Palladium-Aminopyridine Catalyzed C−H Oxygenation: Probing the Nature of Metal Based Oxidant

A mechanistic study of direct selective oxidation of benzylic C(sp³)−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 C−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 C−H abstraction/oxygen rebound pathway. For the ketones formation, O−H abstraction/в-scission mechanism has been proposed.