Hydrogenation of (E)-2-methyl-2-butenoic acid over cinchona-modified Pd catalyst in the presence of achiral amines: Solvent and modifier effect

• The hydrogenation of 2-methyl-2-butenoic acid was studied over Pd-cinchona alkaloid.
• The effect of dilution showed the involvement of acid dimers.
• Amine additives altered the cinchonidine concentration effect on the ee.
• The results indicted the participation of the amine in the surface reaction.
• Possible structures of the intermediate are suggested.

The effect of the solvent, modifier structure and concentration on the enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid over Pd/Al2O3 modified by cinchona alkaloids was influenced by the addition of achiral amines to the reaction slurry. The solvent dependence in the presence of achiral amines showed that additives are involved in the rate determinant step of the reaction, whereas the effect of the dilution pointed to the presence of the acid dimers in the intermediate responsible for enantioselection. The dependence of the enantioselectivity on the cinchonidine concentration in the presence of amines and results obtained using cinchona derivatives and mixtures thereof were in line with our earlier assumptions related to the participation of the amine additive in the formation of the surface intermediate. Based on these and previously published results possible structures of this intermediate are suggested.

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