A Ruthenium Protic N-Heterocyclic Carbene Complex as a Precatalyst for the Efficient Transfer Hydrogenation of Aryl Ketones

Abstract

A neutral azole precursor to a protic N-heterocyclic carbene (pNHC) ligand, 6-((4,5-diphenyl-1H-imidazol-1-yl)methyl)-2,2′-bipyridine (3), was prepared from 6-(bromomethyl)-2,2′-bipyridine (2) and 4,5-diphenylimidazole. Complex [RuCl(pNHC-bpy)(PPh3)2](PF6) (4) bearing a protic, bipyridine-tethered NHC ligand was prepared by refluxing 3 with RuCl2(PPh3)3 and KPF6 in methanol and was characterized by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, elemental analysis, and a single-crystal X-ray diffraction study. The hydrido complex [RuH(pNHC-bpy)(PPh3)2](PF6) (5) was prepared by reaction of 4 with NaBH4 in ethanol and characterized by NMR and Fourier transform infrared spectroscopy. Complex 5 was used as the catalyst (0.1 mol % loading) in the transfer hydrogenation of a range of alkyl/aryl ketones in basic iso-propanol at 60 °C. Bulky alkyl groups or ortho-substituted aryl groups at the ketones slowed down or inhibited the catalytic transformation. The addition of an excess of triphenylphosphine also slowed the catalysis, providing an indication for a mechanism involving phosphine dissociation, while the addition of an excess of elemental mercury had only a small effect on the conversion. The importance of potassium cations in the mechanism is consistent with the observation of reduced catalytic conversion when [2,2,2]-cryptand was present or when 1,8-diazabicyclo[5.4.0]undec-7-ene was used as the base. A plausible homogeneous catalysis mechanism involving the innersphere addition of hydride to the substrate in the transition state TS1 is supported by density functional theory calculations where the potassium ion has replaced the hydrogen atom of the N–H group in a protic NHC.