ELECTROCATALYTIC REDUCTION OF VICINAL DIBROMIDES BY VITAMIN B12

Article abstract of DOI:10.1021/j100321a023

Alkyl vicinal dibromides are reduced by cob(I)alamin generated at a glassy carbon cathode in weakly acidic aqueous acetonitrile in a two-electron electrocatalytic cycle with a lowering in overpotential of 0.84 V compared to direct reduction.Unlike reactions of primary alkyl halides with cob(I)alamin that yield stable alkylcobalamins, 1,2-dibromobutane and 1,2-dibromoethane are reduced completely to alkenes at potentials somewhat positive of the E⁰' of Co(II)/Co(I).Spectroelectrochemistry showed that the reaction between Co(I) and 1,2-dibromobutane is spontaneous and regenerates Co(II).Analysis of voltammetric data by digital simulation/nonlinear regression gave a rate constant of 6*1E6 L mol^-1 s^-1 for this rate-determining reaction.A previously developed simulation model for second-order kinetics was used, extended to include intermediate rates of heterogeneous charge transfer and a difference in diffusion coefficients for substrate and catalyst.Intermediacy of a (β-bromoalkyl)cobalamin is likely but was kinetically invisible on the millisecond time scale of our experiments.At large excess of substrate, trace crossing observed in cyclic voltammograms may suggest formation of a readily reducible bromobutyl radical in an autocatalytic process.