Chiral Bidentate (Phosphinophenyl)benzoxazine Ligands in Asymmetric Catalysis

Abstract

Dedicated to the memory of Prof. Luigi M. VenanziThe new chiral bidentate (phosphinoaryl)benzoxazine ligands 2 were applied in asymmetric catalysis. Rhodium and copper complexes catalyzed the hydrosilylation of acetophenone and [4 2] cycloadditions with moderate enantioselectivity. Iridium complexes were used to hyrogenate di-, tri-, and tetrasubstituted alkenes, giving products with moderate to high enantiomer excesses. Enantioselective allylic substitution and Heck reactions catalyzed by [(phosphinoaryl)benzoxazine]palladium complexes occurred with high enantioselectivities. The results were similar to those obtained with the corresponding dihydro(phosphinoaryl)oxazole ligands. Comparison of the structures of (diphenylallyl)(benzoxazine)palladium and (diphenylallyl)(dihydrooxazole)palladium complexes showed that the coordination geometries and the chiral environments of the metal centers are very similar, which explains why the enantioselectivities induced by the two ligand classes are in the same range. Scheme 21 ) It has been previously shown that, although the use of TFPB À as counterion results in higher catalyst activity and stability compared to PF 6 À , the enantioselectivity is not influenced by the counterion [3]. Scheme 3Scheme 6