Kinetic deuterium isotope effect profiles and substituent effects in the oxidative N-demethylation of N,N-dimethylanilines catalyzed by tetrakis(pentafluorophenyl)porphyrin iron(III) chloride

The mechanistic dichotomy (hydrogen atom transfer or electron-transfer mechanism) in the oxidative N-dealkylation of a series 4-X-N,N-dimethylanilines (X = MeO, Me, H, Br, CF3, CN, NO2) by PhIO, catalyzed by tetrakis(pentafluorophenyl)porphyrin iron(LII) chloride (FeTPFPPCl), was investigated in CH2Cl2 by determining both the intra- and the intermolecular kinetic deuterium isotope effects and the effect of substituents on reactivity. The results were as follows: (a) The values of k(H)/k(D)(intra), obtained by the study of 4-X-N-methyl-N-trideuteriomethylanilines [2.0 (X = NO2), 2.0 (X = CN), 2.6 (X = Br), 3.1 (X = H), 3.2 (X = Me), 3.3 (X MeO)], regularly decreased on going from electron donating to electron withdrawing substituents, a trend exactly contrary to that found for the hydrogen atom transfer reactions of some of the same substrates with tert-butoxyl radicals. (b) The intermolecular kinetic deuterium isotope effects, k(H)/k(D)(inter), determined by competitive experiments with 4-X-substituted N,N-dimethyl- and N,N-bis(trideuteriomethyl)anilines [k(H)/k(D)(inter) for X H, Br, and MeO, 1.6, 1.5 and 1.9, respectively], were significantly different from the corresponding k(H)/k(D)(intra) values. (c) The relative reactivities of 4-X-substituted N,N-dimethylanilines, determined by competitive kinetics, spanned a reactivity range of 25 (from X = NO2 to X = MeO) and were nicely correlated by the substituent constants sigma(+). A rho value of -0.88 (r(2) = 0.98) was determined by this correlation. The relative reactivity can also be fitted to the Rehm-Welter equation for electron-transfer reactions. A value of 47 kcal mol(-1) for the reorganization energy was calculated. Altogether, the above results, and particularly points (a) and (b), allow us to dismiss the operation of a hydrogen atom transfer mechanism. A one electron transfer mechanism is instead consistent with these results and appears therefore the most likely pathway for the oxidative N-demethylation of N,N-dimethylanilines catalyzed by iron porphyrins. The intramolecular kinetic deuterium isotope effect profile is a useful tool for distinguishing electron transfer from hydrogen atom transfer mechanisms.