized that if one were to assume the separate existence – however fleeting – of [the resonance hybrids of benzene, see Figure 79], the conversion of the one into the other might be regarded as a chemical reaction, in my mind – in fact an addition reaction of an olefin to a diene. Thus was born – in my mind – one of the first chemical reactions of my own design . . . ” [20, 21]

That Woodward would lean backward and use a graphic and concept from a 1968 draft of the Angewandte treatise [226] for his Cope Award address illustrates that like Hoffmann—he was also a salvager (but a much more selective salvager than Hoffmann). Perhaps all successful scientists are salvagers in their very own idiosyncratic ways.

During a review of the final version of this manuscript before its submission, Hoffmann wrote in the margin of the draft:

“But should we believe Woodward? Was he a precocious 10‐year‐old, able to encompass the ideas of aromaticity and the Diels‐Alder reaction? It seems too much. But Woodward was Woodward, and our science has not had a talent like his ever since” [163].

The penultimate paragraph in Woodward's Cope Award address was:

“After a brief false start in extending these ideas [Woodward's earlier and wrong solutions to the no‐mechanism problem], attributed clearly to my gaucherie in the details of quantum chemistry, I very soon realized that I needed more help than was available in my immediate circle, and I sought out Roald Hoffmann, who was well ‐known to me, though by reputation only, as a brilliant young theoretician, at that time, as I have already mentioned, a member of the Society of Fellows. I told him my story, and then, essentially, put to him, the question, ‘Can you make this respectable in more sophisticated theoretical terms?’ He could, and did, AND THUS began our immensely pleasurable, rewarding, and fruitful collaboration. . . . ” [21]

I say nothing further about Woodward's lecture, as the focus of this publication is the history of the chemistry of the W‐H rules, not Woodward's biographical experiences prior to 1964. In fact, that topic was discussed in one of the previous publications in this series entitled “Five Decades of Dispute. A Triptych: Why Woodward?” [34]. A second publication in this series entitled “Why Woodward and Hoffmann? And Why 1965?” [57] is further expansive of Woodward's pre‐May 5, 1964, experiences relevant to orbital symmetry control.

Hoffmann's Cope Award lecture on bonding in cyclopropanes was never published. That lecture is one of a very few of Hoffmann's major lectures that was not turned into an article (a counter‐example of his being a salvager). He has accumulated a number of lectures toward the end of his career that were never written up (but may be found, at least some of them, on that modern time machine, YouTube). An example is a lecture he has given for 30 years with the title “Chemistry's Essential Tensions: Three Views of a Science in Culture” [439]. Hoffmann has just recently reused many ideas from that lecture in a book coauthored with Aleksandr P. Svitin, to be published in 2026, entitled Pebbles: The Commonalities and Divergences of Art and Science” [440].

12. Woodward‐Hoffmann and the Social Construction of Science

12.1. The Effect of Quantum Chemistry on the Sociology of Chemistry

The 1960s was an era of “full steam ahead” for basic research in chemistry worldwide. The Russians’ success with Sputnik opened the floodgates of America's government funding of science, in general, and of chemistry, in particular. Other countries followed suit. Students flocked into STEM (science, technology, engineering, and mathematics) curricula. Universities expanded their chemistry departments, and funding for new instrumentation was readily available. Furthermore, the needs of World War II had resulted in advances in electronics and eventually in such novel instrumentation and previously unimaginable computer power that by the 1960s, the face of chemical research had advanced by a generation beyond that of the 1940s.

By the mid‐1960s, two revolutions of chemistry had become evident: the Instrumental Revolution [23, 441, 442, 443, 444, 445, 446, 447] and the Quantum Chemistry Revolution [23, 447]. All of this resulted in an avalanche of new chemistry being published and the appearance a host of unsolved challenges in chemistry. One of these was the no‐mechanism problem [86, 448, 449], and the opportunity was extraordinarily ripe for the invention of the Woodward‐Hoffmann rules.

