Asymmetric Hydrgenation Catalyzed by Cationic Ferrocenylphosphine Rhodium(I) Complexes and the Crystal Structure of a Catalyst Precursor

Article abstract of DOI:10.1021/ja00523a014

The asymmetric hydrogenation of acylaminocinnamic acid and acylaminoacrylic acid is catalyzed by the cationic rhodium(I) complexes <((+)- or (-)-PPFA)Rh(diene)>X<(+)- or (-)-PPFA = (S,R)- or (R,S)-α-(2-diphenylphosphinoferrocenyl)ethyldimethylamine, diene = norbornadiene or cyclooctadiene, X = ClO4, PF6, B(C6H5)4, BF4, but not all combinations>.The reactions take place in alcohol solutions at 32 deg C (pH2 = 1 atm).The cinnamic acid derivative is hydrogenated in high chemical (ca.90percent) and optical yield (ca.80percent).The acrylic acid is hydrogenated faster but the optical yields are lower. <(PPFA)Rh(NBD)>PF6 crystallizes in the space group P2₁/c with a = 9.406 (6) Angstroem, b = 29.68 (2) Angstroem, c = 11.600 (8) Angstroem, β = 94.55 (3) deg, and Z = 4.The structure was solved by heavy-atom procedures and refined to a final R₁ of 0.083.The ligand chelates to the metal center through the N and P and the geometry around the rhodium atom is square planar.Models based on this structure indicate that the diastereotopic discrimination necessary for asymmetric reduction to occur probably arises because of steric interactions between the olefin substrate and the bound PPFA ligand.This model successlully predicts the absolute configuration of the products.