Enantioselective hydrogenation of itaconic acid over cinchona alkaloid modified supported palladium catalyst

Abstract

The enantioselective hydrogenation of itaconic acid was studied over cinchona alkaloid-modified supported palladium catalyst in the presence of benzylamine as achiral base additive. The effect of several reaction parameters was investigated, such as the addition order of the additive, modifier and itaconic acid, hydrogen pressure, reaction temperature, amount of additive and modifier. Enantioselectivities up to 58% could be obtained by setting the appropriate conditions. The above results along with studies on the structure of the achiral amine additive, the chiral modifier and the substrate led to an interpretation of the increased enantioselection observed in the hydrogenation of itaconic acid in presence of benzylamine. According to this benzylamine plays role both on the surface of the catalyst and in the liquid phase by transforming the acid in bis-benzylammonium salt. Due to the salt formation the interaction with cinchonidine occurred through the conjugated carboxylic group, which led to increased enantioselectivity. Based on modifications in the substrate structure the effect of the unconjugated carboxylic group was also discussed.