Palladacyclic Imidazoline-Naphthalene Complexes: Synthesis and Catalytic Performance in Pd(II)-Catalyzed Enantioselective Reactions of Allylic Trichloroacetimidates

Abstract

A new family of air- and moisture-stable enantiopure C,N-palladacycles (PIN-acac complexes) were prepared in good overall yield in three steps from 2-iodo-1-naphthoic acid and enantiopure β-amino alcohols. Three of these PIN complexes were characterized by single-crystal X-ray analysis. As anticipated, the naphthalene and imidazoline rings of PIN-acac complexes 18a and 18b were canted significantly from planarity and projected the imidazoline substituents R¹ and R² on opposite faces of the palladium square plane. Fifteen PIN complexes were evaluated as catalysts for the rearrangement of prochiral (E)-allylic trichloroacetimidate 19 (eq 2) and the S_N2′ allylic substitution of acetic acid with prochiral (Z)-allylic trichloroacetimidate 23. Although these complexes were kinetically poor catalysts for the Overman rearrangement, they were good catalysts for the allylic substitution reaction, providing branched allylic esters in high yield. However, enantioselectivities were low to moderate and significantly less than that realized with palladacyclic complexes of the COP family. Computational studies support an anti-acetoxypalladation/syn-deoxypalladation mechanism analogous to that observed with COP catalysts. The computational study further suggests that optimizing steric influence in the vicinity of the carbon ligand of a chiral C,N-palladacycle, rather than near the nitrogen heterocycle, is the direction to pursue in future development of improved enantioselective catalysts of this motif.

Introduction

Enantioselective allylic substitution reactions are an essential class of catalytic asymmetric processes that are used to prepare a wide variety of enantioenriched chiral organic molecules.¹ For a number of years we have been engaged in the development of catalytic enantioselective transformations of allylic substrates that employ palladium(II) complexes.² Among these are allylic alkylation reactions of trichloroacetimidate derivatives of prochiral 2-alken-1-ols with carboxylic acid³ and phenol^3c,4 nucleophiles (Figure 1). These reactions take place with extraordinarily high branched-to-linear ratios (>100:1) and have been shown to not proceed via η³-allylpalladium(0) intermediates.^3c Of the many palladium(II) complexes investigated for the reaction of (Z)-allylic trichloroacetimidates 1 with carboxylic acids and phenols, the commercially available catalysts [(R_p,S and S_p,R)-COP-OAc]₂⁵ (5 and ent-5) were found to be optimal. For the transformation of (E)-allylic trichloroacetimidates 4 to branched allylic phenols 3, the di-μ-amidate dipalladium complexes [(R_p,S and S_p,R)-COP-NHCOCCl₃]₂ (6 and ent-6) are preferred, because they are kinetically poor catalysts for the competing allylic rearrangement to form 3-trichloroacetamido-1-alkenes.^4b

Figure 1.

Enantioselective Reactions of Allylic Trichloroacetimidates Catalyzed by COP Complexes 5 and 6.

Since our initial disclosure in 1997,⁶ a wide variety of palladium(II) complexes have been evaluated as catalysts for various transformations of allylic imidates.⁷ Among these, palladacyclic catalysts,⁸ in particular C,N-palladacycles such as the COP (5 and 6)⁵ and FOP complexes (e.g., 7)⁹ and related complexes such as 8 developed by Peters¹⁰ have proven to be of particular value. A structural feature of the C,N-palladacycle catalyst motifs 5–8 that is believed to be important for achieving high levels of enantioselectivity is the projection of steric bulk both above and below the palladium square-plane coordination sphere. Sterically differentiating the faces of the palladium square plane is thought to control which prochiral face of the coordinated alkene is activated in the enantiodetermining step. As a result, most enantioselective palladacycle catalysts rely on a planar chiral design to place steric elements perpendicular to the cyclopalladated ring.¹¹ However, methods for preparing non-racemic planar-chiral palladacycles are multi-step, typically requiring diastereoselective cyclopalladation of an auxiliary-appended metallocene or resolution of a racemic complex.¹² Consequently, this structural moiety adds complexity to catalyst synthesis and limits the accessibility of analogs.

With the aforementioned considerations in mind, we have explored the C,N-palladacyclic imidazoline naphthalene (PIN) catalyst motif 9 (Figure 2). The modular nature of this bidentate palladacycle—derived from a naphthalene and an easily modified imidazoline ring system—should allow rapid access to an array of catalyst structures. Moreover, both the steric and electronic environment around the metal center can be systemically altered by variation of the imidazoline substituents (R¹, R², and R³), the aromatic carbon ligand, and the ancillary ligands (L) on palladium. Of central importance, we envisioned creating an adjustable element of chirality by perturbation of the steric interaction between the naphthalene ring and imidazoline substituent R². This approach would capitalize on two potential steric contacts: 1) the interaction between R¹ and R³ and the ancillary palladium ligands L; and 2) the interaction between R² and the peri-C–H bond of the naphthalene ring. When R¹ and R² are large (and R³ small), these steric interactions should result in the cyclopalladated complex being non-planar, with the naphthalene and imidazoline rings canted out of plane and the substituents R¹ and R² oriented in an anti fashion. Furthermore, we anticipated that the nitrogen substituent R² would provide a handle to tune the basicity of the donating nitrogen atom, allowing easy adjustment of the electronic properties of the palladium center.¹³ In this article we report the preparation and structural characterization of a family of PIN complexes 9 and an evaluation of their performance as catalysts for the enantioselective transformation of (Z)-allylic trichloroacetimidates 1 to branched allylic esters 2 and phenols 3.

Figure 2.

Design of the Palladacycle Imidazoline-Naphthalene (PIN) Catalyst Motif.

Results and Discussion

Synthesis of PIN Complexes

Nitrogen-directed carbopalladation of an aromatic ring is the most direct route to C,N-palladacycles;^8,14 therefore, we began our investigation by exploring the C–H activation of imidazoline 12 with various Pd(II) electrophiles (Scheme 1). Starting from (S)-valinol-derived β-hydroxyamide 10, enantiopure 1-naphthylimidazoline 12 was prepared by a conventional sequence that began by the conversion of amide 10 to imidoyl chloride 11 by reaction with SOCl₂ at 85 °C.¹⁵ After evaporation of excess SOCl₂ under reduced pressure, crude 11 was allowed to react with 1.1 equiv of isopropylamine in 5:1 CH₂Cl₂-Et₃N at room temperature. This two-step sequence provided imidazoline 12 in 70% yield and >98% ee.¹⁶ However, attempted reaction of 12 with stoichiometric amounts of various Pd(II) electrophiles under common cyclopalladation conditions did not generate palladacycle 13.⁸ For example, attempted cyclopalladation of 12 with Pd(OAc)₂ in refluxing acetic acid⁵ resulted in decomposition of the imidazoline and formation of palladium black, whereas attempted reaction at ambient temperature afforded a dark, intractable reaction mixture. Attempted cyclopalladation of imidazoline 12 using Pd(OAc)₂, Na₂PdCl₄ or PdCl₂(MeCN)₂ under both neutral and basic reaction conditions either returned 12 or resulted in decomposition. We reasoned that the desired cyclopalladation reaction was complicated by the steric strain associated with bringing the naphthyl and imidazoline rings into the near coplanar orientation required to promote C–H activation.

Scheme 1.

Attempted Synthesis of PIN Complexes by Direct Cyclopalladation.

The inability to cyclopalladate imidazoline 12 led us to explore forming palladacyclic PIN complexes by the oxidative addition of Pd(0) with halogen-containing precursors. Accordingly, 1-cyanonaphthalene (14) was deprotonated at low temperature with lithium 2,2,6,6-tetramethylpiperide and the resultant aryllithium species was quenched with I₂ as reported by Fraser to give 1-cyano-2-iodonaphthalene (Scheme 2).¹⁷ Hydrolysis of this crude product in refluxing HOAc containing concentrated H₂SO₄ yielded the primary aryl amide, which was subsequently dissolved in MeCN and treated with excess NaNO₂ and 70% H₂SO₄ at room temperature to provide 2-iodo-1-naphthoic acid (15).¹⁸ This three step sequence was preformed routinely on 5–10 g scales without purification of intermediates to give iodoacid 15 in 65% overall yield after recrystallization. Conversion of 15 to the corresponding acid chloride and subsequent reaction with either (S)-valinol or (S)-tert-leucinol gave respectively β-hydroxyamides 16a (R¹ = i-Pr) and 16b (R¹ = t-Bu) in high yields as colorless solids after recrystallization.

Scheme 2.

Synthesis of (S)-PIN-acac Complexes 18.

Hydroxy amides 16a,b were converted in 43–76% yield to imidazolines 17a–m by the two-step sequence utilized to prepare imidazoline 12 (Scheme 2, Table 1). In addition, imidazolines 17o (R² = Ts) and 17p (R² = COt-Bu) were prepared in good yield from N–H imidazoline 17n by reaction with TsCl or pivaloyl chloride at room temperature. With the exception of imidazoline 17n, these products were 1:1 mixtures of atropisomers.¹⁹

Table 1.

Prepartion of (S)-PIN-acac Complexes 18 from Imidazoline Precursors 17.

entry	17	R1	R2	18	yield (%)a
1	17a	i-Pr	i-Pr	18a	70
2	17b	t-Bu	i-Pr	18b	68
3b	17c	i-Pr	CH2Ph	18c	53
4b	17d	i-Pr	CH(Ph)2	18d	62
5b	17e	i-Pr	Cy	18e	57
6	17f	i-Pr	t-Bu	18f	68
7b	17g	t-Bu	t-Bu	18g	41
8	17h	t-Bu	1-adamantyl	18h	63
9	17i	i-Pr	Ph	18i	72
10b	17j	t-Bu	Ph	18j	52
11b	17k	i-Pr	p-OMeC6H4	18k	54
12b	17l	i-Pr	p-CF3C6H4	18l	58
13b	17m	i-Pr	mesityl	18m	65
14	17o	i-Pr	Ts	18o	64
15	17p	i-Pr	C(O)t-Bu	18p	50

^aIsolated yield.

^bPrepared directly from amides 16a,b without purification of the imidazoline intermediate. Reflects overall isolated yield (3 steps).

Having developed an efficient route to imidazolines 17, several conditions for promoting their conversion to palladacycles were explored. The optimal condition was found to be reaction of the imidazoline with 0.5 equiv Pd₂dba₃ in refluxing toluene,²⁰ which furnished iodide-bridged palladacyclic products as yellow-orange solids. NMR analysis showed that these products were mixtures of head-to-head and head-to-tail isomers. Separation of these dimeric complexes was complicated by their decomposition upon exposure to silica gel. As a result, the crude mixtures of iodide-bridged dimers were allowed to react with silver acetylacetonate in CH₂Cl₂ at room temperature to give (S)-PIN-acac complexes 18a–m,o–p as pale yellow solids in 41–68% yield (Table 1). Alternatively, the acetylacetonate complexes could be prepared in comparable overall yields directly from amides 16a,b without purification of the labile imidazoline ligand. These monomeric complexes displayed a high degree of stability, withstanding silica gel chromatography and prolonged exposure to air and moisture at room temperature. Palladacycles 18 were generally more soluble in common organic solvents such as Et₂O and toluene than COP complexes 5 and 6. Analysis of the NMR spectra of (S)-PIN-acac complexes 18 revealed that most were produced as single stereoisomers; the exception was (S)-PIN-acac complex 18h (R² = 1-adamantyl), which was isolated in 41% yield as a ~1:1 mixture of inseparable stereoisomers.

