A Mixed N-Heterocyclic Carbene/2,2-Bipyridine-Supported Robust Ruthenium(II) Oxidation Precatalyst for Benzylic C-H Oxidation

Catalytic oxidation of hydrocarbons to the oxygenated derivatives has been an important research area for a long time and the search for well-defined, tunable, efficient, selective, and robust oxidation (pre)catalysts is still on. To further develop the applicability of N-heterocyclic carbene (NHC) ligands, herein we report a mixed NHC/bipyridine-supported ruthenium(II) oxidation precatalyst, [(N-py<^>C-NHC)Ru-II(N-py<^>N-py)(CH3CN)(2)](PF6)(CF3SO3), 1 (N-py<^>C-NHC=a bidentate 2-pyridyl-NHC neutral ligand, N-py<^>N-py=2,2-bipyridine) consisting of two cis-disposed loosely bound solvent ligands, which was successfully utilized for catalytic benzylic C-H oxidation of a variety of aromatic hydrocarbons to the corresponding ketones in 65-90% yields with the mild sacrificial oxidant NaIO4. The ligand backbone was found to be robust under oxidizing conditions. The presence of these ligands around the ruthenium center was crucial for the system, as the known RuCl3/NaIO4 and [Ru(para-cymene)Cl-2](2)/NaIO4 combinations were proved to be completely ineffective precatalysts under similar conditions. The scope and mechanistic aspects of this precatalyst were also evaluated.