Catalytic transfer hydrogenation of ketones by the use of ruthenium complexes incorporating the new tridentate ligand, bis(2-oxazolin-2-ylmethyl)phenylphosphine

Abstract

The new heterofunctional phosphine ligand bis(2-oxazolin-2-ylmethyl)phenylphosphine (N,P,N) has been prepared and has allowed the synthesis of the ruthenium complexes fac-[RuCl₂(DMSO)(N,P,N)] 1, fac-[RuCl₂(PPh₃)(N,P,N)] 2, [RuCl(η⁶-C₆H₆)(N,P,N)][O₃SCF₃] 3 and [Ru(η⁶-C₆H₆)(N,P,N)][O₃SCF₃]₂ 4. When tridentate, as in 1, 2, and 4, this ligand co-ordinates in a facial-type mode. In complex 3, it acts as a P,N-chelate with a dangling oxazoline ring. The structures of the ligand, 2·CH₂Cl₂·0.25C₆H₁₄ and 3 have been determined by X-ray diffraction. Complexes 1–4 catalyse the transfer hydrogenation reaction between propan-2-ol and ketones. Only small differences in reactivity were observed between 3 and 4, despite the different ligand bonding mode in these complexes. For the best catalyst, 2, yields up to 97% were obtained and turnover frequencies may be as high as 112 000 h^–1.