Structural Properties of PIN-acac Complexes

Two PIN-acac complexes having alkyl substituents, 18a (R¹ = R² = i-Pr) and 18b (R¹ = t-Bu, R² = i-Pr), provided single crystals suitable for X-ray diffraction by slow evaporation from solutions of 10% CH₂Cl₂-cyclohexane. Palladacycle 18a crystallizes in the orthorhombic space group P2₁2₁2₁, whereas palladacycle 18b crystallizes in the monoclinic space group P2₁. Representations of their X-ray models are shown in Figure 3. The bond lengths and angles involving the atoms coordinated to palladium are nearly identical in the two complexes, with the ligands about palladium being approximately square-planar (Table 2). Bond lengths and angles involving Pd, the coordinated imidazoline nitrogen atom N(1), and the cyclometallated carbon C(1) are similar to those found in other C,N-palladacycles.^8b The Pd–O bond lengths of the acetylacetonate ligand are longer for the bond trans to carbon [Pd–O(2) bond length; 18a: 2.0796(11) Å; 18b: 2.0848(13) Å] than that trans to nitrogen [Pd–O(1) bond length; 18a: 2.0122(11) Å; 18b: 2.0050(14) Å], consistent with a larger trans influence for the anionic naphthyl ligand.

Figure 3.

X-ray models of (S)-PIN-acac complexes 18a and 18b.

Table 2.

Selected Bond Lengths and Bond Angles for Three PIN-acac Complexes and COP-hexafluoroacetylacetonate.

	bond length (Å)				bond angle (deg)		dihedral angle (deg)
complex	Pd–C	Pd–N	Pd–O(1)a	Pd–O(2)b	C–Pd–N	O–Pd–O	τd
18a	1.9800(16)	1.9970(13)	2.0122(11)	2.0769(11)	80.74(6)	92.48(5)	18.11(18)
18b	1.9693(16)	2.0262(16)	2.0050(14)	2.0848(13)	80.97(7)	92.72(5)	24.6(3)
18i	1.964(2)	1.9817(18)	2.0186(16)	2.0791(17)	79.65(8)	91.55(7)	−5.9(4)
(Rp,S)-COP-hfacacc	1.962	2.026	2.020	2.102	80.78	92.76	n/a

^aThe Pd–O(1) bond is trans to the Pd–N bond.

^bThe Pd–O(2) bond is trans to the Pd–C bond.

^cCrystalographic data for the COP hexafluoroacetylacetonate complex (COD 4021070) is available from the Crystallographic Open Database.

^dThe dihedral angle C(1)-C(2)-C(3)-N(1).

As we had anticipated, the imidazoline and naphthalene fragments are twisted out of plane in the X-ray models of PIN-acac complexes 18a and 18b, with the two alkyl substituents on opposite sides of the palladium square plane. The torsion angles around the C(2)–C(3) bond provide a measure of the degree to which the imidazoline substituents would be expected to influence coordination of an alkene π-bond to the Pd(II) center. This angle is 18.1° for diisopropyl complex 18a and slightly larger, 24.6°, for the isopropyl/tert-butyl complex 18b.

Although the imidazoline substituents of PIN-acac complexes 18a and 18b reside on opposite sides of the palladium square plane in the solid state, the configurational stability of the N-isopropyl substituent in solution was less certain (Scheme 3). Inversion at this site, which would require bringing the bulky isopropyl substituent into an unfavorable coplanar orientation with the naphthalene ring, could result in both alkyl substituents residing on the same face of the palladium square plane. The configurational stability of 18a in solution was examined by variable temperature ¹H NMR from −80 °C to 110 °C.²¹ Within this temperature range, the ¹H NMR spectrum of 18a was unchanged. This observation, and DFT calculations (b3-lyp/def2-TZVP) that estimate the barrier to be <15 kcal/mol,^22,23 are consistent with the N(2) nitrogen stereocenter undergoing rapid inversion at room temperature.

Scheme 3.

Nitrogen Inversion of PIN Complexes.

The structural features and catalytic properties of PIN-acac complex 18i in which R² is phenyl and R¹ isopropyl are quite different than those of complexes 18a and 18b. The X-ray model of complex 18i (Figure 4) shows that the geometry around the palladium center is approximately square planar with the bond lengths and angles involving the palladium atom being quite similar to those of complexes 18a and 18b (Table 2). However, the imidazoline substituents reside on the same face of the palladium coordination sphere and N(2) is nearly sp³ hybridized. The overall shape of this complex is quite flat with the torsion angle around the C(2)–C(3) bond being −5.9°.²⁴ The phenyl substituent is twisted perpendicular to the imidazoline ring, thus minimizing destabilizing steric interactions when the naphthalene and imidazoline fragments are oriented nearly coplanar.²²

Figure 4.

X-ray structure of (S)-PIN-acac complex 18i.

Catalytic Performance of PIN-acac Complexes

(S)-PIN-acac complexes 18 were examined as enantioselective catalysts for the [3,3]-rearrangement and allylic substitution reactions of allylic trichloroacetimidates. Initial investigations were carried out with complex 18a, which was found to be a poor catalyst for the rearrangement of (E)-allylic imidate 19. For example, after 24 h allylic trichloroacetamide 20 was formed in only 10% yield when 19 was exposed to 10 mol % of 18a (1.0 M substrate concentration) at 38 °C, with the bulk of the starting allylic imidate remaining unchanged (eq 1)

(1)

In contrast, PIN-acac complexes proved to be kinetically excellent catalysts for the allylic esterification reaction. Initial experiments were again conducted with complex 18a, which at 10 mol % transformed (Z)-allylic trichloroacetimidate 21 cleanly to branched allylic acetate 22 within 8 h at room temperature (eq 2). This reaction proceeded with no detectable formation of rearrangement product 20; however, allylic acetate 22 was produced in only 21% ee. Attempts to improve enantioselectivity by carrying out the reaction at 0 °C resulted only in prolonged reaction times without any improvement in enantioselectivity. In addition, changes in the solvent had only a small influence on catalytic efficiency or enantioselectivity (yield of 22, reaction time at 23 °C, ee): CH₂Cl₂ (95%, 8 h, 21%) ≈ MeCN (92%, 8 h, 20%) > toluene (88%, 10 h, 10%) > Et₂O (94%, 16 h, 20%) ≈ THF (94%, 18 h, 18%).

(2)

These initial experiments indicated that PIN-acac complexes might be useful catalysts for enantioselective allylic esterification of (Z)-allylic trichloroacetimidates if stereoinduction could be improved by modification of the imidazoline substituents. Accordingly, our small library of PIN catalysts was surveyed for the enantioselective conversion of (Z)-allylic trichloroacetimidate 23 to 3-acetoxy-5-phenyl-1-pentene (24) (Table 3).

Table 3.

Survey of (S)-PIN-acac Complexes for the Catalytic Asymmetric Synthesis of Branched Allylic Acetates.

entry	catalyst	R1	R2	time(h)	yield (%)a	ee (%)b
1	18a	i-Pr	i-Pr	18	95	28
2	18b	t-Bu	i-Pr	18	96	34
3	18c	i-Pr	CH2Ph	18	93	30
4	18d	i-Pr	CH(Ph)2	18	90	38
5	18e	i-Pr	Cy	18	95	25
6	18f	i-Pr	t-Bu	24	96	48
7	18g	t-Bu	t-Bu	48	95	57
8c,d	18h	t-Bu	1-adamantyl	18	65	28
9	18i	i-Pr	Ph	18	97	20
10	18j	t-Bu	Ph	18	96	23
11	18k	i-Pr	p-OMeC6H4	18	94	18
12	18l	i-Pr	p-CF3C6H4	18	96	21
13	18m	i-Pr	mesityl	36	90	42
14	18o	i-Pr	Ts	12	90	16
15	18p	i-Pr	C(O)t-Bu	18	91	10
16e	[COP-OAc]2 (5)	---	---	16	88	91

^aDuplicate experiments (±3%).

^bDetermined by GC analysis of duplicate experiments (±2%).

^cComplex 18h was used as a 1:1 mixture of atropisomers.

^dThe remaining mass was imidate 23.

^eCatalyst loading of 5 was 1.5 mol % (3 mol % Pd).

Several trends in the data summarized in Table 3 are apparent. Palladacycles containing bulky alkyl substituents on the imidazoline nitrogen (18a–h) provided 3-acetoxy-1-pentene (24) in excellent yields (70–95%) and moderate enantioselectivities (25–57% ee). Increasing the size of the R¹ substituent from i-Pr to t-Bu translated into only a marginal increase in enantioselectivity (entries 1 and 2), whereas increasing the size of the imidazoline nitrogen substituent R² proved to be more critical to the stereochemical outcome (entries 3–7). The most effective R² substituent was t-Bu, with PIN catalysts 18f and 18g providing acetate 24 in 48% and 57% ee, respectively (entries 6 and 7). Notably, catalyst 18g (R¹ = t-Bu, R² = t-Bu), which is expected to have the largest degree of angular torsion around the C(2)–C(3) bond, provided the highest level of enantioselectivity among the PIN complexes screened (entry 7). However, catalytic rate was somewhat lower with this catalyst, with a reaction time of 48 h being required to give acetate 24 in high yield. It is not surprising that complex 18h which was used as a 1:1 mixture of atropisomers provided acetate 24 in low ee only (entry 8). PIN complexes having N-aryl substituents (entries 9–13) displayed useful catalytic activity, but provided the allylic acetate product in low to moderate ee. The most enantioselective N-aryl PIN variant was 18m (R¹ = t-Bu, R² = Mes), which gave allylic acetate 24 in 90% yield and 42% ee (entry 13). PIN catalysts 18o–p bearing an electron-withdrawing substituent on the imidazoline nitrogen gave product 24 in low ee and exhibited only marginal improvement in reaction rate relative to 18a–m and (entries 14 and 15).

This catalyst survey showed that PIN-acac complexes are useful catalysts for the addition of carboxylic acid nucleophiles to allylic trichloroacetimidates, with catalytic rates being similar to those of COP catalyst 5 (entry 16). Unfortunately, the enantioselectivity achieved with PIN catalysts 18 is significantly lower than that realized with COP catalyst 5.

Computational Modeling

To gain insight into what features of the PIN complex architecture might be modified to increase enantioselectivity of these catalysts, computational modeling was undertaken. The structural relationship between the C,N-palladacycle portions of the PIN and COP catalyst structures and their comparable reactivity suggests that they might operate by a common mechanism. We have recently reported a detailed computational study of the addition of carboxylic acid nucleophiles to (Z)-allylic trichloroacetimidates using COP catalyst 5.^3c The proposed catalytic cycle, which involves antarafacial S_N2′ addition of acetic acid to the double bond, is summarized in Scheme 4.²⁵ On the basis of experimental observations and computational experiments, this earlier study concluded that: 1) oxypalladation (26 → 27) is the rate- and enantio-determining step; 2) bidentate coordination of the allylic trichloroacetimidate to the palladacycle catalyst (e.g., intermediate 26) is favored over other coordination modes; and 3) there is a preference for the alkene π-bond to coordinate cis to the Pd–C σ-bond of the palladacycle.^3c This latter observation is consistent with our mechanistic investigations of the COP-catalyzed [3,3]-rearrangement of (E)-allylic trichloroacetimidates²⁶ and with the strong trans influence observed for d⁸ square-planar Pd(II) complexes.²⁷ The current analysis thus focused on the interactions between the imidate substrate, acetate nucleophile, and C,N-palladacycle framework in the oxypalladation step.

Scheme 4.

Proposed Catalytic Cycle for the Pd(II) Catalyzed Conversion of (Z)-Allylic Trichloroacetimidates (1) to Enantioenriched Allylic Esters (2).

