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 reac tion was catalyzed by Pt/alumina modified with O-methyl-cinchonidine. Chang es in reaction rate, ee, and composition in solution were followed during r eaction, using kinetic analyses and NMR. The conversion dependence of ee an d the effect of preequilibration of the reaction mixture are attributed to the competing hydrogenation of the keto-carbonyl group and the hydrogenolys is of the C-OR bond of the racemic hemiketal formed. We propose that the re action route via the hemiketal is important because the hemiketal/keto mola r ratio is high (up to ca. 500 after equilibration) and the hemiketal posse sses sufficient reactivity toward hydrogenolysis over Pt. This reaction pat hway is less important in the enantioselective hydrogenation of ethyl pyruv ate, an activated ketone often used as a model reactant. (C) 2001 Academic Press.