RECENT ADVANCES IN THE CHEMISTRY OF VITAMIN B12 AND VITAMIN B12 MODEL COMPOUNDS: REDUCTIVE COBALT-CARBON BOND CLEAVAGE REACTIONS

Abstract

The cobalt–carbon bond in alkylcobaloximes and in alkylcobalt derivatives of related chelates is reductively cleaved by thiols in mildly acidic medium, or by carbon monoxide, dithionite and stannite in alkaline solution. The reductants interact initially by trans attack of the cobalt atom, followed by the rate-determining cleavage of the Co–C bond. Cobalt-bound methyl groups are converted into methyl carbanions, or species with the reactivity of methyl carbanions, which react with protons of the medium to form methane. In the presence of CO2 detectable amounts of acetic acid are formed, in accord with this mechanism. The reductive cleavage of organocobalt complexes, derived from vitamin B12 or vitamin B12 model compounds, by thiols is correlated with available enzymological evidence on ribonucleotide reductase of Lactobacillus leichmannii. microbial methane biosynthesis of Methanobacillus omelianskii and acetate biosynthesis by Clostridium thermoaceticum. All three enzymatic processes are envisaged to involve reductive Co–C bond cleavage reactions of coenzyme–substrate intermediates as part of the catalytic cycle.