Flavin Trapping of Carbanion Intermediates as Catalyzed by Cyanide Ion and Cationic Micelle

Benzaldehydes and benzoylformic acids which usually undergo benzoin-type condensation in the presence of cyanide ion were easily oxidized by flavin to the corresponding carboxylic acids under ambient reaction conditions (30-50 °C). Aliphatic substrates such as formaldehyde, butyraldehyde, and pyruvic acid were also oxidized by flavin under similar reaction conditions, although the yields were lower than those of aromatic substrates. The flavin oxidation was facilitated markedly by cationic micelles. Kinetic studies established that the reaction is zero order in flavin and first order in cyanide ion and substrate, indicating the rate-limiting formation of the carbanion intermediates from the cyanide adducts followed by the rapid oxidation by flavin. The results show that flavin is able to trap the transient carbanion intermediates to give the oxidation products. Hence, the reaction is readily diverted from condensation to oxidation. The catalytic role of the cationic micelles was accounted for by synergistic effects such as effective concentration of reactants and local reaction environments. The relevance of these reactions to biological systems and the application to organic synthesis are discussed.