Effect of palladium dispersion on the enantioselective hydrogenation of alpha,beta-unsaturated acids with modified Pd/TiO2 catalysts

The enantioselective hydrogenation of the C=C double bonds in (E)-2-methyl- 2-butenoic acid, (E)-2-methyl-2-pentenoic acid, and (E)-2,3-diphenyl-2-prop enoic acid has been studied using cinchonidine-modified Pd/TiO2 catalysts w ith different metal loadings of between 0.5 to 30 wt%. For all of the subst rates examined, those catalysts with degrees of Pd dispersion lower than 0. 2 have been shown to be rather detrimental for obtaining a high enantiosele ctivity, as opposed to the tendencies reported for the hydrogenations of ke to esters with modified Pt and Ni systems. The hydrogenations of the alipha tic acids are almost structure insensitive with a slight decrease in the en antioselectivity at Pd dispersions lower than 0.2, which is in great contra st to the hydrogenation of the aromatic acid. A similar difference in the s tructure sensitivity has been observed with catalysts modified by a larger modifier, (-)-dihydroapovineaminic acid ethyl ester. The influence of the s ubstrate structure on the adsorption mode and the strength of the modifier- substrate complex is discussed in relation to the size of the Pd ensemble o n the catalyst surface.