Abstract
Carbon-based benzvalene undergoes diverse rearrangements upon exposure to external stimuli. In this report, we investigate the thermal and metal-catalyzed rearomatization pathways of BN-benzvalene, a boron–nitrogen-containing valence isomer of benzene. Depending on the choice of metal catalyst, these transformations selectively yield either C5- or C3-functionalized 1,2-azaborines. The C5-isomer formation serves as a proof of concept for a new molecular photoswitch system, while the C3-isomer formation represents the functional group transposition on the 1,2-azaborine scaffold. Mechanistic investigations, such as deuterium labeling studies and DFT calculations, suggest distinct operative pathways for the generation of each isomer.
Graphical Abstract
INTRODUCTION
Benzvalene, first synthesized in 1971, is a prominent member of the strained hydrocarbons and has long fascinated chemists due to its unique structural features, high strain energy, and unusual reactivity.1 As with other bicyclo[1.1.0]butane derivatives,2,3 benzvalene undergoes diverse rearrangements upon exposure to external stimuli, such as heat,4 light,5 and metal-based catalysts.6 For instance, depending on the identity of the zerovalent metal it is exposed to, benzvalene can selectively transform into either benzene or fulvene (Scheme 1A, top).6a In addition, treatment of 1-deuterobenzvalene with silver(I) perchlorate catalyst induces deuterium scrambling in benzvalene prior to the irreversible formation of monodeuter-obenzene (Scheme 1A, bottom).6b These unique metal-catalyzed transformations prompt tantalizing questions such as what would happen if substituents other than deuterium were introduced at the periphery of benzvalene or if heteroatoms were embedded within its skeletal framework? Despite its conceptual appeal, this area has been virtually unexplored in the literature.7 This present study addresses the above questions in the context of a boron–nitrogen-containing benzvalene.
Scheme 1.
Divergent Rearrangement of Benzvalene
We have been developing boron–nitrogen (BN)-containing valence isomers of benzene8 since the initial discovery of its first-in-kind in 2012 in collaboration with the Bettinger Group.9 It has been demonstrated that the photoisomerization of 1,2-azaborine selectively yields its Dewar isomer (BN-Dewar benzene). Moreover, the selective cycloreversion of the BN-Dewar benzene to 1,2-azaborine has been achieved under thermal conditions, as well as through the use of homogeneous Rh-, Cu-, and Ag-based catalysts and a heterogeneous Au-based catalyst (Scheme 1B, left).10 This combination of clean photoisomerization and selective cycloreversion, along with the high energy storage density of up to 0.64 MJ/kg, establishes the 1,2-azaborine/BN-Dewar benzene system as a promising molecular photoswitch pair for molecular solar thermal (MOST) energy storage applications.11
More recently, we reported the synthesis and structural characterization of another BN-valence isomer of benzene, namely, BN-benzvalene,12,13 along with its photochemical rearomatization to a C4-functionalized 1,2-azaborine.14 However, its reactivity toward thermal or metal-catalyzed conditions had yet to be explored (Scheme 1B, right). Inspired by seminal reports in the metal-catalyzed rearrangement chemistry of carbon-based benzvalene and BN-Dewar benzene, we sought to investigate whether BN-benzvalene might exhibit analogous reactivity.
In this article, we present the first reactivity studies of BN-benzvalene under thermal and metal-catalyzed conditions (Scheme 1C). To our surprise, we found that both thermal and metal-catalyzed rearomatization of BN-benzvalene can yield the C3- and C5-isomers of 1,2-azaborine. Moreover, the product selectivity can be precisely controlled by the choice of the metal catalyst.
RESULTS AND DISCUSSION
In our initial investigation, we discovered that heating BN-benzvalene 1a to 100 °C in toluene resulted in the formation of a nearly 1:1 mixture of C5–1,2-azaborine isomer 2a and C3–1,2-azaborine isomer 3a (Scheme 2).
Scheme 2.
