Catalytic and structural studies of the rhodium(I) complexes of the norphos and renorphos ligands

Article abstract of DOI:10.1021/ic50225a007

Previously reported (-)-(R,R)-2-exo-3-endo-bis(diphenylphosphino)bicyclo[2.2.1]heptene (norphos) and its reduced congener (-)-(R,R)-2-exo-3-endo-bis(diphenylphosphino)bicyclo[2.2.1]heptane (renorphos) have been studied as chiral ligands for the Rh(I)-catalyzed reductions of prochiral substrates. It was found that reduction of the norphos to renorphos ligand occurs upon catalyst activation so that the ligand on the operating catalyst is renorphos, regardless of the precursor. High hydrogenation ee's (95%) were obtained with two enamide substrates but only moderate (63%) for itaconic acid. Crystal structure data at -100°C: a = 15.765 (8) A?, b = 20.482 (9) A?, c = 11.317 (4) A?, orthorhombic, P2₁2₁2₁ (No. 16), Z = 4, R = 0.047, R_w = 0.048, absolute configuration determined, 3944 reflections with I > 2.5σ_I. X-ray data were collected at -100°C on a Syntex P2₁ autodiffractometer with monochromated Mo Kα radiation. The norphos ligand chelates the Rh(I) center although it must suffer severe distortions in order to do so. The largest distortion is the change in dihedral angle PCCP from 120 to 64°. The bicyclo[2.2.1]heptene skeleton absorbs this drastic change in dihedral angle in smaller changes in dihedral and bond angles, as well as bond lengths.