Enantioselective hydrogenation in water catalysed by rhodium phosphine complexes bound to polyacrylic acid

The coupling of (2S,4S)-4-diphenylpho sphino-2-diphenylphosphino-methylpyrrolidine (PPM) to polyacrylic acid demonstrates an easy way to prepare a water-soluble analogue of the parent ligand. The water solubility of the macromolecular ligand is high and this property can be varied by changing the phosphine to carboxylate ratio on the polymer. Reaction of the macromolecular ligand with bis-norbornadienerhodium(I) triflate [Rh(NBD)(2)]OTf yield a polymer-bound cationic rhodium phosphine complex catalyst which is active in enantioselective hydrogenation in water or under biphasic conditions (H2O:EtOAc). Hydrogenations of different prochiral enamide precursors give the corresponding aminoacid derivative in moderate to good ee:s, the best enantioselectivity is obtained for alpha-acetamido cinnamic acid (1a) giving N-acetyl-(R)phenylalanine (2a) in 89% ee. The enantioselectivity is dependent on the phosphorus loading on the polymer being high at low loading and low at high loading. The enantioselectivity of the catalysts decreases by increasing H-2 pressure up to 22 bar where it remains constant. The aqueous catalysts solutions are easily recovered and recycled by phase separation with no loss in enantioselectivity. (C) 1999 Elsevier Science B.V. All rights reserved.