Solvent effect on the structure sensitivity in enantioselective hydrogenation of α, β-unsaturated acids with modified palladium catalysts

The solvent effect on the enantioselective hydrogenation of alpha,beta-unsaturated carboxylic acids has been studied using cinchonidine-modified Pd/TiO2 catalysts with different Pd dispersions. The structure sensitivity observed in the hydrogenation of (E)-2,3-diphenyl-2-propenoic acid was found to depend on the property of the solvent used, especially in the dispersion range lower than 0.2. In the solvents with low polarity, the optimal I'd dispersions are much lower than those observed in polar and protic solvents. The catalysts with the lowest dispersions, however, exhibit unexpectedly poor selectivities irrespective of the solvent employed. With the reactions of smaller aliphatic acids: (E)-2-methyl-2-pentenoic acid and (E)-2-methyl-2-butenoic acid, the I'd dispersion dependencies are much milder and independent of the solvents. These variations of the structure sensitivity could be explained by the difference in the steric interference of the metal surface with the modifier-substrate adduct in different solvents. (C) 2003 Elsevier B.V. All rights reserved.