Inversion of enantioselectivity for the hydrogenation of ethyl pyruvate in the gas-phase over Pt/SiO 2 modified with derivatives of hydroquinidine

Abstract

Ethyl pyruvate was hydrogenated as a gas-phase reactant over 2.5% Pt/SiO 2 premodified with hydroquinidine 4-chlorobenzoate at two concentrations (0.85 and 8.5 mM g −1 cat). The sense of enantioselectivity changed as a function of the modifier concentration. At low modifier concentrations, the ( S)-lactate is the preferred product, and at higher concentrations, ( R)-lactate is favoured. The lower concentration yielded an enantiomeric excess (ee) of 15% ( S)-ethyl lactate in contrast to 17% ( R)-lactate for the higher concentration. A range of hydroquinidine and hydroquinine derivatives were evaluated at these concentrations; the results suggest that the carbonyl group of the ester linkage at the C(9) position was required to promote the observed inversion of enantioselectivity. In a subsequent set of experiments, Bi 3+ was adsorbed onto the Pt/SiO 2 catalyst before the premodification step; these ions are considered to adsorb preferentially on high-energy surface sites (e.g., corner sites or step edges). Increasing Bi 3+ concentration decreased the degree of inversion until the effect was lost. On the basis of high-resolution electron microscopy (HREM) comparisons of these Pt/SiO 2 and Bi–Pt/SiO 2 catalysts, the inversion effect is discussed in terms of the interaction of the substrate and modifier with the catalyst surface.