Heterogeneous Enantioselective Hydrogenation of Hydroxy-substituted ( E)-2,3-diphenylpropenoic Acids over Pd/AlO Modified by Cinchonidine.

The enantioselective hydrogenation of ( E)-2,3-diphenylpropenoic acids substituted by hydroxyl group has been studied over Pd/AlO catalyst modified by cinchonidine. The effect of the acidic hydroxyl substituents was compared with that of the methoxy group in the same position. The para-hydroxyl substituent on the 3-phenyl ring had similar effect on the enantioselectivity as the methoxy group, whereas the meta positioned decreased the optical purity of the saturated acid. This was explained by different origin of the increase in the enantioselectivity obtained in the presence of electron releasing substituents in these positions. Although, the para-hydroxyl group on the 2-phenyl ring had beneficial influence on the enantioselectivity of the hydrogenation of the mono-substituted acid, in the presence of fluorine or hydroxyl group on the 3-phenyl ring the effect of the two substituents was not additive. This study demonstrated that the cinchonidine-modified Pd catalyst is appropriate for the preparation of several hydroxy-substituted 2,3-diphenylpropionic acids in good optical purities, extending the scope of this catalytic system to new types of versatile chiral building blocks. Graphical Abstract: Heterogeneous Enantioselective Hydrogenation of Hydroxy-substituted ( E)-2,3-diphenylpropenoic Acids over Pd/AlO Modified by Cinchonidine.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]