In order to gain a better understanding of diastereoselection induced by the chiral PIN catalyst architecture, the oxypalladation step (Scheme 4, 26 → 27) was studied computationally. Calculations modeled the addition of HOAc to the trichloroacetimidate derivative of (Z)-2-butene-1-ol using complex 18a as a catalyst.^28–31 The previously reported enantiodetermining transition-state structure for the reaction of acetic acid with (Z)-2-butenyl trichloroacetimidate catalyzed by COP complex 5 was used as the starting geometry for calculations of PIN transition-state structures 29–32.^3c The results of this computational study are depicted in Figure 5. In transition-state structures 29 and 30, the reacting allylic C–C π-bond is positioned cis to the Pd–C(1) bond, consistent with the predicted preferred coordination geometry of the imidate (e.g. 26, Scheme 4). Conversely, in transition-state structures 31 and 32, the C–C π-bond is coordinated trans to the palladacycle Pd–C(1) bond. Transition-state structures 29 and 31 would afford the observed major R enantiomer of the allylic acetate product, whereas the transition-state structures 30 and 32 would produce the minor S enantiomer. Of the four transition structures, 29 was found to have both the lowest activation energy (ΔE^‡ = 4.2 kcal/mol) and the lowest relative energy (ΔΔE^‡). Structures 31 and 32 having the C–C π-bond coordinated trans to the carbon of the palladacycle are calculated to be 4.0 and 5.4 kcal/mol higher in energy than the corresponding cis complexes. Transition-state structure 30, which has the imidate nitrogen coordinated trans to the Pd–C(1) bond and leads to the formation of the minor enantiomer, is calculated to be only 0.2 kcal/mol higher in energy than the low energy transition-state structure 29.

Figure 5.

Relative Energies of Four Isomeric Transition-state Structures for Oxypalladation using PIN Complex 18a.

Several additional observations from the computational study warrant further comment. In low energy transition-state structures 29 and 30, nucleophilic attack on the activated C–C double bond takes place next to C(19) of the naphthalene ligand. Thus, the enantiodetermining event is occurring relatively far away from the chiral environment provided by the imidazoline substituents R¹ and R². The small energy difference (ΔΔE^‡ = 0.2 kcal/mol) between the transition-state structures 29 (Re face addition) and 30 (Si face addition) is consistent with the observed low level of enantioselectivity realized with PIN complex 18a. Although variation of R¹ and R² can undoubtedly lead to increased torsion around the C(2)–C(3) bond, which would place these groups in somewhat closer proximity to the reacting C–C π-system, it is doubtful that the N(2) substituent could be positioned in a way to effect a high degree of energy separation between transition-state structures 29 and 30. Our computational analysis suggests that the most useful approach to optimizing the PIN catalyst structure would be by appending functionality at C(19) of the naphthalene backbone.

Conclusion

In summary, a new family of enantiopure C,N-palladacycles (PIN-acac complexes 18) were developed and evaluated as asymmetric catalysts for the reaction of allylic trichloroacetimidates with external non-metal bound nucleophiles. These air- and moisture-stable complexes were formed in good overall yield in three steps from 2-iodo-1-naphthoic acid (15) and β-amino alcohols. Three PIN complexes were characterized by single-crystal X-ray analysis. As anticipated, the naphthalene and imidazoline rings of PIN-acac complexes 18a and 18b were canted significantly from planarity and projected the imidazoline substituents R¹ and R² on opposite faces of the palladium square plane. Although these PIN palladacycles displayed useful levels of catalytic activity for the allylic substitution of prochiral allylic trichloroacetimidates with carboxylic acid nucleophiles and exclusively provided branched allylic esters in high yield, enantioselectivities were low to moderate. Nonetheless, the accessible and easily varied naphthalene-imidazoline ligands described here may be useful for the synthesis of related enantiopure axial chiral cyclometalated complexes that might find future applications in asymmetric catalysis.

The experimental and computational results presented here reinforce our previous conclusions with COP catalysts that the alkene π-bond of an allylic imidate substrate is preferentially coordinated cis to the carbon ligand of the palladacycle^3c,26 with attack of an external nucleophile occurring from the least-sterically hindered face in this quadrant. This study further suggests that optimizing steric influence in the vicinity of the carbon ligand of a chiral C,N-palladacycle, rather than near the nitrogen heterocycle, is the direction to pursue in future development of improved enantioselective catalysts in this area.

Experimental Section

General Procedure for the Synthesis of β-Hydroxyamides

Preparation of (S)-N-(1-Hydroxy-3-methylbutan-2-yl)-2-iodonaphthalene-1-carboxamide (16a)

Carboxylic acid 15 (6.00 g, 20.1 mmol) was suspended in CH₂Cl₂ (100 mL) and rapidly stirred. Oxalyl chloride (3.6 mL, 40 mmol) and a catalytic amount of DMF (~ 0.02 mL) were added dropwise at rt (Caution! Gas evolved), and the resultant suspension was stirred until it became homogeneous (approx 2 h). The reaction mixture was then concentrated under reduced pressure, and the resulting yellow residue was dissolved in CH₂Cl₂ (50 mL) and transferred by cannula to a solution of (S)-valinol (2.07 g, 21.1 mmol) and Et₃N (8.5 mL, 61 mmol) in CH₂Cl₂ (150 mL) maintained at 0 °C. The reaction mixture was allowed to warm to rt over 16 h. The reaction then was diluted with CH₂Cl₂ (150 mL) and washed successively with 1 M HCl (2 x 250 mL), 1 M NaOH (2 x 250 mL), and brine. The organic layer was dried with anhydrous MgSO₄, filtered, and concentrated under reduced pressure. The resulting brown residue was recrystallized from CHCl₃ to afford 16a (6.55 g, 17.1 mmol, 85%) as a tan solid: mp 132–136 °C; [a]_D²⁴ = +11.2, [a]₅₇₇²⁴ = +12.1, [a]₅₄₆²⁴= +12.9, [a]₄₃₅²⁴ = +15.3, [a]₄₀₅²⁴= +17.4, (c = 0.2, CHCl₃); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.87 (q, J = 3.0, 1H), 7.80–7.76 (m, 2H), 7.52–7.47 (m, 3H), 6.07 (d, J = 8.3, 1H), 4.07–4.01 (m, 1H), 3.93–3.91 (m, 1H), 3.81–3.79 (m, 1H), 2.50 (br s, 1H), 1.97 (octet, J = 6.8, 1H), 1.04 (d, J = 6.8, 3H), 1.03 (d, J = 6.8, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 170.3 (C), 141.0 (C), 135.2 (CH), 132.5 (C), 131.2 (C), 130.4 (CH), 128.2 (CH), 127.9 (CH), 127.0 (CH), 125.2 (CH), 91.2 (C), 63.6 (CH₂), 57.8 (CH), 29.0 (CH), 19.8 (CH₃), 19.3 (CH₃); IR (thin film) 3406, 2960, 1732, 1076 cm⁻¹; HRMS (ESI) m/z, 406.0271 (406.0280 calcd for C₁₆H₁₈NINaO₂, (M + Na)⁺).

(S)-N-(1-hydroxy-3,3-dimethyl-2-yl)-2-iodonaphthalene-1-carboxamide (16b)

Prepared following the general procedure used to prepare 16a. The crude residue was recrystallized from CHCl₃ to afford amide 16b (2.48 g, 6.24 mmol, 93%) as a light brown solid: mp: 71–75 °C; [a]_D²³ = −10.3, [a]₅₇₇²³ = −11.2, [a]₅₄₆²³= −13.3, [a]₄₃₅²³= −24.6, (c = 0.83, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 8.01 (br s, 1H), 7.83 (d, J = 8.5, 2H), 7.58 (d, J = 8.4, 1H); 7.55–7.53 (m, 2H), 6.02 (d, J = 8.2, 1H), 4.22 (ddd, J = 9.8, 6.8, 3.0, 1H), 4.10 (dd, J = 11.0, 2.4, 1H), 5.38 (m, 1H), 2.48 (br s, 1H), 1.10 (s, 9H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 170.6 (C), 141.2 (C), 135.2 (CH), 132.5 (C), 131.3 (C), 130.4 (CH), 128.2 (CH), 127.9 (CH), 127.0 (CH), 125.3 (CH), 91.2 (C) 63.1 (CH), 60.5 (CH₂), 33.8 (C), 27.5 (CH₃); IR (thin film) 3395, 3278, 3058, 2960, 1746 cm⁻¹; HRMS (ESI) m/z, 398.0625 (398.0619 calcd for C₁₇H₂₁INO₂, (M + H)⁺).

General Procedure for the Synthesis of Imidazolines.³²

Preparation of (S)-1,4-Diisopropyl-2-(naphthalene-1-yl)-4,5-dihydro-1H-imidazole (12)

Amide 10 (0.518 g, 2.00 mmol) was dissolved in freshly distilled SOCl₂ (0.73 mL, 10 mmol) and the solution was warmed to 85 °C. After 5 h, the reaction was cooled to rt, diluted with toluene (10 mL), and concentrated under reduced pressure. The unpurified chloroalkylimidoyl chloride was dissolved in CH₂Cl₂ (5 mL) and any insoluble impurities were removed by filtration. The resulting solution was treated with a solution of Et₃N (0.83 mL, 6.0 mmol) and isopropylamine (0.18 mL, 2.2 mmol) in CH₂Cl₂ (2 mL) at rt. The reaction mixture was maintained at rt for 12 h, then stirred with 2 M NaOH (20 mL) for 30 min. The organic layer was separated and the aqueous layer was extracted with EtOAc (3 x 10 mL). The combined organics were dried with anhydrous MgSO₄, filtered, and concentrated under reduced pressure. The resulting yellow residue was purified by column chromatography (silica gel, 8:1:1 hexanes/Et₂O/Et₃N)³³ to afford imidazoline 12 (0.392 g, 1.40 mmol, 70%) as a colorless foam: [a]_D²⁴ = −37.0, [a]₅₇₇²⁴ = −38.9, [a]₅₄₆²⁴ = −42.6, [a]₄₃₅²⁴= −72.4, [a]₄₀₅²⁴= −85.7, (c = 1.0, CHCl₃); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 8.34 (d, J = 8.3, 1H), 7.95 (d, J = 8.0, 1H), 7.88 (d, J = 7.9, 1H), 7.65 (d, J = 7.9, 1H), 7.56–7.54 (m, 2H), 7.48 (dd, J = 8.0, 7.3, 1H), 6.18 (d, J = 8.7, 1H), 4.11–4.08 (m, 1H), 3.94–3.91 (m, 1H), 3.86 (m, 1H), 2.51 (br s, 1H), 2.05 (septet, J = 7.0, 1H), 1.10 (d, J = 6.8, 3H), 1.09 (d, J = 6.8, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 170.6 (C), 134.7 (CH), 133.7 (CH), 130.6 (C), 130.1 (CH), 128.3 (CH), 127.2 (C), 126.5 (CH), 125.4 (CH), 124.8 (C), 124.7 (CH), 64.0 (CH), 57.6 (CH₂), 29.2 (CH), 19.7 (CH₃), 19.0 (CH₃); IR (thin film) 3296, 3060, 3051, 2931, 1623 cm⁻¹; HRMS (ESI) m/z, 281.2021 (281.2018 calcd for C₁₉H₂₅N₂, (M + H)⁺).

The following compounds were prepared in identical fashion from 16a or 16b employing 1.1 equiv the appropriate alkyl/aryl primary amine.

