Continuous enantioselective hydrogenation of activated ketones

Heterogeneous enantioselective hydrogenation of activated ketones in a fixed-bed reactor was achieved by continuous feeding of minute amounts of chiral modifier to the reactant stream. The potential of this concept is illustrated using the hydrogenation of ketopantolactone and ethyl pyruvate over Pt/alumina modified by cinchonidine. Production rates and enantiomeric excesses tee) achieved without optimization at room temperature and 40 bar were 94 mmol/g(cat).h and 83.4% ee for ketopantolactone, and 23 mmol/g(cat).h and 89.9% ee for ethyl pyruvate. Transient measurements by stopping of the cinchonidine flux indicate that continuous feeding of the modifier in ppm concentration is essential. (C) 1999 Academic Press.