Induction period for the enantioselective hydrogenation of α,β-unsaturated acids over a cinchonidine-modified palladium catalyst

The enantioselective hydrogenation of α-phenylcinnamic acid (PCA) was performed using a cinchonidine (CD)-modified Pd/C catalyst, and the conversion and enantiomeric excess (ee) of the product were monitored during the reaction. Low ee values were observed only during the initial stages of the reaction (<20 % conversion) in the presence of benzylamine as reported, and the use of the 3-ethyl-analogue instead of 3-vinyl natural CD as a modifier resulted in no decline in ee despite the presence of amine. Because the 3-ethyl moiety was generated in situ during the modification process prior to the reaction of PCA, the role of the 3-substituent was unclear, as is the case with Pt/ketone hydrogenation systems. To determine the reasons for the induction, the initial stage of the reaction was mimicked using a very small amount of substrate (stoichiometric catalysis). During this study, a minimum quantity of benzylamine additive was found to be necessary to increase the ee and the reaction rate, indicating that benzylamine functioned during the induction period.