Heteroarene-fused boroles: what governs the antiaromaticity and Lewis acidity of the borole skeleton?

Abstract

A series of benzoheteroarene-fused boroles, containing pyrrole, furan, and thiophene as the heteroarene rings, were synthesized and their structures were determined by X-ray crystallographic analysis. Their antiaromaticity was assessed by calculations of the nucleus-independent chemical shift (NICS) values using the geometries derived from their crystal structures. The large NICS(1)_zz values of the borole ring in the heteroarene-fused structures showed their high antiaromaticity, in contrast to the fact that dibenzoborole has a reduced antiaromaticity. This enhanced antiaromaticity is due to the lower extent of bond alternation in the borole ring with the heteroarene-fused structures compared to that in the parent borole. Theoretical calculations also indicated that the antiaromatic character of the borole is relevant to its LUMO level. Moreover, the experimentally evaluated Lewis acidities of the heteroarene-fused boroles are linearly correlated to their reduction potentials, namely, the LUMO levels. These results shed light on the understanding of how the fused-structures affect the properties and reactivities of the borole moiety. All the heteroarene-fused boroles have broad absorption bands with the comparable absorption maxima around 468–479 nm. In contrast, their reduction potentials are different from one another, dependent on the electronic characteristics of the heteroarene rings.