Efficient and selective oxidation of secondary benzylic C-H bonds to ketones with O2 catalyzed by metalloporphyrins under solvent-free and additive-free conditions

Non-solvent and non-additive oxidation of secondary benzylic C-H bonds to aromatic ketones utilizing simple metalloporphyrins as catalysts and O-2 was investigated systematically. Based on systematical studies on reaction parameters, such as the structures of porphyrins, the types of central metals, the amount of catalyst, the pressure of oxygen, and the temperature, most aromatics with secondary benzylic C-H had been converted efficiently and selectively to their corresponding aromatic ketones catalyzed by the optimized metalloporphyrin catalyst T(2,3,6-triCl)PPCo. And the selectivity towards aromatic ketones reached up to 80 %-95 % with acceptable conversions in lower catalyst-loading (2.4 x 10(-3)%, mol/mol). The highest selectivity was achieved for substrate 1,2,3,4-tetrahydronaphthalene with the selectivity of 95.0 % in the conversion of 46.3 %. The superior catalytic performance of T(2,3,6-triCl)PPCo could be mainly ascribed to its excellent plane structure, superior ability for electron transfer and low positive charge around the central metal. All of above properties promoted the formation of catalytic active species high-valence cobalt-oxo complexes, and facilitated the approach of substrates to the catalytic active centers. This work presented an optimized balance among higher selectivity, milder conditions, lower catalyst-loading, and simpler catalysts compared with current documents, which was not only an efficient, practical and significant strategy to functionalize the widely available aromatics possessing secondary benzylic C-H with high efficiency and selectivity under milder conditions, but also an important instance in the effective functionalization and utilization of other C-H bonds employing optimized catalytic systems based on metalloporphyrins.