Recent advances in azaborine chemistry

Abstract

The chemistry of organoboron compounds has been primarily dominated by their use as powerful reagents in synthetic organic chemistry. Recently, the incorporation of boron as part of a functional target structure has emerged as a useful way to generate diversity in organic compounds. A commonly applied strategy is the replacement of a CC unit with its isoelectronic BN unit. In particular, the BN/CC isosterism of the ubiquitous arene motif has undergone a renaissance in the past decade. The parent molecule of the 1,2-dihydro-1,2-azaborine family has now been isolated. New mono- and polycyclic B,N heterocycles have been synthesized for potential use in biomedical and materials science applications. This review is a tribute to Dewar's first synthesis of a monocyclic 1,2-dihydro-1,2-azaborine 50 years ago and discusses recent advances in the synthesis and characterization of heterocycles that contain carbon, boron, and nitrogen.

Figure 1

Isoelectronic relationship between CC and BN. The notation illustrated on the left column is used throughout this review.

Figure 2

Molecular consequences of BN/CC isosterism.

Figure 3

An intuitive explanation for reduced dipolemoment in aminoborane vs. AB.

Figure 4

Isomeric forms of singly substituted aromatic CBN heterocycles

Figure 5

Bond lengths (in Å) and carbonyl stretching frequencies for 82 and benzene-Cr(CO)₃. Computed values are in parentheses.

Scheme 1

Preparation of boron-substituted 9,10-azaboraphenanthrene derivatives.

Scheme 2

Installation of a p-accepting group at the nitrogen of BN-phenanthrene.

Scheme 3

Synthesis of B-substituted 1,2-azaboranaphthalenes.

Scheme 4

Comparison of hydrolytic stability of BN-naphthalene 11 versus reduced anhydride 14.

Scheme 5

EAS reactivity of BN-naphthalene 12.

Scheme 6

Synthesis of water-soluble BN-naphthalenes.

Scheme 7

Synthesis of bridgehead BN-naphthalene 29.

Scheme 8

Formation of BN-phenalenium cation 30 from BN-naphthalene 29.

Scheme 9

Synthesis of 1,2-azaborine derivative 32 via desulfurization with Raney nickel (Dewar, 1962).

Scheme 10

Dehydrogenation route to 1,2-azaborine 34 (White, 1963).

Scheme 11

Hydroboration-oxidation protocol leading to undesired trimerization to BN-triphenylene 36.

Scheme 12

Hydroboration to stable 1,2-azaboracyclohexane 37.

Scheme 13

Hydroboration-oxidation to generate B-H substituted 1,2-azaborine 39.

Scheme 14

Desulfurization of BN-benzothiophenes with Raney nickel to generate 4- and 5-substituted 1,2-azaborines.

Scheme 15

Mild synthesis of 1,2-azaborine 49 via ring-closing metathesis.

Scheme 16

Ring-expansion route to 1,2-azaborines 53a–c and deuterium labeling.

Scheme 17

Haptotropic migration of Cr-complexes of B-phenyl-1,2-azaboranaphthalene 9.

Scheme 18

Haptotropic migration of Cr-complexes of B-methyl-1,2-azaboranaphthalene 12.

Scheme 19

Synthesis of complexes 66 and 67 from BN-styrene 65.

Scheme 20

Nucleophilic substitution of 69 to generate B-substituted 1,2-azaborines.

Scheme 21

Synthesis of BN-benzonitrile and unexpected isomerization to complex 72.

Scheme 22

Synthesis of 1,2-azaborine cations 74–77.

Scheme 23

Synthesis of 1,2-dihydro-1,2-azaborine 82.

Scheme 24

Catalytic formation of 1,2-azaborine isomers 84–87 from common intermediate 83.

Scheme 25

Experimentally determined resonance stabilization energy of 1,2-azaborine.

Scheme 26

Synthesis of N-t-Bu-BN-indole 93.

Scheme 27

Synthesis of the parent “fused” BN-indole 96.

Scheme 28

Hydrogen storage by CBN heterocycles.

Scheme 29

Regeneration of 1,2-azaborine spent fuel 89 via catalytic hydrogenation of C=C bonds and sequential addition of H⁻/H⁺ across the B–N bond.

Scheme 30

Synthetic route to 1,2-azaboracyclohexane 102 and dehydrogenation/trimerization to form 104.

Scheme 31

Liquid hydrogen storage material based on BN-methylcyclopentane 105.

Scheme 32

Synthesis of BN-tolan analogs 107 and 108.

Scheme 33

Synthesis of the first 1,3-azaborine 114.

Scheme 34

Synthesis of 1,2,-azaborine-fused oligothiophene materials 117 and 120.

Scheme 35

Proposed formation of deborylated polymer 123 from 117 via intermediates 121 and 122.

Scheme 36

Synthesis of Π-congugated 1,2-azaborine material 125.

Scheme 37

Synthesis of BN-pentacene isomers 123 and 125 and BN-heptacene 127.

Scheme 38

General route to BN-anthracenes 135–137 and BN-pentacenes 140a–c.

Scheme 39

Dicationic BN-anthracenes 141 and 142.

Scheme 40

Multistep F⁻ and CN⁻ anion sensing with bis(dimesitylboryl)azaborine 143.

Scheme 41

Π-conjugated dendrimers based on BN-anthracene.

Scheme 42

Synthesis of dinapthoazaborine 151.

Scheme 43

Synthesis of BN-fused polyaromatics 154 and 156.