(S)-2-(2-iodonaphthalen-1-yl)-1,4-diisopropyl-4,5-dihydro-1H-imidazole (17a)

Following the general procedure, the crude residue was purified by column chromatography (silica gel, 6:3:1 hexane/Et₂O/Et₃N) to afford 17a (0.463 g, 1.14 mmol, 76%) as a yellow foam. The product was characterized as a 1:1 mixture of atropisomers: ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.90–7.89 (m, 2H), 7.87 (dd, J = 8.7, 1.7, 2H), 7.80–7.78 (m, 2H), 7.56 (d, J = 8.7, 2H), 7.52–7.47 (m, 4H), 4.13 (dt, J = 11.0, 6.3, 1H), 4.05 (dt, J = 10.6, 3.1, 1H), 3.65–3.60 (m, 2H), 3.35–3.31 (m, 2H), 3.18 (sextet, J = 6.7, 2H), 2.00 (sextet, J = 6.5, 2H), 1.24 (d, J = 6.6, 3H), 1.23 (d, J = 6.6, 3H), 1.16 (d, J = 6.7, 3H), 1.13 (d, J = 6.7, 3H), 1.08 (d, J = 6.6, 3H), 1.07 (d, J = 6.7, 3H), 0.96 (d, J = 6.6, 3H), 0.93 (d, J = 6.6, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 163.30 (C), 163.26 (C), 135.7 (CH), 135.6 (CH), 132.9 (C), 132.7 (C), 132.6 (C), 132.5 (C), 130.2 (CH), 128.1 (CH), 128.0 (CH), 127.3 (CH), 127.2 (CH), 126.7 (CH), 126.6 (CH), 125.8 (CH), 96.3 (C), 95.9 (C), 71.2 (CH), 70.9 (CH), 46.34 (CH), 46.31 (CH), 45.6 (CH₂), 45.1 (CH₂), 33.8 (CH), 33.7 (CH), 21.7 (CH₃), 21.5 (CH₃), 20.8 (CH₃), 20.7 (CH₃), 19.9 (CH₃), 19.4 (CH₃), 18.9 (CH₃); IR (thin film) 3040, 2952, 1712, 1624, cm⁻¹; HRMS (ESI) m/z, 429.0811 (429.0804 calcd for C₁₉H₂₃IN₂Na, (M + Na)⁺).

(S)-4-tert-butyl-2-(2-iodonaphthalen-1-yl)-1-isopropyl-4,5-dihydro-1H-imidazole (17b)

Following the general procedure, the crude residue was purified by column chromatography (silica gel, 7:2:1 hexane/Et₂O/Et₃N) to afford 17b (0.379 g, 0.902 mmol, 43%) as an orange film. The product was characterized as a 1:1 mixture of atropisomers: ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.92–7.89 (m, 2H), 7.86 (app dd, J = 8.7, 2.8, 2H), 7.80–7.77 (m, 2H), 7.55 (app dd, J = 8.7, 2.6, 2H), 7.50–7.46 (m, 4H), 4.08 (t, J = 11.4, 2H), 3.54 (dt, J = 9.5, 4.3, 2H), 3.42–3.36 (m, 2H), 3.19 (sextet, J = 6.4, 2H), 1.22–1.21 (m, 6H), 1.10 (s, 9H), 1.09 (s, 9H), 0.96 (d, J = 6.6, 3H), 0.92 (d, J = 6.5, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 163.43 (C), 163.41 (C), 135.82 (C), 135.80 (C), 135.7 (CH), 135.6 (CH), 133.1 (C), 132.8 (C), 132.6 (C), 132.5 (C), 130.14 (CH), 130.11 (CH), 128.13 (CH), 128.0 (CH), 127.3 (CH), 127.1 (CH), 126.7 (CH), 126.6 (CH), 126.0 (CH), 125.9 (CH), 96.6 (C), 95.6 (C), 74.88 (CH), 74.86 (CH), 46.42 (CH), 46.36 (CH), 43.7 (CH₂), 43.5 (CH₂), 34.7 (C), 34.3 (C), 27.2 (CH₃), 26.6 (CH₃), 22.1 (CH₃), 21.7 (CH₃), 20.5 (CH₃), 20.4 (CH₃); IR (thin film) 3051, 2990, 1661, 1235, cm⁻¹; HRMS (ESI) m/z, 443.0970 (443.0960 calcd for C₂₀H₂₅IN₂Na, (M + Na)⁺)

(S)-1-tert-butyl-2-(2-iodonaphthalen-1-yl)-4-isopropyl-4,5-dihydro-1H-imidazole (17f)

Following the general procedure, the crude residue was purified by column chromatography (silica gel, 8:1:1 hexane/Et₂O/Et₃N) to afford 17f (0.274 g, 0.652 mmol, 71%) as a yellow paste. The product was characterized as a 1:1 mixture of atropisomers: ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.88–7.86 (m, 2H), 7.80 (d, J = 5.3, 1H), 7.78 (d, J = 5.4, 1H), 7.75–7.71 (m, 2H), 7.49–7.43 (m, 6H), 3.96 (dt, J = 11.0, 6.3, 1H), 3.86 (dt, J = 10.8, 7.3, 1H), 3.74 (app ddd, J = 11.0, 9.3, 6.3, 2H), 3.45 (app q, J = 9.3, 2H), 1.97 (m, 2H), 1.12 (d, J = 6.7, 3H), 1.07 (d, J = 6.8, 3H), 1.04–1.02 (m, 24H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 162.8 (C), 162.6 (C), 139.0 (C), 138.98 (C), 135.7 (CH), 135.6 (CH), 133.5 (C), 132.8 (C), 132.5 (C), 132.4 (C), 129.6 (CH), 128.0 (CH), 127.9 (CH), 127.0 (CH), 126.9 (CH), 126.7 (CH), 126.4 (CH), 97.8 (C), 96.7 (C), 69.9 (CH), 69.7 (CH), 54.5 (C), 50.6 (CH₂), 50.0 (CH₂), 33.6 (CH), 33.4 (CH), 29.4 (CH₃), 29.2 (CH₃), 19.9 (CH₃), 19.5 (CH₃), 19.4 (CH₃), 18.8 (CH₃); IR (thin film) 3059, 2980, 1655, 1241 cm⁻¹; HRMS (ESI) m/z, 421.1143 (421.1141 calcd for C₂₀H₂₆IN₂, (M + H)⁺).

(S)-1,4-di-tert-butyl-2-(2-iodonaphthalen-1-yl)-4,5-dihydro-1H-imidazole (17g)

Following the general procedure, the crude residue was purified by column chromatography (silica gel, 8:2:1 hexane/Et₂O/Et₃N) to afford 17g (0.504 g, 1.16 mmol, 75%) as a yellow film. The product was characterized as a 1:1 mixture of atropisomers: ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.89–7.88 (m, 2H), 7.79 (t, J = 8.5, 2H), 7.72 (m, 1H), 7.71 (d, J = 9.0, 1H), 7.49–7.43 (m, 6H), 3.93 (t, J = 11.8, 2H), 3.68 (t, J = 9.3, 1H), 3.66 (t, J = 8.7, 1H), 3.51–3.47 (m, 2H), 1.10 (s, 9H), 1.04 (s, 9H), 1.03 (s, 9H), 1.01 (s, 9H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 163.0 (C), 162.9 (C), 139.1 (C), 139.1 (C,), 135.9 (CH), 135.6 (CH), 133.9 (C,), 132.7 (C), 132.6 (C), 132.4 (C), 129.7 (CH), 129.6 (CH), 128.1 (CH), 127.8 (CH), 127.0 (CH), 126.84 (CH), 126.80 (CH), 126.7 (CH), 126.5 (CH), 98.3 (C), 96.1 (C), 73.5 (CH), 73.5 (CH), 54.7 (C), 54.6 (C), 48.8 (CH₂), 48.6 (CH₂), 34.7 (C), 34.3 (C), 29.5 (CH₃), 29.1 (CH₃), 27.2 (CH₃), 26.6 (CH₃); IR (thin film) 2989, 2908, 1648, 1211, 1023 cm⁻¹; HRMS (ESI) m/z, 435.1294 (435.1297 calcd for C₂₁H₂₈IN₂, (M + H)⁺).

(S)-4-tert-butyl-2-(2-iodonaphthalen-1-yl)-1-(1-adamantyl)-4,5-dihydro-1H-imidazole (17h)

Following the general procedure, the crude residue was purified by column chromatography (silica gel, 8:1:1 hexane/Et₂O/Et₃N) to afford 17h (0.155 g, 0.302 mmol, 60%) as a colorless foam. The product was characterized as a 1:1 mixture of atropisomers: ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.93–7.89 (m, 2H), 7.81 (t, J = 8.6, 2H), 7.79–7.76 (m, 2H), 7.50–7.46 (m, 6H), 3.93 (t, J = 11.8, 1H), 3.92 (t, J = 11.6, 1H), 3.75–3.69 (m, 2H), 3.60–3.56 (m, 2H), 1.85–1.84 (m, 6H), 1.73–1.67 (m, 12H), 1.50–1.38 (m, 8H), 1.09 (s, 9H), 1.06 (s, 9H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 162.48 (C), 162.45, 139.7 (C), 135.8 (CH), 135.6 (CH), 134.0 (C), 132.9 (C), 132.6 (C), 132.4 (C), 129.53 (CH), 129.50 (CH), 128.0 (CH), 127.8 (CH), 127.0 (CH), 126.9 (CH), 126.82 (CH), 126.80 (CH), 126.41 (CH), 126.39 (CH), 98.2 (C), 96.2 (C), 73.30 (CH), 73.28 (CH), 55.74 (C), 55.69 (C), 41.5 (CH₂), 41.1 (CH₂), 36.07 (CH₂), 36.06 (CH₂), 34.8 (C), 34.4 (C), 27.2 (CH₃), 26.6 (CH₃); IR (thin film) 3056, 2994, 2852, 1590, 1260, 1242 cm⁻¹; HRMS (ESI) m/z, 513.1748 (513.1767 calcd for C₂₇H₃₄IN₂, (M + H)⁺).

(S)-2-(2-iodonaphthalen-1-yl)-4-isopropyl-1-phenyl-4,5-dihydro-1H-imidazole (17i)

Following the general procedure, the crude residue was purified by column chromatography (silica gel, 8:1:1 hexane/Et₂O/Et₃N) to afford 17i (0.355 g, 0.806 mmol, 69%) as an orange foam. The product was characterized as a ~1.4:1 mixture of atropisomers. The compound contains some minor impurities, but was used directly in the next step, as decomposition was observed during purification attempts: ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 8.02 (dd, J = 8.0, 2.8, 1H), 7.83–7.80 (m, 5H), 7.79 (d, J = 8.3, 1H), 7.73–7.53 (m, 4H), 7.01–6.96 (m, 3H), 6.83–6.80 (m, 3H), 6.62 (d, J = 8.4, 1H), 6.57–6.55 (m, 3H), 4.26–4.20 (m, 2H), 4.05–4.00 (m, 2H), 3.91 (d, J = 6.2, 1H), 2.17 (septet, J = 6.8, 1H), 2.12 (septet, J = 6.5, 1H), 1.16 (d, J = 6.7, 3H), 1.15 (d, J = 6.7, 3H), 1.12 (d, J = 6.5, 3H), 1.11 (d, J = 6.5, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 160.3 (C), 160.2 (C), 140.4 (C), 140.2 (C), 135.7 (CH), 135.6 (CH), 135.5 (C), 135.2 (C), 133.6 (C), 132.9 (CH), 132.7 (CH), 132.5 (C), 130.1 (CH), 129.5 (CH), 128.7 (CH), 128.1 (CH), 128.0 (CH), 127.9 (CH), 127.3 (CH), 127.2 (CH), 126.7 (CH), 126.6 (CH), 125.9 (CH), 125.3 (CH, 122.3 (CH), 122.2 (CH), 118.3 (CH), 118.2 (CH), 97.0 (C), 96.4 (C), 70.8 (CH), 70.6 (CH), 53.4 (CH₂), 53.1 (CH₂), 33.2 (CH), 33.0 (CH), 20.0 (CH₃), 19.4 (CH₃), 19.2 (CH₃), 19.1 (CH₃); IR (thin film) 3061, 2995, 1675, 1238 cm⁻¹; HRMS (ESI) m/z, 463.0650 (463.0647 calcd for C₂₂H₂₁IN₂Na, (M + Na)⁺).

