Highly Enantioselective Hydrogenation of Enamides and Itaconic Acid in Water in the Presence of Water-Soluble Rhodium(I) Catalyst and Sodium Dodecyl Sulfate

The water-soluble cationic Rh complexes, such as [Rh(α- d -glucopyranosyl-(1,1)-2,3-di- O -(diphenylphosphino)-α- d -glucopyranoside)(cod)]BF 4 ( 1 ) and [Rh(β- d -glucopyranosyl-(1,1)-2,3-di- O -(diphenylphosphino)-β- d -glucopyranoside)(cod)]BF 4 ( 2 ) bearing free hydroxy groups, are effective catalysts in the asymmetric hydrogenation of various enamides and itaconic acid (up to 99.9% ee) in water in the presence of sodium dodecyl sulfate (SDS) (7.5 × 10 -3 to 1.0 × 10 -1 M). The hydrogenation of methyl ( Z )-α-acetamidocinnamate in water using the corresponding Rh complexes of triflate ( 6 ) and d -camphor-10-sulfonate ( 7 ), prepared separately, results in a formation of the product of the same enantioselectivity (48% ee), but the use of SDS can reduce the amount of the catalyst and improve the enantioselectivity to 81% ee. These results show that the counteranions do not directly influence the enantioselectivity. Although the effect of SDS on the enhancement of enantioselectivity remains speculative, the formation of micelle seems to play an important role in improving the enantioselectivity.