New insights into the relationship between conversion and enantioselectivity for the asymmetric hydrogenation of alkyl pyruvate

The initial transient period in the enantioselective hydrogenation of alkyl pyruvate esters is probed using the sequential reactions of ethyl and methyl pyruvate. The reaction of methyl pyruvate, subsequent to the hydrogenation of ethyl pyruvate, led to a higher e.e. when compared to the coreaction of these reactants, or prehydrogenation with methyl pyruvate followed by reaction of ethyl pyruvate. The initial transient effect, in which e.e. increases with conversion, is observed in both periods of the sequential reaction and the origin of this effect is discussed. (C) 2003 Elsevier Inc. All rights reserved.