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. 2024 Oct 1;30(55):e202402544.
doi: 10.1002/chem.202402544. Epub 2024 Sep 17.

Boron-Nitrogen-Containing Benzene Valence Isomers

Tomoya Ozaki  1 , Shih-Yuan Liu  1
Affiliations

Boron-Nitrogen-Containing Benzene Valence Isomers

Tomoya Ozaki et al. Chemistry. .

Abstract

Benzene is one of the most ubiquitous structural motifs in chemistry. The valence isomers of benzene have also attracted synthetic chemists' attention due to their unique structures, bonding, and reactivity. We have been investigating boron-nitrogen-containing benzene valence isomers via photoisomerization of 1,2-azaborines. In this contribution, we summarize recent developments of these highly strained BN-heterocyclic compounds including their synthesis, characterization, proposed mechanism of formation, and their potential applications.

Keywords: Azaborines; BN-heterocycle; Benzvalene; Dewar benzene; Photoisomerization.

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Conflict of interest statement

Conflict of Interests

The authors declare no conflict of interest.

Figures

Scheme 1.
Scheme 1.
BN/CC isosteres of benzene.
Scheme 2.
Scheme 2.
(A) Valence isomers of benzene 4–7. (B) Tamelen’s first synthesis of Dewar benzene 4 (1963). (C) Katz’s first synthesis of benzvalene 5 (1971). (D) Katz’s first synthesis of prismane 6 (1973). (E) Billups’s first synthesis of bicyclopropenyl 7 (1989).
Scheme 3.
Scheme 3.
(A) Photoisomerization products of benzene. (B) A schematic representation of productive photoisomerization pathway of benzene according to nonadiabatic molecular dynamics simulations. CI: conical intersection; MECP: minimal energy crossing point.
Scheme 4.
Scheme 4.
Photoisomerization of 1,2-azaborine.
Scheme 5.
Scheme 5.
(A) Photoisomerization of 1,2-dihydro-1,2-azaborine 1. (B) Other potential photoisomer products 10–13. (C) Relative energies of potential photoisomers of 1,2-dihydro-1,2-azaborine 1. Energies were computed at the MP2/6–311+ +G** level of theory.
Scheme 6.
Scheme 6.
Three possible isomers of BN-Dewar benzene from photolysis of 1,2-azaborine 1 investigated by Su.
Scheme 7.
Scheme 7.
Proposed mechanism of photoisomerization of 1,2-dihydro-1,2-azaborine 1 to BN-Dewar benzene 9.
Scheme 8.
Scheme 8.
Proposed mechanism of ring-opening of BN-Dewar benzene 9 to 1,2-dihydro-1,2-azaborine 1. Relative energies were computed at the MRMP2 and CCSD(T) levels of theory.
Scheme 9.
Scheme 9.
Dimerization pathways from BN-Dewar benzene 9 to its dimer. Relative energies were computed at the SCS-RIMP2/def2-TZVP level of theory.
Scheme 10.
Scheme 10.
Photoisomerization of N-TBS B-Mes-1,2-azaborine 16 to the BN-Dewar isomer 17.
Scheme 11.
Scheme 11.
Proposed mechanism of ring-opening from BN-Dewar benzene 17 to 1,2-azaborine 16. Relative energies are computed at the B3LYP/6–31G* level of theory.
Scheme 12.
Scheme 12.
Photodecomposition of BN-Dewar benzene 17 to 1,3,2,4-diazadiboretidine 18.
Scheme 13.
Scheme 13.
Rh-Catalyzed cycloreversion of 17 to 16.
Scheme 14.
Scheme 14.
1,2-Azaborine/BN-Dewar benzene system for MOST application.
Scheme 15.
Scheme 15.
ORTEP illustrations of [Dewar•AgSbF6]2 with 50% thermal probability ellipsoids. Hydrogen atoms are omitted for clarity.
Scheme 16.
Scheme 16.
(A) Diels alder reaction of 1,2-azaborine. (B) BN-Dewar benzene as a 4 C+1 N+1B synthon for aminoborylated cyclobutane.
Scheme 17.
Scheme 17.
The synthesis of aminoborylated cyclobutane 24.
Scheme 18.
Scheme 18.
The synthesis of trisubstituted aminoborylated cyclobutane 25–29.
Scheme 19.
Scheme 19.
Select derivatized products from 24.
Scheme 20.
Scheme 20.
Ring opening metathesis polymerization of 17.
Scheme 21.
Scheme 21.
Photoisomerization of C5-aryl-substituted 1,2-azaborines to BN-benzvalenes.
Scheme 22.
Scheme 22.
ORTEP illustrations of 37 and 44 with 50% thermal probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths in Å for 37 are: N–B=1.419(3), C3–C4=1.531(3), C3–C5=1.546(3), C4–C6=1.494(3), C5–C6=1.497(3). Selected bond lengths in Å for 44 are: N–B=1.446(3), C3–C4=1.355(3), C5–C6=1.355(3).
Scheme 23.
Scheme 23.
The synthesis of BN-bicyclo[2.1.1]hexane 45.
Scheme 24.
Scheme 24.
A schematic representation of bond connections over the course of the photoisomerization reaction from 46 to 35.
Scheme 25.
Scheme 25.
Proposed mechanism.
See this image and copyright information in PMC

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