Enantioselective hydrogenation of α-ketoesters: Preparation and catalytic behavior of different alumina-supported platinum catalysts modified with cinchonidine

Abstract

The enantioselective hydrogenation of ethyl pyruvate has been investigated over different alumina-supported platinum catalysts modified with cinchonidine. The influence of various preparation parameters of the catalysts on their platinum dispersion and hydrogenation activity and enantioselectivity is described. The following preparation variables were investigated: support material, platinum precursor, platinum loading and reduction procedure. The platinum salt used to impregnate the supports had a strong influence on the catalyst performance. Highest enantioselectivities were obtained with catalysts prepared with H 2PtCl 6 and K 2PtCl 6. The type of support and the platinum loading also had a noticeable effect. Good results were obtained with aluminas prepared from AlCl 3 with platinum contents > 2%. The reduction procedure used was found to be the most critical preparation variable. Catalysts with highest enantioselectivity were obtained by reducing the catalyst precursor in aqueous solution with Na(HCOO), K(HCOO) or CH 2O. Catalysts obtained by reduction with hydrogen exhibited generally high dispersions but low enantioselectivities. Applying these different preparation procedures, it was possible to vary the platinum dispersions between < 0.05 and 0.78. The enantioselectivity of the ethyl pyruvate hydrogenation increased with increasing platinum particle size, reaching 80% with platinum particles larger than about 3–4 nm in diameter. These studies reveal an interesting interrelationship between optical yield and turnover frequency for the different catalysts. In general, good enantioselectivity is observed for catalysts with high turnover frequency.