The Woodward‐Hoffmann rules precipitated the Quantum Chemistry Revolution, with its attendant research and sociological consequences [23]. Characteristics of the Quantum Chemistry Revolution included a well‐spring of synergy between experimental chemistry and theoretical chemistry; the beginning and solidification of a new subdiscipline of chemistry, namely computational chemistry; the insertion of molecular orbital theory into the working vocabulary of the majority of experimental chemists; an evolution in the practice of physical organic chemistry, from the “Hammett‐equation era” to the “quantum chemistry era”; and the origination of many new types of research programs that were hardly envisioned in the 1950s, e.g., the construction of multidimensional reaction potential energy surfaces in both the ground and excited states. Even a peripheral examination of the Woodward‐Hoffmann story reveals those giants’ fingerprints on all of these chemical themes.

12.2. The Matthew Effect

The Matthew effect is a sociological theory that describes how those with initial advantages tend to accumulate more over time, while those with disadvantages fall further behind. The Matthew effect is also known as “cumulative advantage,” this concept is famously summarized as “the rich get richer and the poor get poorer” [293, 392, 393, 450, 451]. With regard to the W‐H story, one might assume that Woodward was the “rich” and everyone else, especially Oosterhoff, Fukui, and Hoffmann, were the “poor.” How did this turn out?

It is quite true that orbital symmetry control of certain valence isomerizations was discovered before Woodward and Hoffmann in 1965 [2, 3, 4, 5, 6]. Oosterhoff did so in December 1960 [370, 371, 412] and Fukui in 1964 [192, 285, 286]. R. Stephen Berry's mechanism of the Diels‐Alder reaction in 1962 [39, 448] was a clear precursor of orbital symmetry control. These precedents garnered little interest. Indeed, they garnered no interest—until Woodward and Hoffmann's publications pointing to them [6, 13].

It is also quite clear that the five Woodward‐Hoffmann communications [2, 3, 4, 5, 6] in JACS in 1965 drew the immediate attention of the chemistry community. No doubt this was in large measure due to what those publications said and how they said it. But the fact that one of the authors was R. B. Woodward surely brought (at least) initial attention to these publications. This is an example of the Matthew effect in science [293, 392, 393, 394, 451], where those who have more recognition and other research advantages tend to accumulate even more recognition and other research advantages. In this instance, I refer to the chemistry community paying more attention, at least initially, to publications being authored by Woodward than those being authored by Hoffmann. It is a fool's errand to try to determine which of Woodward and Hoffmann gained the most by being the authors and promulgators of the Woodward‐Hoffmann rules. Arguments could be made in both directions.

Did things ever change in the second W‐H publication [3]! Hoffmann put his name first in a draft, and Woodward let it remain there. The world did change for Hoffmann at that point, but not for Woodward. The community of chemists noticed the author order for sure. The name “Roald Hoffmann” began to draw the attention of chemists, even if it would be spelled incorrectly for some time. Would Hoffmann have objected if Woodward changed the author order on W‐H 2, on cycloadditions. I doubt it—he didn’t object when in the process of writing their long paper, Woodward indeed changed the original order of authors that Hoffmann had drafted for the Angewandte treatise (See Figure 68).

But it soon became clear to the chemistry community that the Woodward‐Hoffmann collaboration was one of equals, at least in terms of the Woodward‐Hoffmann rules. As has been demonstrated in the previous publications in this series [7, 110, 111, 112, 136], Hoffmann's leadership, creativity, discoveries, and work product was surely 50% of the total if not far more than Woodward's. One might well conclude that the Matthew effect had little relevance regarding the W‐H rules. If anything, it kept Woodward's name in the spotlight while bringing Hoffmann's to the fore.

12.3. The Consequences

12.3.1. On Woodward

Could Woodward's reputation have grown even greater than it was prior to the W‐H rules? Of course, yes, especially decades after his death in 1979. Surely more chemists, especially younger chemists, know of and use the W‐H rules far more than any other of Woodward's structure determinations or total syntheses. Indeed, the relevance of the former has given way to modern instrumental analyses and the latter—methodology of synthetic chemistry—has evolved enormously since Woodward's death almost 50 years ago. But the W‐H rules are as valid and contemporary as they were in the 1960s, and research continues on the breadth and validity of the concept [253, 256].

