Oxidation of alkanes by iodosylbenzene (PhIO) catalysed by supported Mn(III) porphyrins: Activity and mechanism

Manganese(III) tetrakis(2,6-dichloro-3-sulfonatophenyl)porphyrin (1) and manganese(III) tetrakis[tetrafluoro-4-(trimethylammoniumyl)phenyl]porphyrin (2), in homogeneous solution and supported on modified silica surfaces, have been used as catalysts for the oxidation of pentane and cyclohexane with iodosylbenzene. The effects of reaction solvent, catalyst support and dioxygen on the oxidation yields and product distributions have been studied.The reactions under nitrogen show a high selectivity for alcohol formation whereas in air ketones become significant products, indicating the participation of dioxygen in the formation of the ketones. Competitive oxidations of pentane and perdeuteropentane give kinetic isotope effects (kH/kD) of 6.9 ± 1.3 for both pentan-2-ol and -3-ol formation. The results are in accord with a mechanism in which an initial hydrogen atom-abstraction from the alkane by the active oxidant, an oxomanganese(V) species, gives a solvent-caged alkyl radical and a hydroxymanganese(IV) porphyrin. All the products can be accounted by partitioning of the carbon radicals between ‘in cage’ reaction to give alcohol and ‘cage escape’ leading to products typical of free alkyl radicals. The effect of solvent viscosity on this partitioning of the carbon radicals has been probed with the radical trap bromotrichloromethane.

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