BINAP-Ru and -Rh catalysts covalently immobilised on silica and their repeated application in asymmetric hydrogenation

Abstract

We present the facile immobilisation of a chiral diphosphine ligand, BINAP, on a silica (high pore volume, low surface area). The protected ligand has been immobilised as a phosphine oxide and deprotected on the surface to prevent side reactions of unprotected phosphines with surface silanol groups. The resulting diphosphine ligand on silica was converted to both rhodium and ruthenium complexes. The novel materials were characterised using solid-state IR-DRIFT and 29Si and 31P CP-MAS NMR techniques as well as elemental content measurements. Ruthenium and rhodium catalysed asymmetric hydrogenation of various enamides, β-keto-esters and aromatic ketones is presented using immobilised BINAP ligands. The repeated use of the immobilised catalyst in five recycles demonstrates ‘homogeneous’ catalysis with ‘heterogeneous’ catalysts, thus reducing solvent waste, and loss of precious metal and or ligand.