BF3-activated oxidation of alkanes by MnO4 -

The oxidation of alkanes and arylalkanes by KMnO₄ in CH ₃CN is greatly accelerated by the presence of just a few equivalents of BF₃, the reaction occurring readily at room temperature. Carbonyl compounds are the predominant products in the oxidation of secondary C-H bonds. Spectrophotometric and kinetics studies show that BF₃ forms an adduct with KMnO₄ in CH₃CN, [BF₃·MnO ₄]^-, which is the active species responsible for the oxidation of C-H bonds. The rate constant for the oxidation of toluene by [BF₃·MnO₄]^- is over 7 orders of magnitude faster than by MnO₄^- alone. The kinetic isotope effects for the oxidation of cyclohexane, toluene, and ethylbenzene at 25.0 °C are as follows: k_C6H12/k_C6D12 = 5.3 ± 0.6, k_C7H8/k_C7D8 = 6.8 ± 0.5, k_C8H10/k _C8D10 = 7.1 ± 0.5. The rate-limiting step for all of these reactions is most likely hydrogen-atom transfer from the substrate to an oxo group of the adduct. A good linear correlation between log(rate constant) and C-H bond energies of the hydrocarbons is found. The accelerating effect of BF₃ on the oxidation of methane by MnO₄ has been studied computationally by the Density Functional Theory (DFT) method. A significant decrease in the reaction barrier results from BF₃ coordination to MnO₄^-. The BF₃ coordination increases the ability of the Mn metal center to achieve a d¹ Mn(VI) electron configuration in the transition state. Calculations also indicate that the species [2BF₃·MnO₄]^- is more reactive than [BF₃·MnO₄]^-. © 2006 American Chemical Society.