Initial Discovery
To improve selectivity and lower the required reaction temperature, we next investigated a variety of metal-based and Lewis acid catalysts (Table 1, see Supporting Information for the complete list). We found that Cu(OTf)2 and AlCl3 effectively facilitated rearomatization, enabling the formation of a 1:1 mixture of 2a and 3a at room temperature (entries 1 and 2). To our delight, Rh-based catalysts and [Pd(C3H5)Cl]2 promoted the selective formation of 2a (entries 3–6). Given that 1a is produced by the photoisomerization of 2a, this result serves as an example of quantitative cycloreversion of BN-benzvalene and provides a proof of concept for defining the azaborine/BN-benzvalene pair as a new molecular photoswitch system.
Table 1.
Investigation of the Metal-Based Catalysts
| ||||
|---|---|---|---|---|
| entry | catalyst | Conv. (%)a | 2aa | 3aa |
| 1 | Cu(OTf)2 | >95 | 46 | 52 |
| 2 | AlCl3 | >95 | 45 | 50 |
| 3 | [Rh(nbd)Cl]2 | 78 | 80 | <1 |
| 4 | (cod)2RhBF4 | 86 | 83 | <1 |
| 5 | [Rh(CO)2Cl]2 | >95 | >95 | <1 |
| 6 | [Pd(C3H5)Cl]2 | >95 | >95 | <1 |
| 7 | AgBF4 | >95 | <1 | >95 |
| 8 | AgSbF6 | >95 | <1 | >95 |
| 9 | AgClO4 | >95 | <1 | >95 |
| 10 | AgOTf | 15 | <1 | 14 |
| 11 | AgOTs | <1 | <1 | <1 |
| 12 | AgNO3 | <1 | <1 | <1 |
| 13 | AgCl | <1 | <1 | <1 |
Yields were determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as an internal standard.
The use of Ag-based catalysts resulted in the rapid and quantitative formation of C3-isomer 3a. The catalytic activity was found to be highly dependent on the coordinating ability of the counteranion: Ag (I) catalysts with weakly coordinating anions such as BF4−, SbF6−, and ClO4− (entries 7–9) exhibited high reactivity, whereas catalysts bearing more coordinating anions such as OTf−, OTs−, NO3−, and Cl− (entries 10–13) showed little or no activity.15 Although mechanistically distinct, a similar counteranion-dependent reactivity trend was previously observed in Bettinger’s Ag-catalyzed cycloreversion studies of BN-Dewar benzene.10a
Although desirable for diversity-oriented synthesis,16 functional group transposition on aromatic rings remains rare.17 By combining the photoisomerization of 2a with a silver-catalyzed selective rearomatization, we have achieved a C5-to-C3 transposition for the 1,2-azaborine scaffold, affording 3a in 85% isolated yield and thereby expanding the synthetic toolbox for accessing substituted monocyclic 1,2-azaborines (Scheme 3).18 Notably, both steps were performed in THF, eliminating the need for a solvent concentration or exchange between steps.
Scheme 3.
Optimized Conditions for C5-to-C3 Transposition
To evaluate the generality of our transposition strategy, we examined the reactivity of various C5-substituted-1,2-azaborines under optimized reaction conditions (Scheme 4). The reaction displayed broad functional group tolerance across ortho-, meta-, and para-substitution patterns. Aryl groups bearing alkyl groups (3a–3c), a trifluoromethoxy group (3d), halogens (3e–3g), and a methoxy group (3j) were all tolerated. Additionally, heteroaromatic substituents, thiophene and thiazole (3k and 3l), as well as a vinyl group (3m) were also compatible with the reaction conditions without greatly diminishing the yield. Finally, a dimeric substrate containing two C5-aryl-1,2-azaborine units also afforded a C3-isomer (3n). In this process, the boron atom and the mesityl substituent relocate from the para-position relative to the phenyl linker in 1n to the ortho-position in 3n, resulting in a formal boron migration.
Scheme 4. Substrate Scope.
a Yield of isolated products: average of two runs. Scale: 0.10 mmol. b1st Step: hexane was used as a solvent. c10 mol% AgClO4, 16 h.
