Coadsorption of Cinchona Alkaloids on Supported Palladium: Nonlinear Effects in Asymmetric Hydrogenation and Resistance of Alkaloids Against Hydrogenation
The transient behavior of the adsorption of cinchona alkaloid modifiers on Pd/TiO2 has been investigated in situ during the enantioselective hydrogenation of 4-methoxy-6-methyl-2-pyrone (1). Modifier mixtures consisting of pairs of alkaloids that alone afford the opposite enantiomers in comparable excess were applied to probe the adsorption behavior and possible nonlinear phenomena. Complementary information has been gathered from an indirect UV-vis study of the adsorption and hydrogenation of cinchonidine and quinidine on Pd/TiO2. The striking nonlinear behavior of cinchonidine-quinidine and cinchonine-quinine pairs in the hydrogenation of 1, and in the competitive saturation of the quinoline rings of the alkaloids, is attributed to differences in the adsorption strength and geometry of the alkaloids. The results are in good agreement with our former mechanistic model assuming that the quinoline ring of cinchona alkaloid and 1 adsorb parallel to the Pd surface during the enantiodifferentiating ste
