Enantioselective Hydrogenation Of Prochiral Carbonyl Compounds On Chirally-modified Pt/Support Catalysts

Enantiomerically pure alcohols can be synthesized through the enantioselective hydrogenations of prochiral carbonyl compounds, and they are widely used in the synthesis of pharmaceuticals, pesticides, flavors and other fine chemicals. Heterogeneous chiral catalysis has received much attention in past decade. Cinchonidine-modified Pt/γ-Al2O3 catalysts show excellent performance in the enantioselective hydrogenations of α-ketoesters. In order to further understand the key factors of Pt/γ-Al2O3 catalysts, particularly the Pt particle size and its interaction with support, we studied the chirally catalytic performance of Pt/γ-Al2O3 catalysts prepared by different methods for the enantioselective hydrogenations.In the first part, Pt/γ-Al2O3 catalysts were prepared using different methods and characterized by XRD, TEM and CO chemisorption. The Pt particle sizes range in 2.4-6.3 ran for 5.0% Pt/γ-Al2O3 catalysts prepared by Dryness method. The average Pt particles sizes are in the range of 3.9-24.4 nm for 5.0% Pt/γ-Al2O3 catalysts prepared by Wetness method. The catalysts were also characterized by FT-IR spectroscopy using CO as a probe molecule before and after the chiral modification with cinchonidine. Two IR bands (2077 and 2060 cm-1) are observed due to CO linearly adsorbed on the Pt/γ-Al2O3 catalyst calcined at 500 ℃ before reduction, indicating that the strong Pt-alumina interaction is produced during the calcination at 500 ℃. Whereas only one IR band at ~ 2070 cm'1 is observed for other catalysts. This suggests that Pt/y-Al2O3 catalysts prepared by Dryness method have much stronger interaction between Pt and γ-Al2O3 than those prepared by Wetness method. A red shift of the IR band is observed after the chiral modification of the catalysts, due to the electron donation from cinchonidine to Pt.In the second part, the catalytic performance of the cinchonidine-modified Pt/γ-Al2O3 catalysts was tested for the enantioselective hydrogenations of ethyl pyruvate and ethyl 2-oxo-4-phenylbutyrate (EOPB). The chirally modified Pt/γ-Al2O3 catalysts prepared by Wetness method give much lower performance than those obtained on Pt/γ-A2O3 catalysts prepared by Dryness method in theenantioselective hydrogenation of ethyl pyruvate. The enantioselectivity as high as 95% ee is achieved for this reaction under the optimized conditions. Comparable performance of the chirally modified Pt/γ-Al2O3 catalysts prepared by different methods is obtained in the enantioselective hydrogenation of EOPB and about 83% ee was achieved under the optimal conditions.It is concluded that the interaction of platinum with γ-Al2O3 is a crucial factor for obtaining high activity and that the adsorption abilities (adsorption of reactant, solvent and chiral modifier molecules) of the catalyst surface affect the catalytic performance significantly. While the Pt particle size is important but not the key factor to influence the catalytic performance. The differences between the enantioselective hydrogenations of ethyl pyruvate and EOPB may be caused by the different reaction pathways and kinetics. The enantioselective hydrogenation of EOPB is probably less structure-sensitive than the asymmetric hydrogenation of ethyl pyruvate, and the latter reaction is influenced more significantly by surface properties of Pt/γ-Al2O3 catalysts.In the third part, in order to broaden the support types and to further investigate the effect of the interaction between Pt and support on the chirally catalytic performance, Pt catalysts supported on SiO2, MCM-41 and SBA-15 were prepared. The catalytic performance of these Pt catalysts are much lower than those obtained for Pt/γ-Al2O3 catalysts in the enantioselective hydrogenation of EOPB. Both Rh and Ru catalysts supported on γ-Al2O3 show inferior catalytic results to Pt/γ-Al2O3 catalysts in this reaction. (R)-(+)-BINAP modified Pt/γ-Al2O3 catalyst exhibits very low activity and no chiral induction for the EOPB hydrogenation. Not only the catalytic activity is much lower, but the chiral induction of cinchonidine disappears for the cinchonidine modified Pt/γ-Al2O3 in this reaction after a little TPP is added. Low catalytic performance is obtained on chirally modified Pt/γ-Al2O3 catalysts in the asymmetric hydrogenations of acetophenone and 2-acetylpyridine.