Chiral Oxazoline-NHC Ligands with and without CR2 Bridges: A Comparative Study in Rhodium Hydrosilylation Catalysis

A series of bidentate oxazoline-NHC ligands has been synthesized in which the two heterocycles are connected by a CH2 linker. The corresponding rhodium(I) complexes were prepared by direct deprotonation of the imidazolium halide salts followed by the addition of a solution of [Rh(nbd)Cl](2) at low temperature. The cationic square planar rhodium complexes were generated by halide abstraction via addition of an excess of KPF6 in a CH2Cl2/water solvent system. Alternatively, the deprotonation of the imidazolium hexafluorophosphates and reaction with [Rh(nbd)Cl](2) directly gave the complex cations. These as well as oxazoline-NHC systems, in which the two heterocycles are directly connected or through a CMe2 bridge, were investigated in the rhodium-catalyzed hydrosilylation of acetophenone. The comparison of the three ligand families showed that the catalysts obtained by direct coupling of oxazolines and N-heterocyclic carbenes, generating highly rigid chelate ligands, remain the most efficient systems giving the secondary alcohols in high enantio-selectivity. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)