12.3.2. On Hoffmann

As discussed elsewhere in this series [78, 205], by the time he received his Ph.D. at Harvard in June 1962, Hoffmann was deluged with offers of academic positions (from Caltech, the University of Washington, the University of California at Berkeley)—as a chemical physicist. Hoffmann turned those offers down, instead choosing to accept the 3‐year Junior Fellowship from Harvard's Society of Fellows. But by January 1965, only one position remained available to him, at Cornell [78] (in due course, other positions were offered, but not by the institutions that had previously sought his employ [78]). Hoffmann accepted Cornell's invitation, and as they say, the rest is history. In some respects, Hoffmann's Nobel Prize in 1981 reflected his future, not his past. This is illustrated by his Nobel Lecture, published by coincidence in Angewandte Chemie [437] and available also on the Nobel Foundation's website [436]: “Building Bridges Between Inorganic and Organic Chemistry.” The topic is not the W–H rules.

It was not that Hoffmann's 3 years as a Junior Fellow failed to be productive. Indeed, Hoffmann had published quite a lot in those years. But those publications looked like those of a physical organic chemist, not a chemical physicist. The Woodward‐Hoffmann research, just then appearing in the literature, did not (yet) attract the physical chemical physics faculty in the top‐notch institutions. This is an example of perceptions of one's research discipline influencing the hiring practices of institutions and the lives of those seeking positions. There is a human tendency to simplify complex situations. And those simplifications are typically limiting, not expansive. The consequences of these social considerations can affect the performance and trajectory of science.

12.3.3. On Cornell

On the internet, on the top of the internet home page of Cornell University's Department of Chemistry and Chemical Biology (as of May 17, 2026):

“Building on history to invent the future”

“The Cornell Department of Chemistry and Chemical Biology has a long history of discovery, innovation and scholarship. We are fortunate to count Nobel Prize winners, National Academy Members, MacArthur Fellows and Guggenheim Fellows among our faculty” [452].

Clearly, Cornell values Nobel laureates and their discoveries, innovation, and scholarship. In the case of Roald Hoffmann, they have valued the person first. The prizes were the consequence of thoughtful choices and, yes, of some measure of good fortune.

12.3.4. On Angewandte Chemie

By 1969, Angewandte Chemie was considered a respectable though not top‐tier chemistry journal. Its German edition was in its 21^st volume; its International Edition in English was in its eighth volume. Helmut Grünewald, the editor in chief, was actively soliciting manuscripts from the upper echelons of chemistry. Woodward was just one of the invitees. Certainly Angewandte Chemie benefited greatly from its publication of Woodward and Hoffmann's treatise [13]. Interestingly, Woodward did not seek out Tetrahedron, the journal for which he was the Co‐Chair with Sir Robert Robinson, to publish any of the Woodward‐Hoffmann manuscripts. In terms of production quality, Angewandte Chemie and Verlag Chemie kept all their promises to Woodward. And Angewandte Chemie and Verlag Chemie benefited from publishing Woodward and Hoffmann's treatise. For decades, the Woodward‐Hoffmann Angewandte treatise was among the most accessed publications in Angewandte Chemie, and many thousands of monographs were sold.

13. Overall “Master” Conclusions

This section is a “master conclusion section” intended to cover this entire publication. Conclusions specific for each of these four W‐H “heart‐and‐soul” publications are found at the end of those individual sections (Sections 5g, 6h, 8, and 9c).

• •As illustrated continuously in this publication, every scientific publication embodies hidden components, one often being the human side of science. In the early to mid‐1960s, just as the Woodward‐Hoffmann collaboration was soon to begin, Nobel laureate Peter Medawar posited that every scientific paper is a fraud [453, 454]. By this, Medawar did not mean fabrication or falsification of scientific data. Rather, he meant that scientific publications “misrepresent the processes of thought that accompany or give rise to the work that is described in the paper” [453, 454]. One of the objectives in this publication is to reveal from laboratory notebooks and contemporaneous correspondence how and why Woodward and Hoffmann's “heart‐and‐soul” publications came to be. These are part of the invisible stories of chemistry.
• •There are natural sequences in healthy research programs and publishing norms. First, brief (or not so brief) communications. Next, short overviews. Third, extensive treatises and perspectives. Lastly, review articles and summaries—hopefully avoiding assessments of self‐plagiarism. Then awards and acclaim. Woodward‐Hoffmann had them all, and in that sequence. There is one more phase of the research sequence that Woodward truly did not fully experience, a consequence of his premature death in 1979 at the age of 62. That is, the sweet aftertaste of a seminal publication. Hoffmann has experienced that afterglow for more than 50 years, about which he has written [455]. See Figure 81. for a lovely honor provided by one of the leading organic chemistry journals, 50 years after the publication of the W‐H rules.

