THE MECHANISM OF ASYMMETRIC HOMOGENEOUS HYDROGENATION. SOLVENT COMPLEXES AND DIHYDRIDES FROM RHODIUM DIPHOSPHINE PRECURSORS

Article abstract of DOI:10.1016/S0022-328X(00)85766-6

Bicyclo<2,2,1>hepta-2,5-diene and cycloocta-1,5 diene(biphosphine)rhodium tetrafluoroborates react with hydrogen at 1 atmosphere in methanol or other polar solvents.The initial product may be either a solvated dihydride or a solvate; depending on phosphine structure the equilibrium between these two species varies widely.Dihydrides are normally the stable product when the ligand is a monophosphine although (o-methoxyphenyl)methylphenylphosphine is an exception. cis-Chelating biphosphines normally form solvate complexes with no affinity for hydrogen.R-Phenyl bis-diphenylphosphinoethane falls into this category, but the 31P NMR spectra of its complexes demonstrate an equilibrium between monomeric and dimeric species, and addition of triethylamine gives rise to a trimer. trans-Chelating biphosphines show more variable behaviour, and in the case of bis-1,5-diphenylphosphinopentane, a number of complexes, including one requiring C-H activation, are observed.