Effect of the substituent position on the enantioselective hydrogenation of methoxy-substituted 2,3-diphenylpropenoic acids over palladium catalyst

The enantioselective hydrogenation of mono and dimethoxy-substituted 2,3-diphenylpropenoic acids has been studied over cinchonidine modified supported Pd catalyst. The hydrogenation of the six monosubstituted methoxy derivatives of (E)-2,3-diphenylpropenoic acid showed that the position of the substituent has a decisive influence on the initial reaction rate and the enantioselectivity. High enantioselectivities, 86–90%, were obtained in the hydrogenation of mono-substituted derivatives with a favourable substituent position. The results were rationalized in terms of either the electronic or the steric effects of the methoxy substituent determined by its position. These suggestions were also applicable in interpreting the results obtained in the hydrogenation of substituted (Z)-2,3-diphenylpropenoic acids and selected dimethoxy (E)-2,3-diphenylpropenoic acids. The combined steric and electronic effects of the substituents on the α- and β-phenyl rings ensured the highest enantioselectivities, up to 92% ee, in the hydrogenation of (E)-2-(2-methoxyphenyl)-3-(4-methoxyphenyl)propenoic acid.