Hydroxylation of Alkanes with Molecular Oxygen Catalyzed by a New Ruthenium-Substituted Polyoxometalate, [WZnRu_2~(III) (OH)(H_2O)(ZnW_9O_(34))_2l~(11-)

摘要

The utilization of dioxygen in liquid-phase oxidation of hydrocarbons is currently an important research goal due to its availibility and its benign effect on the environment. Since dioxygen has a triplet ground state, direct oxidation of organic compounds is not possible. Common modes of dioxygen "activation" thus include a) formation of hydrocarbon radicals leading to formation of reactive hydroperoxides by free-radical au-toxidation mechanisms, b) photoexcitation of dioxygen to form highly reactive singlet oxygen, c) use of dioxygen as a secondary oxidant in metal-ion-catalyzed mechanisms as in Wacker-type processes, and d) reductive cleavage of transition-metal dioxygen complexes to form highly active metal - oxo intermediates as in manganese porphyrins and the various Gif systems. A more desirable mode of activation would be the direct use of dioxygen to elicit oxygenation without formation of hydrocarbon radicals, or use of photoexcitation or reducing agents. A few such examples exist including alkene epoxidation with ruthenium porphyrins and alkane hydroxylation with perfluorinated iron porphyrins. although recently evidence has been put forward claiming that the latter is a free-radical autoxidation.