A Study of 1,2-Dihydro-1,2-azaborine in a π-Conjugated System

The reaction of N-Boc-protected bis(5-phenyl-2-pyrrolyl)borane with BF3·OEt2 produced 3-(phenylpyrrolyl)-6-phenyl-1,2-dihydro-1,2-azaborine in moderate yield. This compound showed an absorption band at a longer wavelength compared to that of its benzene analogue and also exhibited an intense red-shifted fluorescence with a high quantum yield close to unity. According to the X-ray structural analysis, cyclic voltammetry, and theoretical calculations, the 1,2-dihydro-1,2-azaborine acts not like a benzene analogue but like a cyclohexadiene analogue in the extended π-conjugated skeleton.