Mechanism of 2,6-Dichloro-4,4′-bipyridine-Catalyzed Diboration of Pyrazines Involving a Bipyridine-Stabilized Boryl Radical

Abstract The mechanism of 4,4′-bipyridine-catalyzed diboration of pyrazines was studied by experimental observation of the intermediates and by theoretical calculations. Intermediary radical species were detected by ESR measurement of the reactions of 2,6-dichloro-4,4′-bipyridines with bis(pinacolato)diboron and identified by simulation. Based on these observations, radical processes involving 4,4′-bipyridine-stabilized boryl radicals were evaluated by DFT calculations combined with single-component artificial force induced reaction (SC-AFIR). The results of calculations indicate that a radical transfer process from 4,4′-bipyridine-stabilized boryl radical to pyrazine is a major pathway in the catalytic reaction. The origin of the high catalytic efficiency of 2,6-dichloro-4,4′-bipyridine is ascribed to the effect of the chlorine atom on the stability of the corresponding N,N′-diboryl-4,4′-bipyridinylidene.