Palladium complexes with tripodal ligands of the tpa family efficiently catalyze benzylic C–H oxidation of various substrates with peracetic acid, affording the corresponding ketones in high yields (up to 100%), at <1 mol% catalyst loadings.
A mechanistic study of direct selective oxidation of benzylic C(sp 3 )À H groups with peracetic acid, catalyzed by palladium complexes with tripodal amino-tris(pyriylmethyl) ligands, is presented. The oxidation of arylalkanes having secondary and tertiary benzylic CÀ H groups, predominantly yields, depending on the substrate and conditions, either the corresponding ketones or alcohols. One of the three 2-pyriylmethyl moieties, which is pending in the starting catalyst, apparently, facilitates the active species formation and takes part in stabilization of the high-valent Pd center in the active species, occupying the axial coordination site of palladium. The catalytic, as well as isotopic labeling experiments, in combination with ESI-MS data and DFT calculations, point out palladium oxyl species as possible catalytically active sites, operating essentially via CÀ H abstraction/oxygen rebound pathway. For the ketones formation, OÀ H abstraction/в-scission mechanism has been proposed.ChemCatChem
The development of methods for the selective oxidation of C–H groups of organic compounds resulting in C–O bond formation is a challenging problem in modern synthetic organic chemistry. Versatile methods for the oxofunctionalization of these groups are currently lacking. Approaches for the oxidation of benzylic C–H groups having a relatively weak C–H bond (the homolytic bond dissociation energy is ∼80–90 kcal mol−1) differ from the methods used for the oxidation of aliphatic and aromatic C–H groups with stronger C–H bonds. The review summarizes homogeneous catalytic and non-catalytic methods for the selective oxofunctionalization of benzylic groups of organic compounds developed in the past 40 years. Reactions using various oxidizing agents in the presence or in the absence of transition metal compounds are considered. Putative mechanisms of selected transformations are discussed. Particular focus is placed on the selective oxidation of various substrates.
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