Hydrogen Atom Abstraction by Permanganate: Oxidations of Arylalkanes in Organic Solvents

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Abstract
Oxidations of arylalkanes by nBu4NMnO4 have been studied in toluene solvent: toluene, ethylbenzene, diphenylmethane, triphenylmethane, 9,10-dihydroanthracene, xanthene, and fluorene. Toluene is oxidized to benzoic acid and a small amount of benzaldehyde; other substrates give oxygenated and/or dehydrogenated products. The manganese product of all of the reactions is colloidal MnO2. The kinetics of the reactions, monitored by UV/vis spectrometry, show that the initial reactions are first order in the concentrations of both nBu4NMnO4 and substrate. No induction periods are observed. The same rate constants for toluene oxidation are observed in neat toluene and in o-dichlorobenzene solvent, within experimental errors. The presence of O2 increases the rate of nBu4NMnO4 disappearance. The reactions of toluene and dihydroanthracene exhibit primary isotope effects: kC7H8/kC7D8 = 6 (±1) at 45 °C and kC14H12/kC14D12 = 3.0 (±0.6) at 25 °C. The rates of oxidation of substituted toluenes show only small substituent effects. In the reactions of dihydroanthracene and fluorene, the MnO2 product is consumed in a subsequent reaction that appears to form a charge-transfer complex. The rate-limiting step in all of the reactions is hydrogen atom transfer from the substrate to a permanganate oxo group. The enthalpies of activation for the different substrates are directly proportional to the ΔH° for the hydrogen atom transfer step, as is typical of organic radical reactions. The ability of permanganate to abstract a hydrogen atom is explained on the basis of its ability to form an 80 ± 3 kcal/mol bond to H•, as calculated from a thermochemical cycle. (This bond strength is slightly lower than given in earlier calculations.) Rates of H• abstraction by nBu4NMnO4 correlate with rates of abstraction by oxygen radicals.

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