Chiral Phosphino(sulfinylmethyl)triarylphosphonium Ylide Ligands: Rhodium Complexes and Catalytic Properties

A novel phosphine−phosphonium ylide ligand bearing a chiral sulfinyl moiety was prepared by reaction of (o-diphenylphosphinophenyl)diphenylphosphonium methylide with (S)-menthyl p-tolylsulfinate. Reaction of this ylide with [Rh(cod)2][PF6] gave stable cationic disymmetrically P,C-chelated rhodium complexes with an asymmetric ylidic carbon atom anchored to the metal center. The configuration of this carbon is controlled by the S-configuration of the adjacent sulfinyl group. The stereoselectivity of the complexation is reversed from 9:1 at 20 °C to 1:9 at −45 °C. An X-ray diffraction analysis of the thermodynamic complex shows that the stereoselectivity is not directed by chelation of the SO group which lies at a nonbonding distance from the rhodium center. In the presence of triethylamine, epimerization occurs via a putative neutral P,C-chelated complex bearing an yldiide ligand. In the presence of HPF6 and PPh3, cleavage of the ylidic carbon−rhodium bond takes place simultaneously with displacement of the phosphino−phosphonium ligand by PPh3. The phosphine end of the ligand is intended to preserve at least one phosphine−rhodium bond, which is a common feature of all the rhodium catalysts derived from the Wilkinson complex. Indeed, we found that the phosphino−phosphonium ylide complexes are activethough poorly enantioselectiveas catalysts for hydrogenation of (Z)-α-acetamidocinnamic acid and hydrosilylation of acetophenone.