The solvent effect in the enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid with cinchonidine doped Pd/Al2O3

The hydrogenation of (E)-2-methyl-2-butenoic acid (tiglic acid), using a commercial 5 wt% Pd/Al2O3 (Engelhard) doped with cinchonidine (CD) was investigated in depth to evidence the nature of the interaction CD:acid and the specific role of the solvent polarity on the enantiomeric excess (e.e.). A linear correlation between the dielectric constant of the solvent (ɛ) and the e.e. was observed, with a decrease in e.e. by increasing ɛ. The highest enantiomeric excess (e.e. = 48%) was obtained with n-hexane.