Radical vs Anionic Pathway in Mediated Electrochemical Reduction of Benzyl Bromide in a Bicontinuous Microemulsion

Products derived from benzyl radicals or anions were obtained by reduction of benzyl bromide mediated by vitamin B 12, Co(salen), and cobalt phthalocyaninetetrasulfonate in a bicontinuous microemulsion and homogeneous solvent. The reactions begin by electrochemical generation of a Co(I) complex (Co I L), followed by oxidative addition of benzyl bromide to give benzyl−Co III L. Reductive cleavage of the Co−C bond yields benzyl radicals or anions depending on the potential of reduction of benzyl−Co III L. Rate constants of Co−C bond formation ( k 1 ) are correlated with the formal potential E °‘ Co(II)/Co(I), indicative of control by the inherent activation free energy of the oxidative addition rather than by reactant distribution between phases in the microemulsion. Vitamin B 12 gave a benzyl−Co III L intermediate reduced at −1.1 V vs SCE and yielded bibenzyl as the sole product of a radical pathway. The reduction potential of benzyl−Co III (salen) is negative enough to reduce the benzyl radical to an anion, so reduction of benzyl bromide mediated by Co(salen) gave toluene in the microemulsion and a mixture of toluene and bibenzyl in DMF. Although Co(salen) reacts very rapidly with benzyl bromide, a slow rate of reductive cleavage of benzyl−Co III (salen) creates a bottleneck in the catalytic pathway. The one-electron catalyst vitamin B 12, with a smaller rate of oxidative addition, gives faster catalytic reduction for a given k 1 relative to the two-electron catalyst Co(salen). Thus, a radical or anionic pathway can be chosen by controlling the potential of the benzyl−Co III reduction. The facile formation of bibenzyl in the microemulsion suggests that it should be applicable to electro-organic syntheses featuring radical-based formation of carbon−carbon bonds.