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

Oxidations of arylalkanes by n Bu4NMnO4 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 n Bu4NMnO4 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 n Bu4NMnO4 disappearance. The reactions of toluene and dihydroanthracene exhibit primary isotope effects: k C 7 H 8 /k C 7 D 8 = 6 (±1) at 45 °C and k C 14 H 12 /k C 14 D 12 = 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 n Bu4NMnO4 correlate with rates of abstraction by oxygen radicals.