Zinc-Promoted Reactions. 1. Mechanism of the Clemmensen Reaction. Reduction of Benzophenone in Glacial Acetic Acid

Article abstract of DOI:10.1021/jo00013a036

The mechanism of the Clemmensen reduction of diaryl ketones was investigated by reducing benzophenone, benzhydryl chloride, and dichlorodiphenylmethane in AcOH under a variety of conditions.Besides diphenylmethane, dimeric products were isolated that were indicative of the formation of radical species.Different product distributions were obtained from reactions run under different conditions.The reduction of deuteriated benzhydryl chloride was also performed.A quite complicated mechanistic pattern, involving ionic and nonionic reactions, emerged from the experimental p icture.Two pathways, connected through the protonated substrate, were recognized.According to the first pathway the reduction is promoted by a SET from Zn to the substrate, leading to the formation of a carbon radical having one zinc atom bound to the oxygen of the carbonyl group.Benzhydryl chloride, benzhydryl acetate, and dichlorodiphenylmethane are involved in the process.The product distributions suggest the occurrence of several SETs, which involve the formation of different radical species.Ionic reactions are responsible for the second route to the reduced products.Nucleophilic displacements also participate to the complex mechanism.