(S)-2-(2-iodonaphthalen-1-yl)-4-isopropyl-4,5-dihydro-1H-imidazole (17n)

The general procedure was followed with the exception that the unpurified chloroalkylimidoyl chloride was suspended in a saturated solution of NH₃ in CHCl₃ (15 mL) at rt. The crude residue was purified by column chromatography (silica gel, 6:3:1 hexane/Et₂O/Et₃N) to afford 17n (0.379 g, 1.04 mmol, 90%) as a colorless solid: mp = 159–163 °C; [a]_D²⁴ = −12.8, [a]₅₇₇²⁴ = −13.6, [a]₅₄₆²⁴ = −15.9, [a]₄₃₅²⁴ = −30.4, [a]₄₀₅²⁴ = −36.9, (c = 1.0, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 330 °K) δ 7.96 (m, 1H), 7.84 (d, J = 8.6, 1 H), 7.87 (m, 1H), 7.55 (d, J = 8.6, 1H), 7.41–7.40 (m, 2H), 4.82 (br s, 1H), 3.97 (m, 2 H), 3.66 (br s, 1H), 2.00–1.99 (m, 1H), 1.10 (d, J = 6.0, 3H), 1.05 (d, J = 6.3, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 164.3 (C), 136.0 (C), 135.2, (CH), 132.6 (C), 132.5 (C), 130.4 (CH), 127.9 (CH), 127.5 (CH), 126.8 (CH), 125.8 (CH), 94.8 (C), 33.1 (CH), 29.8 (CH₂), 19.7 (CH₃), 19.1 (CH₃); IR (thin film) 3142, 3064, 2955, 2869, 1610, 1504 cm⁻¹; HRMS (ESI) m/z, 365.0511 (365.0515 calcd for C₁₆H₁₈IN₂, (M + H)⁺).

Preparation of (S)-2-(2-iodonaphthalen-1-yl)-4-isopropyl-1-tosyl-4,5-dihydro-1H-imidazole (17o)

A solution of 17n (0.150 g, 0.441 mmol) and Et₃N (0.18 mL, 1.3 mmol) in CH₂Cl₂ (2 mL) was cooled to 0 °C, a solution of TsCl (0.100 g, 0.534 mmol) in CH₂Cl₂ (1 mL) was added, and the reaction was allowed to warm to rt. After 16 h, the reaction was diluted with CH₂Cl₂ (25 mL) and washed with saturated aqueous NaHCO₃ (2 x 20 mL). The organic layer was separated, dried with anhydrous MgSO₄, filtered, and concentrated under reduced pressure to afford 17o (0.217 g, 0.419 mmol, 95%) as a colorless foam. No purification was necessary. The product was characterized as a 1:1 mixture of atropisomers: ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.87–7.79 (m, 4H), 7.63 (d, J = 8.7, 2H), 7.57–7.54 (m, 2H), 7.50–7.46 (m, 2H), 7.35 (appr t, J = 7.6, 2 H), 7.30–7.25 (m, 4H), 7.19–7.14 (m, 1H), 7.08 (appr d, J = 8.0, 3H), 4.23–4.09 (m, 4H), 3.89–3.85 (m, 2H), 2.38 (s, 3H), 2.37 (s, 3H), 2.06 (septet, J = 6.7, 1H), 2.02 (septet, J = 6.9, 1H), 1.16 (d, J = 6.7, 3H), 1.16–1.05 (m, 9H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 156.1 (C), 156.0 (C), 144.4 (C), 144.3 (C), 135.5 (C), 135.3 (C), 135.2 (CH), 135.2 (CH), 133.8 (C), 133.7 (C), 133.0 (C), 132.8 (C), 132.2 (C), 132.2 (C), 130.9 (CH), 129.5 (CH), 129.5 (CH), 129.1 (CH), 128.3 (CH), 128.2 (CH), 128.1 (CH), 127.8 (CH), 127.7 (CH), 127.2 (CH), 126.5 (CH), 126.5 (CH), 125.6 (CH), 125.5 (CH), 97.6 (C), 97.2 (C), 71.7 (CH), 71.4 (CH), 51.4 (CH₂), 50.9 (CH₂), 33.2 (CH), 33.0 (CH), 21.7 (CH₃), 21.6 (CH₃), 19.7 (CH₃), 19.4 (CH₃), 19.1 (CH₃), 18.9 (CH₃); IR (thin film) 3058, 2959, 2872, 1640, 1360 cm⁻¹; HRMS (ESI) m/z, 519.0610 (519.0605 calcd for C₂₃H₂₄IN₂O₂S, (M + H)⁺).

(S)-1-(2-(2-iodonaphthalen-1-yl)-4-isopropyl-4,5-1H-imidazol-1-yl)-2,2-dimethylpropan-1-one (17p)

Prepared by the procedure used to prepare 17o using 1.1 equiv of pivaloyl chloride as the electrophile. The crude residue was purified by column chromatography (9:1:1 hexane/Et₂O/Et₃N) to furnish 17p (0.182 g, 0.406 mmol, 92%) as a yellow foam. The product was characterized as a 1:1 mixture of atropisomers: ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.80–7.77 (m, 4H), 7.75–7.73 (m, 2H), 7.53 (d, J = 8.6, 2H), 7.48–7.46 (m, 2H), 4.33–4.21 (m, 4H), 4.07 (dd, J = 8.6, 5.6, 1H), 3.94 (t, J = 9.4, 1H), 2.14–2.06 (m, 2H), 1.31–1.30 (m, 18H), 1.81 (app t, J = 6.9, 6H), 1.14–1.20 (m, 6H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 174.8 (C), 174.7 (C), 160.8 (C), 160.7 (C), 137.9 (C), 137.7 (C), 135.0 (CH), 134.9 (CH), 132.7 (C), 132.6 (C), 132.5 (C), 132.4 (C), 129.7 (CH), 129.7 (CH), 128.4 (CH), 128.3 (CH), 127.3 (CH), 127.2 (CH), 126.4 (CH), 126.3 (CH), 125.7 (CH), 125.4 (CH), 94.1 (C), 93.5 (C), 73.6 (CH), 73.4 (CH), 50.7 (CH₂), 50.2 (CH₂), 40.4 (C), 33.0 (CH), 32.7 (CH), 27.6 (CH₃), 27.5 (CH₃), 19.7 (CH₃), 19.3 (CH₃), 19.2 (CH₃); IR (thin film) 3056, 2960, 2872, 1668, 1334 cm⁻¹; HRMS (ESI) m/z, 449.1089 (449.1092 calcd for C₂₁H₂₆IN₂O, (M + H)⁺).

General Procedure for the Synthesis of (S)-PIN-acac Complexes from Imidazolines 17

Preparation of Acetylacetonato{(S)-2-[2-(1-isopropyl)-4-isopropyl)-4,5-dihydro-1H-imidizyl]-naphthyl-C,N}palladium(II) (18a)

Imidazoline 17a (0.406 g, 1.00 mmol) and Pd₂dba₃ (0.458 g, 0.500 mmol) were dissolved in toluene (3 mL), and the resulting dark red solution was heated to 120 °C. After 12 h, the reaction mixture was concentrated under reduced pressure to furnish a labile mixture of iodine-bridged palladacycle dimers a dark brown residue.

The dark solid prepared above was dissolved in CH₂Cl₂ (5 mL) and silver acetylacetonate (0.217 g, 1.05 mmol) was added in a single portion. The resulting black suspension was stirred at rt for 4 h, then filtered through a pad of Celite^®. The filter cake was washed with CH₂Cl₂ (50 mL) and the filtrate was concentrated to afford a yellow residue. The crude product was purified by column chromatography (silica gel, 10:1 hexanes:acetone) to afford palladacycle 18a (0.339 g, 0.70 mmol, 70%) as a yellow solid: mp = 150–152 °C; [α]_D²⁴+22.7, [α]₅₇₇²⁴+23.1, [α]₅₄₆²⁴+24.2, [α]₄₃₅²⁴+25.5, [α]₄₃₅²⁴ +28.1 (c = 0.2, CHCl₃); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.85 (d, J = 8.4, 2H), 7.77 (d, J = 8.0, 1H), 7.63 (d, J = 8.4, 1H), 7.39 (t, J = 7.4, 1H), 7.30 (t, J = 7.4, 1H), 5.35 (s, 1H), 4.34 (septet, J = 6.6, 1H), 4.12 (ddd, J = 10.5, 8.0, 5.1, 1H), 3.59 (t, J = 10.5, 1H), 3.36 (dd, J = 10.5, 8.0, 1H), 2.50–2.49 (m, 1H), 2.07 (s, 3H), 1.95 (s, 3H), 1.25 (d, J = 6.7, 3H), 1.01 (d, J = 6.7, 3H), 0.95 (d, J = 7.0, 3H), 0.91 (d, J = 7.0, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.0 (C), 186.1 (C), 176.8 (C), 156.0 (C), 132.0 (C), 129.7 (C), 129.6 (C), 129.1 (CH), 129.0 (CH), 128.8 (CH), 125.7 (CH), 123.6 (CH), 123.5 (CH), 100.3 (CH), 67.5 (CH), 50.5 (CH₂), 43.9 (CH), 30.9 (CH), 29.8 (CH₃), 27.9 (CH₃), 27.7 (CH₃), 21.0 (CH₃), 18.9 (CH₃), 18.7 (CH₃), 15.8 (CH₃); IR (thin film) 3053, 2960, 1572, 1514, 1263 cm⁻¹; HRMS (ESI) m/z, 507.1242 (507.1249 calcd for C₂₄H₃₀N₂O₂PdNa, (M + Na)⁺); Anal. calcd for C₂₄H₃₀N₂O₂Pd: C, 59.44; H, 6.24; N, 5.78. Found: C, 59.29; H, 6.22; N, 5.76.

The following compounds were prepared in identical fashion from the corresponding imidazoline.

Acetylacetonato{(S)-2-[2-(1-isopropyl)-4-tert-butyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18b)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 10:1 hexanes:acetone) to afford palladacycle 18b (0.220 g, 0.441 mmol, 68%) as a yellow solid: mp = 123–126 °C; [α]_D²⁴ −22.6, [α]₅₇₇²⁴ −22.9, [α]₅₄₆²⁴ −24.7, [α]₄₃₅²⁴ −30.1, [α]₄₃₅²⁴ −36.3 (c = 1.0, CHCl₃); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.86 (d, J = 8.4, 1H), 7.81 (d, J = 8.1, 1H), 7.72 (d, J = 8.4, 1H), 7.65 (d, J = 8.1, 1H), 7.41 (dt, J = 6.8, 1.2, 1H), 7.35 (dt, J = 6.8, 1.2, 1H), 5.38 (s, 1H), 4.47 (septet, J = 6.6, 1H), 3.84 (dd, J = 10.6, 3.8, 1H), 3.73 (t, J = 10.6, 1H), 3.49 (dd, J = 10.6, 3.8, 1H), 2.10 (s, 3H), 2.00 (s, 1H), 2.06 (s, 3H), 1.34 (d, J = 6.6, 3H), 1.01 (s, 9H), 0.99 (d, J = 6.6, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 187.9 (C), 185.9 (C), 176.5 (C), 153.6 (C), 132.0 (C), 130.2 (C), 129.4 (C), 129.2 (CH), 128.6 (CH), 128.5 (CH), 125.7 (CH), 123.7 (CH), 123.5 (CH), 100.2 (CH), 69.6 (CH), 49.7 (CH), 45.3 (CH₂), 36.1 (C), 28.0 (CH₃), 27.6 (CH₃), 25.7 (CH₃), 21.1 (CH₃), 18.3 (CH₃); IR (thin film) 3111, 2901, 2780, 1761, 1654, 1031 cm⁻¹; HRMS (ESI) m/z, 521.1405 (521.1406 calcd for C₂₅H₃₂N₂O₂PdNa, (M + Na)⁺). Anal. calcd for C₂₅H₃₂N₂O₂Pd: C, 60.18; H, 6.46; N, 5.61. Found: C, 59.98; H, 6.48; N, 5.59.

