HYDROGEN-ATOM ABSTRACTION BY PERMANGANATE - OXIDATIONS OF ARYLALKANESIN ORGANIC-SOLVENTS

Oxidations of arylalkanes by (Bu4NMnO4)-Bu-n have been studied in tolu ene solvent: toluene, ethylbenzene, diphenylmethane, triphenylmethane, 9,10-dihydroanthracene, xanthene, and fluorene. Toluene is oxidized t o benzoic acid and a small amount of benzaldehyde; other substrates gi ve 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 ar e first order in the concentrations of both (Bu4NMnO4)-Bu-n and substr ate. No induction periods are observed. The same rate constants for to luene oxidation are observed in neat toluene and in o-dichlorobenzene solvent, within experimental errors. The presence of O-2 increases the rate of (Bu4NMnO4)-Bu-n disappearance. The reactions of toluene and d ihydroanthracene exhibit primary isotope effects: k(C7HS)/k(C7D8) = 6 (+/-1) at 45 degrees C and k(C14H12)/k(C14D12) = 3.0 (+/-0.6) at 25 de grees C. The rates of oxidation of substituted toluenes show only smal l substituent effects. In the reactions of dihydroanthracene and fluor ene, the MnO2 product is consumed in a subsequent reaction that appear s to form a charge-transfer complex. The rate-limiting step in all of the reactions is hydrogen atom transfer from the substrate to a perman ganate oxo group. The enthalpies of activation for the different subst rates;ire directly proportional to the Delta H-o for the hydrogen atom transfer step, as is typical of organic radical reactions. The abilit y of permanganate to abstract a hydrogen atom is explained on the basi s of its ability to form an 80 +/- 3 kcal/mol bond to H-., as calculat ed from a thermochemical cycle. (This bond strength is slightly lower than given in earlier calculations.) Rates of H-. abstraction by (Bu4N MnO4)-Bu-n correlate with rates of abstraction by oxygen radicals.