Regioselective Photoisomerization/C-C Bond Formation of Asymmetric B(ppy)(Mes)(Ar): The Role of the Aryl Groups on Boron.

Asymmetric N,C-chelate organoboron compounds bearing two different aryl groups at the boron center undergo photoisomerization reactions that involve exclusively the less bulky aryl group, generating various strongly colored "dark isomers". These species thermally isomerize to 4bH-azaborepin molecules by direct hydrogen atom transfer from a borirane cycle to the pyridyl moiety and ring expansion. Mechanistic insight into these highly regioselective transformations was obtained from kinetic data and through computational studies.