Acetylacetonato{(S)-2-[2-(1-tert-butyl)-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18f)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 9:1 hexanes:acetone) to afford palladacycle 18f (0.230 g, 0.461 mmol, 68%) as a yellow solid: mp 148–151 °C; [α]_D²⁴ +43.7, [α]₅₇₇²⁴ +44.1, [α]₅₄₆²⁴ +44.8, [α]₄₃₅²⁴ +47.5, [α]₄₃₅²⁴ +51.2 (c = 0.1, CHCl₃); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 8.64 (d, J = 8.4, 1H), 7.99 (d, J = 8.4, 1H), 7.79 (d, J = 8.2, 1H), 7.66 (d, J = 8.4, 1H), 7.41 (t, J = 8.2, 1H), 7.35 (d, J = 8.2, 1H), 5.54 (s, 1H), 4.20 (ddd, J = 12.1, 7.4, 4.8, 1H), 3.67 (dd, J = 12.7, 7.5, 1H), 3.42 (t, J = 12.1, 1H), 2.98–2.96 (m, 1H), 2.12 (s, 3H), 2.04 (s, 3H), 1.35 (s, 9H), 1.01 (d, J = 7.1, 3H), 0.89 (d, J = 7.1, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.1 (C), 186.0 (C), 179.7 (C), 154.8 (C), 133.7 (C), 132.0 (C), 131.7 (C), 128.6 (CH), 128.5 (CH), 128.4 (CH), 125.4 (CH), 124.7 (CH), 123.5 (CH), 100.3 (CH), 67.9 (CH), 61.6 (CH₂), 47.4 (C), 28.9 (CH₃), 28.4 (CH), 27.9 (CH₃), 27.7 (CH₃), 19.1 (CH₃), 15.3 (CH₃); IR (thin film) 3051, 2968, 2933, 1729, 1582, 1264 cm⁻¹; HRMS (ESI) m/z, 499.1582 (499.1578 calcd for C₂₅H₃₃N₂O₂Pd, (M + H)⁺); Anal. calcd for C₂₅H₃₂N₂O₂Pd: C, 60.18; H, 6.46; N, 5.61. Found: C, 60.09; H, 6.47; N, 5.60.

Acetylacetonato{(S)-2-[2-(1-1-adamantyl)-4-tert-butyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18h)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 15:1 hexanes:acetone) to afford palladacycle 18h (0.186 g, 0.315 mmol, 63%) as a yellow solid. The resulting compound was an inseparable 1:1 mixture of isomers: mp = 89–92 °C; ¹H NMR (500 MHz, toluene-d₈, 298 °K) δ 9.09 (d, J = 8.4, 1H), 8.51 (d, J = 8.4, 1H), 8.23 (d, J = 8.3, 1H), 8.13 (d, J = 8.5, 1H), 7.61 (d, J = 8.2, 1H), 7.54 (d, J = 8.3, 1H), 7.53 (d, J = 8.4, 1H), 7.52 (d, J = 8.3, 1H), 7.47 (t, J = 7.0, 1H), 7.39–7.36 (m, 2H), 7.20 (m, 1H), 5.24 (s, 1H), 5.21 (s, 1H), 4.01 (dd, J = 10.1, 7.0, 1H), 3.95 (t, J = 9.3, 1H), 3.86 (dd, J = 13.0, 10.3, 1H), 3.77 (dd, J = 13.0, 7.0, 1H), 3.51 (app t, J = 11.4, 1H), 3.12 (dd, J = 11.5, 8.3, 1H), 2.16–2.06 (m, 8H), 1.98–1.95 (m, 2H), 1.94–1.91 (m, 4H), 1.90–1.84 (m, 13H), 1.78–1.74 (m, 2H), 1.73 (s, 1H), 1.67 (s, 3H), 1.60 (s, 3H), 1.00 9 (s, 9H), 0.99 (s, 9H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.1 (C), 188.0 (C), 185.9 (C), 185.7 (C), 177.6 (C), 165.4 (C), 151.5 (C), 147.3 (C). 134.8 (C), 132.4 (C), 131.9 (C), 131.8 (C), 131.7 (C), 129.0 (C), 128.4 (CH), 127.9 (CH), 127.7 (CH), 127.3 (CH), 127.1 (CH), 126. (CH), 125.7 (CH), 125.6 (CH), 124.9 (CH), 124.4 (CH), 124.3 (CH), 123.4 (CH), 78.8 (CH), 70.3 (CH), 67.9 (CH₂), 62.5 (CH₂), 53.0 (CH₂), 47.1 (CH₂), 42.3 (CH₂), 40.5 (CH₂), 35.95 (CH₂), 35.90 (CH₂), 34.4 (C), 30.2 (CH₃), 29.81 (CH₃), 29.77 (C), 28.3 (CH), 28.0 (CH), 27.62 (CH), 27.55 (CH), 26.5 (CH₃), 26.2 (CH₃); IR (thin film) 3092, 2898, 1701, 1594, 1234 cm⁻¹; HRMS (ESI) m/z, 591.2197 (591.2203 calcd for C₃₂H₄₁N₂O₂Pd, (M + H)⁺); Anal. calcd for C₃₂H₄₀N₂O₂Pd: C, 65.02; H, 6.82; N, 4.74. Found: C, 65.18; H, 6.81; N, 4.73.

Acetylacetonato{(S)-2-[2-(1-phenyl)-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18i)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 9:1 hexanes:acetone) to afford palladacycle 18i (0.618 g, 1.19 mmol, 72%) as a yellow solid: mp = 182–185 °C; [α]_D²⁴+96.6, [α]₅₇₇²⁴+102.7, [α]₅₄₆²⁴+109.8, [α]₄₃₅²⁴+114.3 (c = 1.0, CHCl₃); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.91 (d, J = 8.4, 1H), 7.65–7.62 (m, 2H), 7.35 (d, J = 8.6, 1H), 7.11–7.07 (m, 3H), 6.98 (t, J = 7.4, 1H), 6.87 (d, J = 7.4, 2H), 6.82 (t, J = 7.4, 1H), 5.39 (s, 1H), 4.28–4.22 (m, 2H), 3.92 (appr d, J = 5.9, 1H), 2.63–2.59 (m, 1H), 2.10 (s, 3H), 1.98 (s, 3H), 0.92 (d, J = 7.1, 3H), 0.84 (d, J = 7.1, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.0 (C), 186.4 (C), 172.4 (C), 157.9 (C), 144.7 (C), 132.0 (C), 130.0 (CH), 129.5 (C), 129.2 (CH), 128.9 (C), 128.7 (CH), 128.6 (CH), 125.1 (CH), 124.6 (CH), 124.2 (CH), 123.3 (CH), 122.8 (CH), 100.4 (CH), 66.3 (CH), 55.0 (CH₂), 29.3 (CH), 27.9 (CH₃), 27.8 (CH)₃, 18.9 (CH₃), 15.3 (CH₃); IR (thin film) 3434, 3053, 2960, 1572, 1514, 1263 cm⁻¹; HRMS (ESI) m/z, 541.1080 (541.1083 calcd for C₂₇H₂₈N₂O₂PdNa, (M + Na)⁺); Anal. calcd for C₂₇H₂₈N₂O₂Pd: C, 62.49; H, 5.44; N, 5.40. Found: C, 62.33; H, 5.42; N, 5.42.

Acetylacetonato{(S)-2-[2-(1-tosyl)-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18o)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 5:1 hexanes:acetone) to afford palladacycle 18o (0.184 g, 0.309 mmol, 64%) as a orange solid: mp = 162–166 °C; [α]_D²⁴+41.2, [α]₅₇₇²⁴+43.4, [α]₅₄₆²⁴+56.7, [α]₄₃₅²⁴+64.3 (c = 1.1, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 8.62 (d, J = 8.5, 1H), 7.88 (d, J = 8.4, 1H), 7.76 (d, J = 8.1, 1H), 7.74 (d, J = 8.4, 3H), 7.47 (t, J = 8.1, 1H), 7.39–7.36 (m, 3H), 5.40 (s, 1H), 3.95 (dd, J = 12.6, 7.7, 1H), 3.18–3.13 (m, 1H), 2.50–2.47 (m, 1H), 2.15 (s, 3H), 2.08 (s, 3H), 1.92 (s, 3H), 0.78 (d, J = 6.8, 6H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.1 (C), 186.2 (C), 171.6 (C), 158.2 (C), 145.6 (C), 133.1 (C), 132.2 (C) 131.8 (C), 131.0 (CH), 130.0 (C), 128.6 (CH), 128.5 (CH), 127.8 (CH), 125.8 (CH), 125.6 (CH), 124.3 (CH), 100.4 (CH), 67.1 (CH), 49.0 (CH₂), 28.8 (CH₃), 27.7 (CH), 27.6 (CH₃), 21.9 (CH₃), 18.7 (CH₃), 14.8 (CH₃); IR (thin film) 3061, 2944, 1642, 1579, 1241 cm⁻¹; HRMS (ESI) m/z, 619.0857 (619.0859 calcd for C₂₈H₃₀N₂O₄PdSNa, (M + Na)⁺); Anal. calcd for C₂₈H₃₀N₂O₄PdS: C, 56.33; H, 5.06; N, 4.69. Found: C, 56.55; H, 5.04; N, 4.68.

Acetylacetonato{(S)-2-[2-(1-pivoloyl)-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18p)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 19:1→5:1 hexanes:acetone) to afford palladacycle 18p (0.147 g, 0.279 mmol, 50%) as a yellow solid: mp 131–136 °C (decomp); [α]_D²⁴+111.2, [α]₅₇₇²⁴+112.4, [α]₅₄₆²⁴ +115.2, [α]₄₃₅²⁴ +118.8 (c = 1.5, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.90 (d, J = 8.3, 1H), 7.81 (br s, 1H), 7.74 (d, J = 8.4, 1H), 7.61 (d, J = 4.3, 1H), 7.34–7.28 (m, 2H), 5.41 (s, 1H), 4.14 (br s, 1H), 4.08–4.06 (m, 2H), 2.29 (br s, 1H), 2.12 (s, 3H), 2.00 (s, 3H), 1.44 (s, 9H), 1.04 (d, J = 6.7, 3H), 0.83 (d, J = 5.2, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 187.9 (C), 186.4 (C), 180.3 (C), 173.7 (C), 131.9 (C), 130.1 (C), 130.0 (CH), 129.8 (C), 129.0 (CH), 128.8 (C), 128.2 (CH), 125.4 (CH), 123.5 (CH), 100.3 (CH), 47.8 (CH), 40.8 (CH₂), 28.7 (C), 28.2 (CH₃), 27.8 (CH₃), 27.7 (CH₃), 25.4 (CH), 19.6 (CH₃), 15.1 (CH₃); IR (thin film) 3072, 2955, 1729, 1629, 1252 cm⁻¹; HRMS (ESI) m/z, 549.1352 (549.1345 calcd for C₂₆H₃₂N₂O₃PdNa, (M + Na)⁺); Anal. calcd for C₂₆H₃₂N₂O₃Pd: C, 59.26; H, 6.12; N, 5.32. Found: C, 59.33; H, 6.14; N, 5.31.

General Procedure for the Synthesis of (S)-PIN-acac Complexes Directly from Amide 16

Preparation of Acetylacetonato{(S)-2-[2-(1-benzyl)-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18c)

Amide 16a (0.350 g, 0.913 mmol) was dissolved in freshly distilled SOCl₂ (0.33 mL, 4.6 mmol) and the resulting solution was heated to 85 °C. After 5 h, the reaction was diluted with toluene (15 mL) and concentrated under reduced pressure to afford the crude chloroalkylimidoyl chloride. The residue was dissolved in CH₂Cl₂ (2 mL) and treated with a solution of benzylamine (0.11 mL, 1.0 mmol) and Et₃N (0.38 mL, 2.7 mmol) in CH₂Cl₂ (2 mL). The reaction was maintained at rt for 6 h, then stirred with 2 M NaOH (10 mL) for 20 min. The organic layer was separated and flushed through a small pad of silica gel. The filter cake was washed with 1:1 Et₂O/hexane (50 mL) and the combined organics were concentrated under reduced pressure to afford the corresponding imidazoline as an orange foam of sufficient purity to be used directly in the next step.

