Structural requirements for substrate in highly enantioselective hydrogenation over the cinchonidine-modified Pd/C

Relationship between substrate structure and enantioselectivity is studied for the asymmetric hydrogenation of 42 different (E)-image-disubstituted acrylic acids (propenoic acids) over cinchonidine-modified Pd/C. The β-phenyl group is indispensable for high enantioselectivity of α-phenylcinnamic acid (2,3-diphenylpropenoic acid, 81% ee), and substitution on this group affects markedly the selectivity. The high ee up to 92% was achieved by the image-alkoxyphenyl substitution, and the selectivity is ascribed mainly to stronger interaction of the substrate with the chiral modifier on the catalyst surface. In contrast, substitution on the α-phenyl group does not affect notably the enantioselectivity (80–82% ee) or even the α-phenyl group itself is not indispensable but replaceable with a properly bulky group for the high enantioselectivity.