The effect of several organic acids and bases used as solvents and additives for the enantioselective hydrogenation of alpha-ketoesters with cinchona-modified platinum catalysts is described. Carboxylic acids have a positive influence on the enantioselectivity of the reaction, both as solvents and as additives. The best results are observed in acetic acid where the optical yields increase between 8 and 12% compared to toluene or ethanol as solvents. In addition, extremely low modifier concentrations are necessary. For the hydrogenation of ethyl pyruvate, using a special wide pore Pt/Al2O3 catalyst and 10,11-dihydro-O-methylcinchonidine as modifier, we obtained 95% enantioselectivity, the highest value ever reported for any chiral heterogeneous catalyst. Bases are not suitable as solvents but in some cases they have a beneficial effect as additives at low concentration.
