Asymmetric transfer hydrogenation of 2,3-diphenylpropenoic acids over heterogeneous palladium catalysts modified by cinchona alkaloids

Abstract

Asymmetric catalytic methods are among the convenient syntheses procedures of optically pure organic compounds, many of which are valuable pharmaceutical intermediates. Applying heterogeneous catalysts may contribute to their economic and environmentally benign synthesis. Among the easiest and often used methods of introducing chiral centres in molecules are the reduction of prochiral unsaturated compounds. Although, several heterogeneous catalytic systems are known in which in situ chirally-modified metals are used, such catalysts have never been applied in asymmetric transfer hydrogenations. Here we report the enantioselective transfer hydrogenations of (E)-2,3-diphenylpropenoic acids over supported palladium catalysts modified by dihydrocinchonidine carried out in ordinary laboratory glassware using easily available, cheap hydrogen donor, i.e. potassium formate. High conversions and good enantioselectivities were obtained in transfer hydrogenations of various acids substituted on phenyl rings. Results obtained with various acid derivatives and with few cinchona alkaloid modifiers indicated that the transfer hydrogenations occur through a similar surface intermediate as the enantioselective hydrogenations with molecular hydrogen. A deuterium tracer study showed low deuterium incorporation in the product possibly due to dissociation of formate and exchange with the surface mobile hydrogens, however, these processes are strongly influenced by the donor cation. The asymmetric transfer hydrogenation may be carried out at over 2 g scale, thus may be used as an advantageous laboratory synthetic procedure without applying special precautions characteristic of gaseous reactants.