Synthesis of Chiral Bis(phosphinite) Ligands with a Tetrahydrothiophene Backbone:  Use in Asymmetric Hydrogenation

A series of novel chiral nonracemic bis(phosphinites) derived from ( R, R )- trans -2,5-tetrahydrothiophenedimethanol has been prepared. Reaction of these ligands, 6, with Rh(COD) 2 X (COD = cyclooctadiene, X = OS(O) 2 CF 3, SbF 6 ) yields rhodium complexes which have been tested in the asymmetric hydrogenation of methyl α-acetamidocinnamate, providing enantioselectivities of up to 55%. The racemic bis(diphenylphosphinite) ligand 6c binds in an unusual tridentate mode, forming the thioether-bridged species (( 6c )Rh) 2 (OTf) 2 ( 7 ). This was characterized by conventional spectroscopic methods and by single-crystal X-ray analysis.