Kinetics and mechanism of the reaction of cyanocobalamin with potassium hydroxide in non-aqueous media

Abstract

The reaction of cyanocobalamin (CNCbl) with potassium hydroxide (KOH) was studied in isopropyl alcohol (iPrOH) and dimethyl sulfoxide (DMSO) under anaerobic conditions. The reaction occurs in a few steps. In the first step, the deprotonation of CNCbl by KOH proceeds to form the cyanocobalamin carbanion (CNCbl⁻) and water (H₂O). The rate of this step increases in the following order of solvents: iPrOH, DMSO. The second step occurs in two parallel paths. The contribution of each path depends on [KOH]. In the absence of a KOH excess, the carbanion CNCbl⁻ decomposes to form dehydrocyanocobalamin containing a lactone ring at the C8 position. With an excess of KOH, the carbanion CNCbl⁻ reacts with a second molecule of KOH to form dehydrocyanocobalamin without a lactone ring. The accumulation of a super-reduced Co¹⁺ and hydroxyl radical (OH˙) precedes the final products.