Participation of Oligovanadates in Alkane Oxidation with H₂O₂Catalyzed by Vanadate Anion in Acidified Acetonitrile: Kinetic and DFT Studies

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Abstract

The efficient oxidation of alkanes (cyclohexane, n-heptane, methylcyclohexane, isooctane, cis- and trans-1,2-dimethylcyclohexane) to the corresponding alkyl hydroperoxides with the system (n-Bu4N)[VO3]/H2O2/trifluoroacetic acid/MeCN–H2O was investigated in detail by 51V NMR, kinetic and theoretical methods. It has been established on the basis of the selectivity parameters in the oxidation of linear and branched alkanes and the kinetic peculiarities of the cyclohexane oxidation that the reaction mechanism includes the formation of HO• radicals. The presence of acid (TFA) is the crucial factor in this process, since in the neutral medium, monovanadate is inactive as a catalyst. The role of the acid is explained by the formation of oligovanadates in acidic medium, which exhibit higher catalytic activity compared with the monovanadate. The theoretical DFT calculations that used an oxodivanadate as a model of the catalytically active species revealed that the key factor of the higher activity of oligovanadates is the modification of the reaction mechanism upon the introduction of the second vanadium fragment into the catalyst molecule. As a result, the model divanadate catalyst is more active, by 4.2 kcal/mol (i.e., by a factor of ca. 1200), than the simple monovanadate species. Less energetically demanding routes (H-transfer to the oxo-ligand in the rate-limiting step) are opened for the di(poly)vanadate catalysts but not accessible for the monovanadate catalyst. Moreover, the second vanadium fragment plays the role of a stabilizer of key transition states due to the formation of 6-membered cyclic structures.

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