Exhaustive Reduction of Esters, Carboxylic Acids and Carbamates to Methyl Groups Catalyzed by Boronic Acids

Carboxy to methyl reduction is an important transformation in organic synthesis, yet existing methodologies often require multi-step procedures or use hazardous metal hydrides. Herein, a metal-free catalytic system is reported for the one-step reduction of esters, carboxylic acids, and carbamates to a methyl group, in the presence of catalytic amounts of boronic acids. By using ammonia borane as a hydrogen donor, a wide range of products bearing different functional groups can be obtained in high yields under relatively mild conditions. Mechanistic studies and control experiments elucidate the complexity of the mechanism and provide an explanation for the observed selectivity. The multi-step reduction of carboxy groups to methyl groups commonly used in organic synthesis is simplified by a new metal-free catalyst system. It uses boronic acids to catalyze the single-step reduction of esters, carboxylic acids and carbamates to methyl groups. High-yield, multifunctional product formation is achieved under mild conditions using ammonia borane as the hydrogen donor.image