For most substrates, the second step proceeded quantitatively and rapidly (5 mol % catalyst, rt, 30 min). However, the first photoisomerization step can be low-yielding or unsuccessful for certain substrates, resulting in poor overall performance (see Scheme S2 for details).
We further demonstrated the applicability of our strategy by incorporating more structurally complex bioactive molecules, namely, phenylalanine and caffeine (3o and 3p) (Scheme 5, right). When combined with our previously reported C5-to-C4 transposition chemistry (Scheme 5, left),14 the divergent positional isomerization of starting materials 2o and 2p provides a proof of concept for enabling rapid structure–activity relationship (SAR) studies for the potential BN-based drug lead compounds.19
Scheme 5. Divergent Functional Group Transposition.
aYield of isolated products: average of two runs. Scale: 0.10 mmol. b1st Step: 0.033 M.
Finally, we explored C3,C5-difunctionalized 1,2-azaborines (Scheme 6). Upon photoisomerization, these substrates first generate the corresponding BN-benzvalenes (6a–6c), which then undergo selective rearomatization to yield the products (7a–7c). This sequence effectively enables a formal switch of substituents between the C3 and C5 positions selectively, contributing to a rare example of this type of structural rearrangement in the literature.20,21
Scheme 6. Substituent Switch at the C3 and C5 Positions.
aYield of isolated products: average of two runs. Scale: 0.10 mmol. b10 mol% AgClO4, 16 h.
To elucidate the reaction mechanism of the thermal- and silver-catalyzed rearomatization, we performed deuterium labeling studies using deuterated BN-benzvalene analogues 8a and 11a, as illustrated in Scheme 7. In both the thermal- and silver-catalyzed pathways, the deuterium at the α-amino position remained unchanged (Scheme 7, eqs 1 and 3). On the other hand, the bridgehead deuterium migrates to the C3 or C5 position, resulting in the formation of C3D-C5-p-Tol-1,2-azaborine 12a or C5D-C3-p-Tol-1,2-azaborine 13a, respectively (Scheme 7, eqs 2 and 4). The formation of 13a in eq 4 is also consistent with products 7a–7c shown in Scheme 6. Notably, no deuterium scrambling into other ring positions was observed in these reactions.
Scheme 7. Deuterium Labeling Studiesa.
aYields were determined by 1H NMR analysis using 1,3,5-trimethoxybenzene as an internal standard.
For the thermal cycloreversion of BN-benzvalene 1 to C5–1,2-azaborine 2, we considered three possible mechanisms as illustrated in Scheme 8. Mechanism 1 involves concerted ring opening through simultaneous cleavage of the C3–C5 and C4–C6 bonds. Although this mechanism has been proposed for carbon-based benzvalene systems,4,22 it is inconsistent with our deuterium labeling results. Mechanism 2 begins with the migration of the C4–C6 bond to form a C4–B bond, assisted by the nitrogen lone pair, leading to a new zwitterionic BN-benzvalene intermediate. From this intermediate, concerted cleavage of the C3–B bond and the C4–C5 bond would yield the product 2.
Scheme 8.
Proposed Mechanisms for C5-Isomer Formation
However, if this mechanism were operative, then deuterium scrambling at the C3 and C4 position would be expected, as they occupy enantiotopic positions in the intermediate. Since no such scrambling was observed, this pathway is also ruled out. Finally, Mechanism 3, which proceeds via a BN-Dewar benzene intermediate, is consistent with the deuterium labeling studies and therefore tentatively proposed as the operative pathway.23
For the thermal rearomatization of BN-benzvalene 1 to C3–1,2-azaborine 3, we considered two mechanistic pathways as outlined in Scheme 9. Both pathways closely parallel the mechanisms shown in Scheme 8, proceeding through either a BN-Dewar benzene intermediate or a zwitterionic BN-benzvalene intermediate.
Scheme 9.
