The effect of water on the enantioselective hydrogenation of ethyl pyruvate and butane-2,3-dione using cinchona-modified Pt/Al2O3

A detailed study of Pt/alumina modified with cinchona alkaloids for the catalytic enantioselective hydrogenation of ethyl pyruvate to ethyl lactate and of butane-2,3-dione to 3-hydroxybutan-2-one is reported. Catalytic and 1H NMR spectroscopic studies have been carried out on both systems to investigate the influence of water on modifier conformation and enantioselectivity. Interestingly, the presence of small amounts of water has been shown to result in an increase the proportion of the open 3 conformer of cinchonidine when it is present in dilute solutions. Increased enantioselectivity, coupled with an increase in the proportion of the open 3 conformation of cinchonidine, is observed after pretreatment of the pyruvate ester with the modifier prior to admission to the reaction vessel. In contrast, no analogous effects are observed with butane-2,3-dione. It is proposed that the enhancements in enantioselectivity are a consequence of hydrolysis of ethyl pyruvate by a process that is catalysed by the basic cinchona modifier. The pyruvic acid formed during the hydrolysis interacts with the modifier and this leads to a further enhancement in the concentration of open 3 conformer present. The results contribute to an understanding of the widely variable enantioselectivities reported in the literature for this reaction.