The imidazoline ligand prepared above was dissolved in toluene (2 mL) and Pd₂dba₃ (0.418 g, 0.457 mmol) was added in a single portion. The resulting red solution was heated to 120 °C. After 16 h, the reaction was concentrated under reduced pressure. The resulting solid was immediately dissolved in CH₂Cl₂ (4 mL) and silver acetylacetonate (0.207 g, 1.00 mmol) was added in a single portion. The resulting black suspension was stirred at rt overnight then filtered through a pad of Celite^®. The filter cake was washed with CH₂Cl₂ (50 mL) and the filtrate was concentrated to afford a yellow residue. The crude product was purified by column chromatography (silica gel, 10:1 hexanes:acetone) to afford palladacycle 18c (0.258 g, 0.484 mmol, 53%) as a yellow solid: mp = 122–128 °C (decomp); [α]_D²⁴ −110.3, [α]₅₇₇²⁴ −110.9, [α]₅₄₆²⁴ −112.7, [α]₄₃₅²⁴ −114.4, [α]₄₃₅²⁴ −118.6 (c = 1.2, CH₂Cl₂); ¹H NMR (500 MHz, toluene-d₈, 298 °K) δ 8.20 (d, J = 8.4, 1H), 7.89 (d, J = 8.1, 1H), 7.56 (d, J = 7.8, 1H), 7.51 (d, J = 8.4, 1H), 7.11–7.08 (m, 5H), 7.07–7.01 (m, 2H), 5.24 (s, 1H), 4.39 (d, J = 16.3, 1H), 4.11 (d, J = 16.3, 1H), 4.02–4.00 (m, 1H), 3.22–3.19 (m, 1H), 3.09 (t, J = 10.8, 1H), 2.54–2.51 (m, 1H), 1.92 (s, 3H), 1.84 (s, 3H), 0.88 (d, J = 6.8, 3H), 0.83 (d, J = 7.0, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.0 (C), 186.2 (C), 177.0 (C), 157.0 (C), 136.9 (C), 132.0 (C), 129.6 (C), 129.3 (CH), 129.2 (CH), 129.0 (CH), 128.9 (CH), 127.6 (CH), 127.1 (CH), 125.9 (CH), 123.6 (CH), 100.3 (CH), 67.1 (CH), 56.5 (CH₂), 52.2 (CH₂), 30.4 (CH), 27.9 (CH₃), 27.7 (CH₃), 18.7 (CH₃), 15.8 (CH₃); IR (thin film) 3072, 3019, 2951, 1668, 1592, 1255 cm⁻¹; HRMS (ESI) m/z, 555.1236 (555.1251 calcd for C₂₈H₃₀N₂O₂PdNa, (M + Na)⁺); Anal. calcd for C₂₈H₃₀N₂O₂Pd: C, 63.10; H, 5.67; N, 5.26. Found: C, 62.90; H, 5.68; N, 5.27.

The following compounds were prepared in identical fashion from amides 16a or 16b without purification of the intermediate imidazoline.

Acetylacetonato{(S)-2-[2-(1-benzhydryl)-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18d)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 12:1 hexanes:acetone) to afford palladacycle 18d (0.195 g, 0.320 mmol, 62%) as a yellow solid: mp = 197–201 °C; [α]_D²⁴ +12.7, [α]₅₇₇²⁴ +12.9, [α]₅₄₆²⁴ +14.1, [α]₄₃₅²⁴ +18.6, [α]₄₃₅²⁴ +19.9 (c = 1.3, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 8.02 (d, J = 8.3, 1H), 7.84 (d, J = 8.1, 1H), 7.81 (d, J = 8.6, 1H), 7.77 (d, J = 8.4, 1H), 7.52–7.49 (m, 2H), 7.45 (d, J = 7.1, 1H), 7.37–7.32 (m, 4H), 7.26 (t, J = 8.1, 1H), 7.03–7.01 (m, 2H), 6.97 (t, J = 7.1, 1H), 6.61 (s, 1H), 5.44 (s, 1H), 3.85–3.81 (m, 1H), 3.65 (dd, J = 10.5, 7.9, 1H), 3.84 (t, J = 10.5, 1H), 2.59–2.55 (m, 1H), 2.18 (s, 3H), 2.00 (s, 3H), 0.93–0.89 (m, 6H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.1 (C), 186.1 (C), 176.8 (C), 156.9 (C), 139.1 (C), 138.9 (C), 131.9 (C), 129.8 (C), 129.5 (CH), 129.4 (C), 129.1 (CH), 128.8 (CH), 128.7 (CH), 128.6 (CH), 128.3 (CH), 128.0 (CH), 127.5 (CH), 125.8 (CH), 124.0 (CH), 123.7 (CH), 100.3 (CH), 67.6 (CH), 67.2 (CH), 46.4 (CH₂), 30.0 (CH), 27.9 (CH₃), 27.7 (CH₃), 18.7 (CH₃), 15.5 (CH₃); IR (thin film) 3058, 3029, 2960, 1768, 1582, 1264 cm⁻¹; HRMS (ESI) m/z, 609.1740 (609.1733 calcd for C₃₄H₃₅N₂O₂Pd, (M + H)⁺); Anal. calcd for C₃₄H₃₄N₂O₂Pd: C, 67.05; H, 5.63; N, 4.60. Found: C, 66.89; H, 5.64; N, 4.59.

Acetylacetonato{(S)-2-[2-(1-cyclohexyl)-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18e)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 15:1 hexanes:acetone) to afford palladacycle 18e (0.134 g, 0.255 mmol, 57%) as a orange solid: mp = 187–190 °C; [α]_D²⁴ −71.7, [α]₅₇₇²⁴ −72.9, [α]₅₄₆²⁴ −74.1, [α]₄₃₅²⁴ −78.6, [α]₄₃₅²⁴ −82.4 (c = 1.5, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.89 (d, J = 8.5, 1H), 7.85 (d, J = 8.5, 1H), 7.80 (d, J = 7.7, 1H), 7.64 (d, J = 8.3, 1H), 7.40 (dt, J = 6.8, 1.3, 1H), 7.33 (dt, J = 6.9, 1.1, 1H), 5.37 (s, 1H), 4.13 (ddd, J = 8.0, 4.5, 1.3, 1H), 3.96–3.90 (m, 1H), 3.67 (t, J = 10.6, 1H), 3.42 (dd, J = 10.6, 8.0, 1H), 2.57–2.51 (m, 1H), 2.09 (s, 3H), 1.98 (s, 3H), 1.85–1.80 (m, 1H), 1.66–1.63 (m, 1H), 1.56–1.47 (m, 5H), 1.23–1.14 (m, 2H), 0.97 (d, J = 7.0, 3H), 0.93 (d, J = 6.8, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 187.9 (C), 186.1 (C), 176.5 (C), 155.9 (C), 132.1 (C), 129.8 (C), 129.6 (C), 129.1 (CH), 128.9 (CH), 128.8 (CH), 125.5 (CH), 123.6 (CH), 123.5 (CH), 100.2 (CH), 67.6 (CH), 58.7 (CH), 45.2 (CH₂), 31.9 (CH₂), 30.8 (CH), 29.5 (CH₂), 27.9 (CH₃), 27.7 (CH₃), 25.9 (CH₂), 25.4 (CH₂), 25.3 (CH₂), 18.7 (CH₃), 15.8 (CH₃); IR (thin film) 3045, 2950, 1718, 1612, 1258 cm⁻¹; HRMS (ESI) m/z, 547.1568 (547.1564 calcd for C₂₇H₃₄N₂O₂PdNa, (M + Na)⁺); Anal. calcd for C₂₇H₃₄N₂O₂Pd: C, 61.77; H, 6.53; N, 5.34. Found: C, 61.84; H, 6.54; N, 5.33.

Acetylacetonato{(S)-2-[2-(1-tert-butyl)-4-tert-butyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18g)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 10:1 hexanes:acetone) to afford palladacycle 18g (72 mg, 0.14 mmol, 41%) as a yellow film: mp = 123–126 °C; [α]_D²⁴ −82.4, [α]₅₇₇²⁴ −83.3, [α]₅₄₆²⁴ −85.1, [α]₄₃₅²⁴ −89.6 (c = 0.5, CH₂Cl₂); ¹H NMR (500 MHz, toluene-d₈, 298 °K) δ 8.36 (d, J = 8.4, 1H), 8.23 (d, J = 8.4, 1H), 7.53 (d, J = 8.4, 1H), 7.47 (d, J = 8.4, 1H), 7.18 (t, J = 7.0, 1H), 7.10 (t, J = 7.0, 1H), 5.24 (s, 1H), 3.91 (dd, J = 9.6, 7.8, 1H), 3.28 (dd, J = 11.4, 9.6, 1H), 3.15 (dd, J = 11.4, 7.8, 1H), 1.92 (s, 3H), 1.87 (s, 3H), 0.97 (s, 9H), 0.95 (s, 9H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.0 (C), 185.7 (C), 177.8 (C), 151.5 (C), 134.3 (C), 131.8 (C), 131.6 (C). 128.5 (CH), 127.9 (CH), 127.7 (CH), 125.6 (CH), 124.5 (CH), 123.4 (CH), 100.2 (CH), 70.0 (CH), 61.1 (C). 49.1 (CH₂), 35.8 (C), 29.8 (CH₃), 28.9 (CH₃), 27.5 (CH₃), 26.0 (CH₃); IR (thin film) 3101, 2979, 1690, 1584 cm⁻¹; HRMS (ESI) m/z, 535.1549 (535.1553 calcd for C₂₆H₃₄N₂O₂PdNa, (M + Na)⁺); Anal. calcd for C₂₆H₃₄N₂O₂Pd: C, 60.88; H, 6.68; N, 5.46. Found: C, 60.84; H, 6.70; N, 5.45.

Acetylacetonato{(S)-2-[2-(1-phenyl)-4-tert-butyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18j)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 10:1 hexanes:acetone) to afford palladacycle 18j (0.119 g, 0.223 mmol, 52%) as a yellow film: mp = 98–101 °C (decomp); [α]_D²⁴ −8.4, [α]₅₇₇²⁴−8.9, [α]₅₄₆²⁴−10.7, [α]₄₃₅²⁴−15.3, [α]₄₀₅²⁴ −17.7 (c = 1.9, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.92 (d, J = 8.3, 1H), 7.65 (t, J = 8.8, 2H), 7.30 (d, J = 8.3, 1H), 7.11–7.09 (m, 3H), 7.04 (t, J = 8.1, 2H), 7.02 (br s, 1H), 6.84 (t, J = 6.84, 1H), 5.41 (s, 1H), 4.31 (dd, J = 10.1, 3.0, 1H), 4.04 (dd, J = 10.6, 3.0, 1H), 3.92 (t, J = 10.1, 1H), 2.13 (s, 3H), 2.01 (s, 3H), 1.10 (s, 9H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 187.9 (C), 186.1 (C), 173.4 (C). 156.2 (C), 144.4 (C), 131.9 (C), 129.8 (C), 129.5 (CH), 129.3 (C), 128.7 (CH), 128.6 (CH), 128.4 (CH), 125.5 (CH), 125.0 (CH), 124.4 (CH), 123.9 (CH), 123.3 (CH), 100.3 (CH), 69.1 (CH), 57.5 (CH₂), 36.1 (C). 28.0 (CH₃), 27.7 (CH₃), 26.0 (CH₃); IR (thin film) 3444, 3063, 2961, 1562, 1513, 1260 cm⁻¹; HRMS (ESI) m/z, 533.1423 (533.1420 calcd for C₂₈H₃₁N₂O₂Pd, (M + H)⁺). Anal. calcd for C₂₈H₃₀N₂O₂Pd: C, 63.10; H, 5.67; N, 5.26. Found: C, 63.19; H, 5.68; N, 5.26.