Proposed Mechanisms for C3-Isomer Formation
Because both proposed mechanisms are consistent with our deuterium labeling results, we turned to density functional theory (DFT) calculations to distinguish among them. To simplify the conformational analysis, the TBS group was replaced with a TMS group. Both reaction pathways leading to product were successfully identified computationally (Mechanism 2 shown in Figure 1b and Mechanism 1 shown in SI). Mechanism 2 is favored, exhibiting a substantially lower activation free energy barrier for the rate-determining transition state (ΔG‡ = 34.1 kcal/mol, TS1) than Mechanism 1 (ΔG‡ = 44.5 kcal/mol, TS-A, Figure S5). Figure 1b depicts the complete free energy profile for the uncatalyzed pathway according to Mechanism 2. The transition state TS1 was computationally identified as the rate-determining step, leading to zwitterionic BN-benzvalene Int1. From Int1, a second transition state (TS2) was located, which affords the PDT via concerted cleavage of the C3–B and C4–C5 bonds. Detailed examination of TS2 reveals a pronounced asynchronicity in the bond-breaking process, as reflected by the markedly different bond lengths (C3–B: 1.695 Å, C4–C5: 2.118 Å), indicating that C4–C5 bond cleavage precedes C3–B bond rupture (see Figure S7 for IRC analysis). Despite extensive exploration of the potential energy surface, no additional zwitterionic intermediate could be identified between Int1 and TS2. Thus, although asynchronous, the transformation from Int1 to the product can be best described as concerted.
Figure 1.
Energy diagram for C3–1,2-azaborine PDT Formation from BN-benzvalene SM. (a) Silver-catalyzed pathway computed at the SMD(THF)-B3LYP-D3(BJ)/lanl2dz(Ag), 6–311+G**(other atoms)//B3LYP-D3(BJ)/lanl2dz(Ag), 6–31G*(other atoms) level of theory. (b) Uncatalyzed pathway computed at the SMD(toluene)-B3LYP-D3(BJ)/6–311+G**//B3LYP-D3(BJ)/6–31G* level of theory.
To elucidate the role of the silver catalyst, we computed the corresponding reaction pathway in the presence of a silver cation (Figure 1a). Initial coordination of the p-tolyl and mesityl substituents of BN-benzvalene SM to the silver cation affords a more stable intermediate Int2 (–5.0 kcal/mol). The reaction then proceeds through rate-determining transition state TS3 to form zwitterionic BN-benzvalene Int3. Coordination to the silver cation significantly lowers the activation free energy of this step (SM → TS1: 34.1 kcal/mol vs Int2 → TS3: 21.3 kcal/mol), primarily due to increased electrophilicity at the boron center induced by mesityl–Ag coordination. From Int3, progression through TS4 leads to the formation of silver-bound C3–1,2-azaborine isomer Int4. This step shows no significant rate acceleration by the silver catalyst (Int1 → TS2: 10.9 kcal/mol; Int3 → TS4: 12.3 kcal/mol). Finally, dissociation of the silver cation from Int4 furnishes the product. This step is endergonic by 20.7 kcal/mol, suggesting that silver dissociation may constitute a competing rate-limiting step in the catalytic cycle.
To probe that dissociation of the silver cation from intermediate Int4 is facile and does not impair catalytic efficiency, we independently synthesized the silver complex 14a by treating C3-p-Tol-1,2-azaborine 3a with 1.0 equiv of AgClO4 (Scheme 10, eq 1). The crystal structure of 14a was successfully obtained, and its η2-coordination of both the p-tolyl and mesityl groups to the silver cation closely matches the coordination mode predicted in our computational studies. The isolated crystal 14a was subsequently employed as a catalyst (5 mol %) and treated with BN-benzvalene 1a (Scheme 10, eq 2). As a result, product 3a was quantitatively formed within 30 min at room temperature, supporting that 14a is a chemically and likely also kinetically competent catalyst.
Scheme 10.
