Transition Metal-Catalyzed Regio-selective Aromatic C-H Bond Oxidation for C-O Bond Formation

Recent years the great progress in transition metal-catalyzed direct aromatic C-H oxidation has been witnessed, which has been utilized in the preparation of various phenolic compounds. These transformations employ inter alia palladium, copper, ruthenium, iridium, etc. as the transition metal catalysts, and hypervalent iodine, persulfate, or oxygen as the oxidants. There have been several reviews in which the C-H oxidations with specific transition metal or oxidant was discussed. This review focuses specifically on transition metal-catalyzed aromatic C-H oxidations with ortho-, meta-, or para-selectivity, and rationalizes the possible generation mechanism of regio-selectivities, which might be controlled by the directing group via chelation-assistance, the ligand, or intrinsic properties of the substrate. The discussion section indicated the existing problems of transition metal-catalyzed aromatic C-H oxidations, as well as the possible limiting factors for the development and application of this strategy.