Enantioselective Hydrogenation Using Cinchona-Modified Pt/γ-Al2O3 Catalysts: Comparison of the Reaction of Ethyl Pyruvate and Buta-2,3-dione

The enantioselective hydrogenation of buta-2,3-dione to 3-hydroxy-buta-2-one and ethyl pyruvate to ethyl lactate are compared using cinchona-modified Pt/γ-Al2O3 catalysts. The reactions were carried out in a range of solvents and both reactants gave the same linear relationship between enantiomeric excess (e.e.) and the dielectric constant of the solvent. The e.e. for the 3-hydroxy-buta-2-one is lower than that for ethyl pyruvate. For both reactants there is an optimal concentration of the cinchona modifier but the optimal concentration required for the hydrogenation of buta-2,3-dione is approximately an order of magnitude higher than that required for the hydrogenation of ethyl pyruvate. The hydrogenation of buta-2,3-dione in acetic acid as solvent leads to a suppression in the e.e. with an enhancement in rate, whereas the reaction of ethyl pyruvate in acetic acid leads to an enhancement in e.e. The e.e. for 3-hydroxy-buta-2-one formation is independent of conversion in the initial period of the reaction which contrasts markedly with the observed initial induction period for ethyl lactate formation. The results are discussed in terms of the interaction of the reactants with the cinchona alkaloid adsorbed on the Pt surface.