Catalytic Competency of Silver Complex 14a
Finally, we experimentally investigated whether Mechanism 1 could operate under silver-catalyzed conditions for the formation of C3–Ar isomer 3. Since the key intermediate in this mechanism is C3-BN-Dewar benzene, this species would be expected to transform into the product 3 under the standard silver-catalyzed conditions if this pathway were operative. To test this hypothesis, we independently synthesized C3-p-Tol-BN-Dewar benzene 15a by photoisomerization of 3a.10c When 15a was subjected to the standard silver-catalyzed conditions, we did not observe any 3a formation and 15a was recovered, thereby ruling out Mechanism 1 under silver-catalyzed conditions (Scheme 11). Although we cannot exclude the possibility that distinct mechanisms operate under uncatalyzed and silver-catalyzed pathways, the combined experimental and computational data presented in this study support Mechanism 2 (involving a zwitterionic BN-benzvalene) as the operative pathway for the formation of the C3–1,2-azaborine isomer 3.
Scheme 11.
Intermediacy Study of C3-p-Tol-BN-Dewar Benzene 15a
CONCLUSIONS
In conclusion, we described the thermal and metal-catalyzed rearomatization of BN-benzvalene into 1,2-azaborines. These transformations enable the selective formation of either C5- or C3-substituted 1,2-azaborines, depending on the choice of metal catalyst. Mechanistic studies revealed distinct reaction pathways for each isomer. Notably, both mechanisms differ fundamentally from classical carbon-based benzvalene’s rear-omatizations, reflecting the unique reactivity imparted by the boron–nitrogen motif. Taken together, we hope that this work lays the groundwork for further exploration of BN-benzvalene chemistry in applications ranging from solar energy storage materials to late-stage diversification of 1,2-azaborines.
Supplementary Material
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.5c23113.
Experimental procedures, compound characterization data, and computational and crystallographic information (PDF)
ACKNOWLEDGMENTS
Research reported in this publication was supported by the National Institute of General Medical Sciences of the National Institutes of Health (NIGMS) under Award Number R35GM153328 and by Boston College internal funds, the Excellence Initiative of Université de Pau et des Pays de l’Adour I-Site E2S UPPA, and by Boston College start-up funds. We also acknowledge the NIH-S10 (award: 1S10OD026910-01A1), the NSF-MRI (award: CHE-2117246) for the support of Boston College’s NMR facilities, and the NIH-S10 (award: S10OD030360) for the support of Boston College’s X-ray facilities. T.O. thanks the Takenaka Scholarship Foundation, the LaMattina Graduate Fellowship in Chemical Synthesis, and Boston College Dissertation Fellowship for support. The “Direction du Numérique” of UPPA and Mésocentre de Calcul Intensif Aquitain (MCIA) are also acknowledged for supporting computational facilities. Y.D. was funded as a Ph.D. student by I-Site E2S-UPPA.
Footnotes
Accession Codes
Deposition Numbers 2516159–2516163 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.
Complete contact information is available at: https://pubs.acs.org/10.1021/jacs.5c23113
The authors declare no competing financial interest.
A reversible three-species photoisomerization of substituted 1,2-azaborines was demonstrated by Bettinger and coworkers while our manuscript was in revision, see J. Am. Chem. Soc. 2026 DOI: 10.1021/jacs.5c20667
Contributor Information
Tomoya Ozaki, Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States.
Yuping Dai, Université de Pau et des Pays de l’Adour, E2S UPPA / CNRS, Institut des Sciences Analytiques et de Physico-Chimie pour l'Environnement et les Matériaux IPREM UMR 5254, 64053 Pau cedex 09, France; Present Address: National Institute of Applied Sciences of Toulouse (INSA Toulouse), 135 Av. de Rangueil, 31400 Toulouse, France.
Bo Li, Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States.
Karinne Miqueu, Université de Pau et des Pays de l’Adour, E2S UPPA / CNRS, Institut des Sciences Analytiques et de Physico-Chimie pour l'Environnement et les Matériaux IPREM UMR 5254, 64053 Pau cedex 09, France.
Shih-Yuan Liu, Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States.
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