FIGURE 81.

Front cover of the December 4, 2015, issue of The Journal of Organic Chemistry [456]. The cover includes an adaptation from the colorful Figure 7 from Woodward and Hoffmann's Angewandte treatise [13], a graphic from the first W‐H communication on electrocyclizations [2], and in light gray background, an excerpt from Hoffmann's February 1965 laboratory notebook on the cycloaddition of two ethylenes [110]. This issue of JOC was dedicated to the 50th anniversary of the W‐H rules and contained 22 research articles, an essay by the special issue editor Barry K. Carpenter [457], a preface by Roald Hoffmann [455], and Publication 11 of this series [136] by myself. Copyright permission to republish this graphic is not required [458].

• •Surely, the Angewandte treatise, in English [13] and in German [14], and the monograph, also in English [15] and German [16], became the bible of pericyclic reactions for decades. The enormous effort expended by Woodward and Hoffmann to author those publications—and of Verlag Chemie and Academic Press to produce them—was well rewarded by readership and the archives of chemistry.
• •In their three shorter perspective articles (in Accounts of Chemical Research, Science, and Chemie in unserer Zeit—and its duplication Kemisk Tidskrift), Hoffmann did all the writing and invited Woodward to be a coauthor in each. Woodward accepted. Some may feel that Hoffmann was being generous. Hoffmann saw it as normative behavior, and believed that Woodward deserved coauthorship [53]. Woodward's immediate acceptance of co‐authorship meant that he felt strongly as to his role in the orbital symmetry story.
• •Were the last three W‐H articles worth the time that went into writing them, these being the Science, Chemie in unserer Zeit (and its duplication Kemisk Tidskrift) publications? We must be careful not to evaluate judgments made “at the time” using criteria and facts known only much later. Hoffmann felt these were important enough for him to write. And Woodward felt these were important enough for him to be a co‐author. But in retrospect, probably not, at least not in terms of the disciplines of the readership, the number of accesses over time, and their archival value. This is a warning to scientists: choose your research projects carefully. Avoid the low hanging fruit whose (likely) best result will be a footnote somewhere. It is as much work and time to conduct research and write a publication on a topic of significant value as on a trivial matter in science.
• •There is no credit given to E. J. Corey in any of the W‐H publications or presentations. I point this out given Corey's claim [24, 25, 223, 401] that Woodward plagiarized his solution to the electrocyclization slice of the orbital symmetry epic. Credit issues will be discussed in detail in the next several publications in this series [30, 31, 32, 33].
• •The community learned that orbital interactions shape potential energy surfaces on which reactions occur, depending on the number of electrons in and the symmetries and energies of the molecular orbitals involved. The “shaping of potential energy surfaces” provides a bridge to dynamic trajectory studies [339, 340] on such surfaces.
• •The W‐H story is replete with examples of the nonlinear manner in which science is discovered and its communication implemented. For example, the antarafacial idea made its way into Hoffmann and Woodward's consciousness through their thinking about the stereochemistry of [1,q] sigmatropic shifts. In time, that idea connected to cycloadditions, then to the much older mechanistic possibility of cis or trans additions to π‐systems, that possibility having been clearly formulated earlier for nonpericyclic stepwise processes.
• •Lastly, in this essay on Woodward and Hoffmann's “heart‐and‐soul” publications, I would like to share the affection that Hoffmann has for what he has called for many years “The Long Paper.” Perhaps it is so deeply personal because it is the only publication that Woodward and Hoffmann created together, as intimate collaborators in an intimate collaboration. They not only wrote the Angewandte treatise together, but during some of the writing, perhaps during the most important moments of that writing, they were actually physically together, in the same room at the same time. This, then, was the apex of their collaboration [206] and what I imagine to be the collaboration that was “Woodward and Hoffmann.”

