Heterogeneous asymmetric reactions, 33. Novel interpretation of the enantioselective hydrogenation of a-ketoesters on Pt/alumina-cinchona alkaloid catalyst system

The enantioselective hydrogenation of ethyl pyruvate (EtPy) was studied in toluene and in acetic acid, under identical reaction conditions (H2pressure 1 bar, Pt-alumina catalyst E 4759, dihydrocinchonidine (DHCD) concentration 0.001- 0.1 mmol/L). The DHCD concentration necessary for achieving maximal enantioselectivity (i.e. 80% ee in toluene and 90% ee in acetic acid) is higher by one order of magnitude in toluene than in acetic acid. This relatively high difference suggests a difference in reaction mechanism. This study calls attention to the formation of new chiral surface sites via chemisorption of DHCD on platinum atoms and the possible role of such sites in enantioselection.