Effect of coacid acidity on the cinchona-modified Pt-catalyzed enantioselective hydrogenations

The effect of acid strength of coacids on the enantioselective hydrogenation of ethyl pyruvate over cinchona-modified Pt/Al₂O₃ catalysts is described. The enantioselectivity passes through a maximum as a function of acid strenght. The best value was obtained using trifluoroacetic acid as additive, which increased the optical yields from 85% to 93% ee. Chiral acids were also studied. It was found, however, that the individual chirality of acids used did not modify the basic characteristics of the outcome of the reactions. The configuration of the products were determined by the cinchona alkaloid. On the basis of separate experiments with cinchona salts a short interpretation of acid-effect is also provided.