| Abstract: | The enantioselective hydrogenation of methoxy‐ and fluorine‐substituted (E)‐2,3‐diphenylpropenoic acid derivatives was studied over cinchonidine‐modified, supported palladium catalysts in the absence and presence of benzylamine as additive. The fluorine substituent in the appropriate position was even more efficient than the methoxy group in increasing the optical purity of the saturated product. High enantioselectivities, up to 96%, were obtained in the hydrogenation of some disubstituted derivatives, unprecedented in the hydrogenation of prochiral unsaturated carboxylic acids over modified heterogeneous catalyst. The best optical purities were reached in the hydrogenation of derivates bearing a para‐substituent on the β phenyl and an ortho‐substituent on the α phenyl ring, respectively. The influence of the substituent on the β phenyl ring was attributed to the increase in the efficiency of the modifier‐substrate interaction by electronic effects or to a decrease in the adsorption strength of the substituted acid over modified surface sites. The beneficial effect of the ortho‐substituent on the α phenyl ring was assumed to be due to the additional interaction of this substituent with the modifier on the surface, its steric hindrance contributing only in a small part to the observed effect. These suggestions were supported by results obtained in the hydrogenations of some methyl‐substituted derivatives. |
