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

Oxidations of arylalkanes by (n)()Bu(4)NMnO(4) 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 MnO(2). The kinetics of the reactions, monitored by UV/vis spectrometry, show that the initial reactions are first order in the concentrations of both (n)()Bu(4)NMnO(4) 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 O(2) increases the rate of (n)()Bu(4)NMnO(4) 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 degrees C and k(C)()14(H)()12/k(C)()14(D)()12 = 3.0 (+/-0.6) at 25 degrees C. The rates of oxidation of substituted toluenes show only small substituent effects. In the reactions of dihydroanthracene and fluorene, the MnO(2) 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 DeltaH degrees 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)()Bu(4)NMnO(4) correlate with rates of abstraction by oxygen radicals.