| Abstract
| - The oxidation of alkanes and arylalkanes by KMnO4 in CH3CN is greatly accelerated by thepresence of just a few equivalents of BF3, the reaction occurring readily at room temperature. Carbonylcompounds are the predominant products in the oxidation of secondary C−H bonds. Spectrophotometricand kinetics studies show that BF3 forms an adduct with KMnO4 in CH3CN, [BF3·MnO4]-, which is theactive species responsible for the oxidation of C−H bonds. The rate constant for the oxidation of tolueneby [BF3·MnO4]- is over 7 orders of magnitude faster than by MnO4- alone. The kinetic isotope effects forthe oxidation of cyclohexane, toluene, and ethylbenzene at 25.0 °C are as follows: kC6H12/kC6D12 = 5.3 ±0.6, kC7H8/kC7D8 = 6.8 ± 0.5, kC8H10/kC8D10 = 7.1 ± 0.5. The rate-limiting step for all of these reactions is mostlikely hydrogen-atom transfer from the substrate to an oxo group of the adduct. A good linear correlationbetween log(rate constant) and C−H bond energies of the hydrocarbons is found. The accelerating effectof BF3 on the oxidation of methane by MnO4- has been studied computationally by the Density FunctionalTheory (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) electronconfiguration in the transition state. Calculations also indicate that the species [2BF3·MnO4]- is more reactivethan [BF3·MnO4]-.
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