Enantioselective Hydrogenation of Pyruvic Acid Oxime to Alanine on Pd/Alumina

The chemo- and enantioselective hydrogenation of pyruvic acid oxime have been studied on Pd/alumina, the latter in the presence of the 1,2-amino alcohol type alkaloids ephedrine, cinchonidine, and cinchonine. High yields of racemic alanine (90–98%) were obtained in the absence of alkaloids in polar solvents at 0–45°C and 10 bar. Enantioselection increased with higher temperature and alkaloid : oxime molar ratio. A 1 : 1 ephedrine : oxime molar ratio afforded the best enantiomeric excess (26%). The presence of alkaloid resulted in a decrease of reaction rate by a factor of up to 140, compared to the racemic hydrogenation. Based on X-ray crystal structure analysis of the alkaloid–pyruvic acid oxime adduct, a mechanism is proposed for the steric course of the reaction. Extended interactions by multiple H bonds between the adsorbed alkaloid–oxime salt units on the Pd surface is assumed to be at the origin of the moderate enantioselectivity and the very low enantioselective hydrogenation rate.