Asymmetric hydrogenation on platinum: nonlinear effect of coadsorbed cinchona alkaloids on enantiodifferentiation

Prominent nonlinear effects in enantioselectivity were observed with a transient technique when ethyl pyruvate was hydrogenated over Pt/Al2O3 in the presence of two cinchona alkaloids, which alone afford the opposite enantiomers of ethyl lactate in excess. The changes in reaction rate and ee, detected after injection of the second alkaloid, varied strongly with type and amount of the alkaloid, and with the order of their addition to the reaction mixture. For example, under ambient conditions in acetic acid cinchonidine (CD) afforded 90% ee to (R)-ethyl lactate and addition of equimolar amount of quinidine (QD) reduced the ee to (R)-ethyl lactate only to 88%, though QD alone provided 94% ee to (S)-lactate in a slightly faster reaction. The stronger adsorption of CD on Pt in the presence of hydrogen and acetic acid was proved by UV–vis spectroscopy. The different adsorption strengths result in an enrichment of CD on the Pt surface and also in a crucial difference in the dominant adsorption geometries. CD is assumed to adsorb preferentially via the quinoline rings laying approximately parallel to the Pt surface. In this position it can interact with ethyl pyruvate during hydrogen uptake and control the enantioselectivity. The weaker adsorbing QD adopts mainly a position with the quinoline plane being tilted relative to the Pt surface and these species are not involved in the enantioselective reaction. Competing hydrogenation of the alkaloid, and steric and electronic interactions among the adsorbed species, can also influence the alkaloid efficiency and the product distribution. Hydrogenation of the quinoline rings at low alkaloid concentration resulted in unprecedented swings in the enantiomeric excess.