Kinetic Analysis of Enantioselective Hydrogenation of 2,3-(E)-Diarylpropenoic Acids over a Chiral Cinchona Alkaloid-Modified Pd/C Catalyst

摘要

Enantioselective hydrogenations of alpha,beta-unsaturated carboxylic acids over cinchona alkaloid-modified Pd metal heterogeneous catalysts have received considerable attention because of scientific importance in molecular recognition catalysis as well as feasibility of industrial applications. In the present study, comprehensive kinetic analysis of the hydrogenation was conducted to disclose the crucial kinetic parameters controlling enantiodifferentiation and reaction rate with the combinations of four kinds of modifier and three kinds of substrate. Despite simplicity of the kinetic model, the present novel kinetic formulation allows us to describe the enantioselectivity as a function of modifier concentration, to estimate intrinsic enantioselectivity at the modified sites, to estimate respective reaction rates at the modified and unmodified sites, and to establish a correlation between the magnitude of ligand acceleration and kinetic parameters. The enantioselectivity is successfully correlated to the reaction rate. The adsorption strength of the modifier on Pd is suggested to decrease in the order, cinchonidine > cinchonine > quinine > quinidine. The roles played by benzyl-ammine and the observed decrease in the selectivity at a high modifier concentration are also discussed. The kinetic model and formulation can be applied to analyze the catalytic behaviors and performance of Pt counterparts.