Achiral amine additives in the enantioselective hydrogenation of aliphatic α,β-unsaturated acids over cinchonidine-modified Pd/Al2O3 catalyst

The effect of the achiral amine additive structure was studied on the enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid and (E)-2-methyl-2-hexenoic acid over Pd/Al2O3 catalyst modified by cinchonidine. It was found that secondary amines are similarly or even more efficient in increasing the enantioselectivity as primary amines. The right basicity of the amine, which may vary in a rather wide range, should be coupled with appropriate steric properties to be effective in increasing the enantioselectivity. The best performing amines was the earlier widely used benzylamine and N-methylbenzylamine. The influence of these amines on the effect of the catalyst amount, H2 pressure and reaction temperature was studied. The effect of the amine amount on the hydrogenation of the two acids was also investigated. Based on the results the involvement of the amine additive in the formation of surface intermediates responsible for enantioselection is proposed. Decrease of the reaction temperature to 273 K increased the enantiomeric excess in the presence of amines, resulting in the hydrogenation of (E)-2-methyl-2-hexenoic acid in up to 71% enantioselectivity in favour of the S product enantiomer, the highest value obtained until now in the enantioselective hydrogenation of aliphatic unsaturated carboxylic acids over chirally modified heterogeneous catalyst.