Distinctive Catalysis of Phosphonium and Sulfonium Compounds in Low-temperature Liquid-phase Hydrocarbon Oxidation

The oxidation catalysis of phosphonium and sulfonium compounds possessing various kinds of counteranions was investigated in the low-temperature homogeneous liquid-phase oxidation of nonpolar tetralin and polar benzaldehyde with particular reference to the initial oxidation mechanism. The detailed approach to the initiation mechanism on the basis of UV spectroscopic measurements of the interaction between the onium catalyst and O2, molecular orbital calculations of onium catalysts, the simulation analysis of the mechanism, and so on led to a conclusion that the partially electron-occupied d-orbitals of phosphorus or sulfur atom in the onium catalyst is favorably deposited for the interaction of O2 so as to activate the latter through the charge transfer from the catalyst to O2. The activated O2 substantially decreased the activation barrier for the abstraction of the active hydrogen of the hydrocarbons.