More than 100,000 Turnovers with Immobilized Ir-Diphosphine Catalysts in an Enantioselective Imine Hydrogenation

Abstract

A modular concept to prepare immobilized enantioselective catalysts is described, consisting of a functionalized xyliphos ligand covalently attached to a support via a linker. Immobilized xyliphos bound to silica and to polystyrene as well as soluble dimeric xyliphos and an extractable analogue were prepared and tested in the Ir-catalyzed hydrogenation of a hindered N-arylimine used for the production of (S)-metolachlor. The best heterogeneous catalyst 9b exhibited TON's > 100,000 and TOF's up to 20,000 h-1, the best values so far for immobilized catalysts. The immobilized catalysts gave similar enantioselectivities but lower activities and higher deactivation rates than the homogeneous analogues. These negative effects were tentatively explained by the higher local catalyst concentration on the support surface leading to an increased tendency to deactivation by irreversible dimer formation. Separation of these catalysts by filtration and extraction is easy and efficient.