Enantioselective hydrogenation of ethyl pyruvate on tin promoted Pt/Al2O3 catalysts

The enantioselective hydrogenation of ethyl pyruvate has been studied on a Pt/Al₂O₃-dihydrocinchonidine catalyst promoted with different amount of tin. The surface reaction between hydrogen adsorbed on Pt and tin tetraalkyls is used for the tin introduction. This reaction leads to the formation of surface organometallic complexes (I), with SnR_(4-x) moieties anchored to the platinum surface. The enantioselectivity of the Pt/Al₂O₃-dihydrocinchonidine catalyst is found to change only slightly upon promotion with tin, while the rate of ethyl pyruvate hydrogenation depends strongly on the amount and the form of tin introduced. The hydrogenation activity is suppressed completely at relatively low tin coverage (Sn/Pt_s < 0.06). The highest hydrogenation rate is measured over catalysts containing complex (I) (Sn/Pt_s = 0.025) on the platinum surface. On Sn-Pt alloy type active sites, which are formed after decomposition of (I) in hydrogen, the rate of hydrogation is considerably lower than on the unpromoted reference Pt/Al₂O₃ catalyst.On leave from the Central Research Institute for Chemistry of the Hungarian Academy of Sciences.