Acetylacetonato{(S)-2-[2-(1-(4-methoxyphenyl))-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18k)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 5:1 toluene/CH₂Cl₂) to afford palladacycle 18k (51 mg, 0.09 mmol, 54%) as a yellow film: [α]_D²⁴+34.4, [α]₅₇₇²⁴ +35.1, [α]₅₄₆²⁴ +35.9, [α]₄₃₅²⁴ +42.3, [α]₄₀₅²⁴ +45.1 (c = 0.9, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.92 (d, J = 8.4, 1H), 7.66 (t, J = 8.6, 2H), 7.42 (d, J = 8.8, 1H), 7.11 (t, J = 7.4, 1H), 6.90–6.85 (m, 3H), 6.66 (d, J = 8.8, 1H), 5.41 (s, 1H), 4.24–4.20 (m, 2H), 3.89 (dd, J = 9.5, 4.2, 1H), 3.70 (s, 3H), 2.66–2.58 (m, 1H), 2.13 (s, 3H), 2.00 (s, 3H), 0.95 (d, J = 7.0, 3H), 0.90 (d, J = 7.0, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.0 (C), 186.3 (C), 172.7 (C), 157.7 (C), 156.7 (C), 138.1 (C), 131.9 (C), 129.8 (CH), 129.5 (C), 129.0 (C), 128.8 (CH), 128.7 (CH), 128.5 (CH), 128.5 (CH), 125.1 (CH), 124.3 (CH), 124.2 (CH), 123.3 (CH), 114.4 (CH), 100.3 (CH), 66.3 (CH), 55.5 (CH₃), 29.5 (CH₂), 27.9 (CH₃), 27.8 (CH₃), 18.9 (CH₃), 15.4 (CH₃); IR (thin film) 3092, 2958, 1662, 1510, 1248 cm⁻¹; HRMS (ESI) m/z, 549.1376 (549.1370 calcd for C₂₈H₃₁N₂O₃Pd, (M + H)⁺); Anal. calcd for C₂₈H₃₀N₂O₃Pd: C, 61.26; H, 5.51; N, 5.10. Found: C, 61.21; H, 5.49; N, 5.10.

Acetylacetonato{(S)-2-[2-(1-(3,4,5-trifluorophenyl))-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18l)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 8:1 toluene/CH₂Cl₂) to afford palladacycle 18l (0.107 g, 0.187 mmol, 58%) as a colorless solid: mp = 210–214 °C; [α]_D²⁴ +68.4, [α]₅₇₇²⁴+69.1, [α]₅₄₆²⁴+71.3, [α]₄₃₅²⁴ +78.9, [α]₄₀₅²⁴ +83.2 (c = 1.5, CH₂Cl₂); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.94 (d, J = 8.4, 1H), 7.73 (d, J = 8.3, 1H), 7.70 (d, J = 8.4, 1H), 7.35 (d, J = 8.6, 1H), 7.21 (t, J = 7.6, 1H), 7.03 (t, J = 8.0, 1H), 6.55–6.52 (m, 2H), 5.42 (s, 1H), 4.31–4.28 (m, 1H), 4.23 (t, J = 10.8, 1H), 3.87 (dd, J = 9.6, 4.3, 1H), 2.69–2.64 (m, 1H), 2.14 (s, 3H), 2.01 (s, 3H), 0.96 (d, J = 7.0, 3H), 0.84 (d, J = 6.8, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.1 (C), 186.4 (C), 171.5 (C), 158.9 (C), 151.2 (d, J = 249 Hz, C), 140.3 (app s, C), 136.8 (d, J = 248 Hz, C), 132.1 (C), 130.6 (CH), 129.2 (CH), 128.8 (C), 128.7 (CH), 128.4 (C), 125.8 (CH), 123.7 (CH), 123.4 (CH), 107.2 (CH), 107.0 (CH), 100.5 (CH), 66.5 (CH), 55.0 (CH₂), 29.0 (CH), 27.9 (CH₃), 27.7 (CH₃), 19.0 (CH₃), 15.2 (CH₃); IR (thin film) 3071, 2955, 1622, 1533, 1241 cm⁻¹; HRMS (ESI) m/z, 595.0801 (595.0811 calcd for C₂₇H₂₅F₃N₂O₂PdNa, (M + Na)⁺); Anal. calcd for C₂₇H₂₅F₃N₂O₂Pd: C, 56.60; H, 4.40; N, 4.89. Found: C, 56.71; H, 4.41; N, 4.88.

Acetylacetonato{(S)-2-[2-(1-mesityl)-4-isopropyl-4,5,-dihydro-1H-imidazyl]-naphthyl-C,N}palladium(II) (18m)

Following the general procedure, the crude product was purified by column chromatography (silica gel, 10:1 hexanes/acetone) to afford palladacycle 18m (0.190 g, 0.339 mmol, 65%) as a yellow solid: mp = 134–139 °C; [α]_D²⁴ +32.6, [α]₅₇₇²⁴ +32.9, [α]₅₄₆²⁴ +33.7, [α]₄₃₅²³ +38.9 (c = 0.1, CHCl₃); ¹H NMR (500 MHz, CDCl₃, 298 °K) δ 7.88 (d, J = 8.4, 1H), 7.59 (d, J = 8.1, 1H), 7.57 (d, J = 8.4, 1H), 7.05–7.02 (m, 2H), 6.94 (s, 1H), 6.74 (t, J = 8.1, 1H), 6.54 (s, 1H), 5.38 (s, 1H), 4.28 (dt, J = 10.4, 5.5, 1H), 3.95 (dd, J = 10.4, 5.7, 1H), 3.63 (t, J = 10.4, 1H), 2.54–2.48 (m, 1H), 2.44 (s, 3H), 2.17 (s, 3H), 2.09 (s, 3H), 1.98 (s, 3H), 1.79 (s, 3H), 1.03 (d, J = 6.8, 3H), 1.00 (d, J = 6.8, 3H); ¹³C NMR (125 MHz, CDCl₃, 298 °K) δ 188.1 (C), 186.3 (C), 174.8 (C), 138.7 (C), 137.4 (C), 135.40 (C), 135.36 (C), 131.8 (C), 130.8 (C), 130.4 (CH), 130.4 (CH), 129.6 (CH), 129.4 (CH), 128.73 (C), 128.70 (CH), 128.4 (CH), 124.2 (CH), 123.3 (CH), 123.1 (CH), 100.4 (C), 66.2 (CH), 54.4 (CH₂), 30.9 (CH), 28.0 (CH₃), 27.8 (CH₃), 20.9 (CH₃), 18.9 (CH₃), 18.7 (CH₃), 18.3 (CH₃), 15.9 (CH₃); IR (thin film) 2958, 2920, 1584, 1398, 1264 cm⁻¹; HRMS (ESI) m/z, 583.1560 (583.1564 calcd for C₃₀H₃₄N₂O₂PdNa, (M + Na)⁺); Anal. calcd for C₃₀H₃₄N₂O₂Pd: C, 64.23; H, 6.11; N, 4.99. Found: C, 64.30; H, 6.13; N, 4.98.

General Procedure for the PIN-Catalyzed Synthesis of Enantiomerically Enriched 3-Acyloxy-1-alkenes

Preparation of 5-Phenylpent-1-en-3-yl Ethanoate (24)

Palladacycle 18a (0.9 mg, 0.02 mmol, 3 mol %) was added in a single portion to a solution of allylic trichloroacetimidate 23 (0.200 g, 0.652 mmol), acetic acid (0.11 mL, 2.0 mmol) and CH₂Cl₂ (0.65 mL). The reaction was sealed under an inert atmosphere (N₂ or Ar) and maintained at rt. After 18 h, the reaction mixture was concentrated under reduced pressure and purified by silica gel chromatography (95:5 pentane/Et₂O) to afford allylic acetate 24 (0.126 g, 0.619 mmol, 95%) as a colorless oil. Enantiomeric excess was determined by GC using a chiral stationary phase [T = 130 °C, hold 40 min; ramp 5 °C to 180 °C, hold 40 min; S (minor) enantiomer t_R = 28.12 min, R (major) enantiomer t_R = 30.09 min; 28% ee]. Spectral data was identical in all respects to previously reported values.^8c

Preparation of 2-iodo-1-naphthoic acid (15).^17,18

A solution of n-BuLi (2.5 M, hexane, 34.5 mL) was added drop wise over 1 h to a solution of 2,2,6,6-tetramethylpiperidine (12.20 g, 86.35 mmol) in THF (300 mL) maintained at 0 °C. After 30 min, the reaction mixture was cooled to −78 °C and a solution of 1-cyanonaphthalene (14, 12.57 g, 82.03 mmol) in THF (25 mL) was added drop wise over 30 min. The resulting dark solution was maintained at −78 °C for 2 h. A solution of I₂ (21.92 g, 86.35 mmol) in THF (250 mL) was added drop wise via addition funnel over 2 h. The reaction mixture was maintained at −78 °C for 2 h then allowed warm to rt overnight. The reaction mixture was quenched with H₂O (400 mL) and the resulting mixture was extracted with EtOAc (2 x 250 mL). The combined organics were washed successively with saturated aqueous Na₂S₂O₃ (3 x 300 mL), 1 M HCl (3 x 300 mL), and brine (1 x 300 mL). The organic layer was dried over MgSO₄, filtered, and concentrated under reduced pressure. The crude residue was recrystallized from CHCl₃/heptane to afford 2-iodo-1-cyanonaphthalene (21.45 g, 76.85 mmol, 89%) as a tan solid. Spectral data was identical in all respects to previously reported values.¹⁷

2-Iodo-1-cyanonaphthalene (5.00 g, 17.9 mmol) was suspended in a solution of water (60 mL) and glacial acetic acid (120 mL). The suspension was cooled to 0 °C and concentrated H₂SO₄ (100 mL) was slowly added drop wise such that the internal temperature was maintained between 45–50 °C. The resulting brown solution was heated to 120 °C. After 16 h, the reaction mixture was cooled to rt and poured into ice water (300 mL). The resulting slurry was extracted with EtOAc (3 x 100 mL) and the combined organic extracts were washed with 3.0 M NaOH (3 x 150 mL). The organic layer was dried over MgSO₄, filtered, and concentrated under reduced pressure to afford a brown paste. The residue was digested in toluene (300 mL) and concentrated under reduced pressure to remove residual AcOH and water. This process was repeated (1–3 times) until a light brown solid formed. The crude solid was recrystallized from MeCN to afford 2-iodonaphthalene-1-carboxamide (5.05 g, 17.0 mmol, 95%) as a tan solid in sufficient purity to be used in the next step. This solid (5.05 g, 17.0 mmol) was dissolved in MeCN (10.0 mL) with external heating. A chilled solution of aqueous H₂SO₄ (70% (w/w), 100 mL) was added and the resulting solution was maintained at rt. NaNO₂ (11.7 g, 170 mmol) was added to the reaction mixture in five equal portions over 1 h. Gas evolution and a color change to red-brown was accompanied by the formation of a precipitate. The resulting suspension was protected from light and stirred at rt. After 16 h, the mixture was diluted with water (100 mL) and extracted with CH₂Cl₂ (3 x 100 mL). The combined organic extracts were washed with 2 M NaOH (3 x 100 mL). The pH of the combined aqueous extracts was adjusted to ~3 with 2 M HCl. The resulting cloudy suspension was extracted with CH₂Cl₂ (3 x 50 mL). The combined organics were dried over Na₂SO₄, filtered, and concentrated under reduced pressure to afford 15 (3.90 g, 13.1 mmol, 77%) as a tan solid. Spectral data was identical in all respects to previously reported values.¹⁸