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.