Mononuclear ruthenium complexes containing chiral aminooxazolines: Syntheses, X-ray studies and catalytic activity

Abstract

The synthesis and characterisation of three novel mononuclear ruthenium(II) complexes containing one of the following chiral auxiliary ligands: 2-amino-(4 R)-phenyl-2-oxazoline (amphox), indanyl-2-amino-(4 R,5 S)-2-oxazoline (aminox) or indanyl-(2′-anilinyl)-(4 R,5 S)-2-oxazoline (aninox) is described using [Ru 2Cl 4(η 6- p-cym) 2] (p-cym = 1-isopropyl-4-methylbenzene) as the Ru starting material. The new complexes have been identified as the neutral derivatives [RuCl 2(η 6- p-cym)(amphox-κ 1 N ox )] ( 1), [RuCl 2(η 6- p-cym)(aminox-κ 1 N ox )] ( 2) and the salt [RuCl(η 6- p-cym)(aninox-κ 2 N, N′)]Cl ( 3). These materials have been fully characterised (elemental analysis, NMR, IR, conductance, MS, etc.) and, in the case of 2 and 3, structurally elucidated in the solid-state using single crystal X-ray diffraction methods. All three complexes show good catalytic activity (max. conversion >99%, TOF = 424 h −1) but only modest enantio-selectivity (max. ee = 40%) for the transfer hydrogenation reaction of acetophenone with isopropyl alcohol. The complexes were also tested in an asymmetric Diels–Alder reaction involving cyclopentadiene and acrolein (max. conversion >99%, TOF = 42 h −1). In this case, the diastereo-selectivity was good to moderate (max. de = 84%), but the ee values were poor (max. ee = 12%).