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.

NHC induced radical formationviahomolytic cleavage of B–B bonds and its role in organic reactions

“…Initially, the groups of Suginome and Ohmura reported the transition metal-free diboration of 1,4-dihydropyrazine derivatives and 4,4′-bipyridine, respectively, simply by reaction with B 2 pin 2 at elevated temperatures. 9,10 While they initially proposed a mechanism proceeding via a nucleophilic pathway, 9 a their more recent reports 9 b ,10 confirm the role of radicals and the homolytic cleavage of the B–B bond in these processes.…”