Anomalous Ligand Acceleration on Cinchona-modified Pd/C during Asymmetric Hydrogenation of Properly Substituted Phenylcinnamic Acid

The hydrogenation of 4,4'-dimethoxy-alpha-phenylcinnamic acid over Pd/C showed a large ligand acceleration effect when a cinchonidine modifier was used; the hydrogen rate increased to 370% relative to the reaction rate on unmodified Pd/C and resulted in a 91% enantiomeric excess (ee). Another good substrate, 4-fluoro-2'-methoxyphenylcinnamic acid, also resulted in a high ee value of 92%; however, the rate increased only to 230% when this modifier was used. On the basis of the difference in the degree to which the reaction rate was accelerated, the ee(max) for each substrate was analyzed.