A New Reaction Pathway in the Enantioselective Hydrogenation of Activated Ketones on Cinchona-Modified Platinum

The mechanism of the heterogenous enantioselective hydrogenation of ethyl-4,4,4-trifluoroacetoacetate has been studied in alcoholic solvents. The reaction was catalyzed by Pt/alumina modified with O-methyl-cinchonidine. Changes in reaction rate, ee, and composition in solution were followed during reaction, using kinetic analyses and NMR. The conversion dependence of ee and the effect of preequilibration of the reaction mixture are attributed to the competing hydrogenation of the keto-carbonyl group and the hydrogenolysis of the C–OR bond of the racemic hemiketal formed. We propose that the reaction route via the hemiketal is important because the hemiketal/keto molar ratio is high (up to ca. 500 after equilibration) and the hemiketal possesses sufficient reactivity toward hydrogenolysis over Pt. This reaction pathway is less important in the enantioselective hydrogenation of ethyl pyruvate, an activated ketone often used as a model reactant.