Enantioselective hydrogenation of aromatic ketones over cinchona-modified rhodium: a new opportunity?

Alumina-supported rhodium modified with cinchonidine has been investigated with regard to its applicability in the enantioselective hydrogenation of various aromatic ketones possessing an α-hydroxy or α-methoxy group. The study revealed that depending on the substrate, rhodium can outperform the catalytic behavior of platinum. With one of the substrates, 2-hydroxy-1-(4-methoxy-phenyl)-ethanone (4), an enantiomeric excess (ee) of 80% at 89% conversion was reached, which is the highest ee reported so far for chirally modified rhodium. However, completely different conditions are required to achieve optimal catalytic performance with rhodium, compared with platinum. Rhodium requires a much higher modifier concentration, and high hydrogen pressure is favorable. The higher modifier concentration required is traced to the much higher activity of rhodium for the hydrogenation of the quinoline ring, which is assumed to be the anchoring moiety of the cinchona modifiers on the platinum group metals. Changing the modifier from cinchonidine to O-phenoxy-cinchonidine resulted in a switch of the major enantiomer of the product, as exemplified for 2-hydroxyacetophenone (1), which showed a switch from 73% ee in favor of the (R)-product to 68% ee for the (S)-product when the modifier was changed from cinchonidine to O-phenoxy-cinchonidine.