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)2X (COD = cyclooctadiene, X = OS(O)2CF3, SbF6) 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.