Supporting Information

Additional supporting information can be found online in the Supporting Information section. Supporting Fig. S1: Joseph F. Bunnett's letter to Roald Hoffmann of September 8, 1967, providing the reviewer's comments and instructions for the submissions ultimate publication in the inaugural issue of Accounts of Chemical Research on January 1, 1968. Supporting Fig. S2: Anonymous reviewer's comments on Woodward and Hoffmann's submission to Accounts of Chemical Research, early September 1967. Supporting Fig. S3: SciFinder abstract for Hoffmann and Woodward's publication in the inaugural issue of Accounts of Chemical Research, January 1, 1968. Supporting Fig. S4: Typed at Cornell from Hoffmann's draft and handwritten corrections by Woodward and Dodie Dyer, from page 41 of Harvard Draft 1 (HD‐1), previously from Cornell Draft 2 (CD‐2). This page illustrates how some of Hoffmann's CD‐2 was inserted directly into the first Harvard draft (i.e., page 41 in CD‐2 became Section X, as seen in red pencil, in HD‐1). Supporting Fig. S5: Hoffmann's handwritten draft of “Eliminations of Small Molecules” section, likely used to prepare Harvard Draft 1 (HD‐1). This page illustrates that, in the preparation of HD‐1, Dolores Dyer (and Woodward) used Hoffmann's graphics by cutting‐and‐pasting them from one draft into the next draft. Supporting Fig. S6: (Left) Hoffmann's handwritten draft of the “The Vinylcyclopropane‐Cyclopentene” Section XVII, also numbered page N1, used to prepare Harvard Draft 1 (HD‐1). This page illustrates that inserts were slow to find their exact place in next major drafts, as this page has several identifiers (N1 and XVII). (Right) Excerpts from the 1969 Angewandte treatise showing that Hoffmann's references found their way into the publication. Supporting Fig. S7: (Left) Hoffmann's handwritten draft of the “Group Transfers and Eliminations” Section XVIII, page R1, used to prepare Harvard Draft 1 (HD‐1). (Right) An excerpt from the 1969 Angewandte treatise showing that Hoffmann's graphics found their way into the publication. Supporting Fig. S8: (Left) Handwritten draft by Hoffmann for HD‐1. Of particular interest in this figure is the manner in which references were added to manuscript drafts in the 1960s, i.e., interpolated within the actual text of the manuscript. Also of note is that Hoffmann used references from 1967 and (mostly) 1968 literature for the Angewandte treatise that was finalized and submitted in December 1968. (Right) Excerpts from Section 13 of the Angewandte publication illustrating that Hoffmann's references were used in the target publication. Supporting Fig. S9: (Left) Page 600 from Woodward and Hoffmann's HD‐2, Section XIII (from HD‐1) renumbered by Woodward as Section 6.5. Note the cut‐and‐paste graphic that was added, then deleted. Note also the instructions to Dolores Dyer in red font at the bottom‐left. (Right) The related text from Section 6.5 of the 1969 Angewandte treatise. These graphics illustrate the major changes in graphics that were still occurring going into the final draft. Supporting Fig. S10: (Left) Page 688 from Woodward and Hoffmann's HD‐2, Section 6.2, renumbered by Woodward. The cut and‐pasted graphics were made by Hoffmann for HD‐1 and added to this page, likely by Dolores Dyer. Note Woodward's revised graphic of the cyclopentenone product in eq. 225 (“class C”) at the bottom of page 688. (Right) The first portion of Section 6.2 from the Angewandte treatise. Supporting Fig. S11: Woodward's notes reveal his attempt to generalize allowed and forbidden reactions by the number of bonding and antibonding orbitals of the same symmetry in correlation diagrams for HD‐2. This analysis is related to his similar thinking regarding the reverse Diels‐Alder reaction in his Aromaticity publication, as discussed in a previous publication in this series. Supporting Fig. S12: Cover page written by an anonymous volunteer who helped organize Woodward's office after his death on July 8, 1979. Supporting Fig. S13: Woodward' handwritten inserts and modifications of the beginning of Section 6, “Theory of Cycloadditions and Cycloreversions” for Harvard draft 2 (HD‐2) along with excerpts from the Angewandte treatise based on those inserts and modifications. Supporting Fig. S14: Section 7: W–H 11b: The Wiley Monograph (the Reprint of the Angewandte Chemie publication). Supporting Fig. S15: Drafts of a publicity