Synthesis and Characterization of B‑Heterocyclic π‑Radical and Its Reactivity as a Boryl Radical

The first isolation and full characterization of the
stable, persistent
diazaboracyclic neutral radical 3 is reported. Reduction
of base-stabilized difluororoborane 2 provided radical 3 as a neutral molecule having a planar sp2 boron
atom attached to one fluorine and two nitrogen atoms. ESR spectroscopy
and DFT calculations indicated that the unpaired electron is delocalized
over the six-membered ring. Because of an electronic transition related
to the singly occupied molecular orbital, 3 has a characteristic
red color, as UV–vis spectroscopy showed an absorption maximum
at 498 nm. Although DFT calculations suggested that radical 3 has relatively low spin density on the boron atom in comparison
with the nitrogen and carbon atoms in the six-membered ring, 3 reacted as a base-stabilized boryl radical when treated
with benzoquinone or benzoyl peroxide.