statement written by Hoffmann and sent to Woodward at the request of the monograph The Conservation of Orbital Symmetry (W–H 11b), August 15, 1969. See Figure S32 for the actual marketing document. Supporting Fig. S16: Woodward was a perfectionist, not just in the research he did but also in its communication. This included his intense interest in the words he used as well as the chemical pictography in his publications. In this section, I focus on Woodward's control over the colors used in the Angewandte treatise and the monograph. One of many letters between Woodward and the Helmut Grünewald and the production teams dealing with color and graphics is shown in this figure. Supporting Fig. S17: Woodward and Hoffmann used colors in their graphics, primarily to designate (relative) phases in the molecular orbitals discussed in their text. It should be no surprise that another key design feature, that the shape of the p‐orbitals, was carefully designed by Woodward (see Figure S18). Woodward is counselling the editor of Angewandte Chemie and a senior executive of the publisher Verlag Chemie about the importance of the chemical pictography of the s and p orbitals. Given that Grünewald was a Ph.D. chemist with many years of publishing experience, such attention may have been supererogatory. Supporting Fig. S18: Dolores Dyer's graphics for color schemes for a p‐orbital. Supporting Fig. S19: Page proofs for Woodward and Hoffmann's “Conservation of Orbital Symmetry.” Supporting Fig. S20: Hand‐drawn graphics of the correlation diagram of the Diels‐Alder reaction of ethylene plus 1,3‐butadiene by Dolores Dyer. Of note is the use of blue and red colors instead of the eventual blue and green in the 1969 Angewandte Chemie treatise. Supporting Fig. S21: Woodward's handwritten graphic of the Diels‐Alder reaction is accompanied by several notes, one of which refers to the relative position of the olefin and the diene. The second note, at the very bottom, “but very poorly drawn” indicates some level of frustration by Woodward as to the difficulty in drawing these graphics perfectly the first go‐around. Supporting Fig. S22: (A) Woodward's mathematical calculations to derive the spacing between the pentagons in these figures. (B and C) Woodward's and possibly Dodie Dyer's drawings{Dyer, 1968 #26917} for the [1,4], [1,2] and [1,3] sigmatropic shifts, as shown in (D) taken from Woodward and Hoffmann's 1969 Angewandte treatise. Supporting Fig. S23: One page from the galley proofs reviewed by Woodward in early August 1969 for the Angewandte treatise in monograph format. The level of Woodward's perfectionism is illustrated by the amount that the “poorly drawn: lines do not appear to meet in a point” is not exact. Supporting Fig. S24: Handwritten page by Hoffmann, likely used as an organizational tool during the drafts of HD‐2. Supporting Fig. S25: Excerpts from two handwritten pages by Woodward, used as organizational tools during the drafts of HD‐1 and/or HD‐2. Supporting Fig. S26: Handwritten organizations list by Woodward of topics to be written by Hoffmann, likely for HD‐2. Supporting Fig. S27: Woodward's organizational notes for the updating of reference numbers, formulae identifiers, and Figure numbers, likely going from Harvard draft 2 (HD‐2) to Harvard draft 3 (HD‐3). Supporting Fig. S28: Dodie Dyer's record of determining the optimum spacing of section titles in the final Woodward‐Hoffmann Angewandte treatise submission, likely for Harvard draft 3 (HD‐3). Supporting Fig. S29: Page proof from Verlag Chemie/Academic Press with Woodward's handwritten corrections. Supporting Fig. S30: Marketing brochure by Verlag Chemie/Academic Press. Supporting Fig. S31: Subramania Ranganathan's late 1963 – early 1964 set of stereospecific reactions that became Woodward's fifth mysterious reaction(s). Supporting Fig. S32: Brief Biography of Helmut Grünewald by Peter Gölitz. Supporting Fig. S33: Letter from Roald Hoffmann to R. B. Woodward, January 21, 1966, Ithaca, NY. See Figure S34. Supporting Fig. S34: “A Catalogue of Concerted Reactions” written by Roald Hoffmann, early January 1966. See Figure S33.

Conflicts of Interest

The author declares no conflicts of interest.

Supporting information

Supplementary Material

Supplementary Material

Seeman Jeffrey I., The Chemical Record 2026, 26, e202500107. 10.1002/tcr.202500107

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.