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-dimeth ylanilines (X = MeO, Me, H, Br, CF3, CN, NO2) by PhIO, catalyzed by te trakis(pentafluorophenyl)porphyrin iron(LII) chloride (FeTPFPPCl), was investigated in CH2Cl2 by determining both the intra- and the intermo lecular kinetic deuterium isotope effects and the effect of substituen ts 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-trideuteriomethyl anilines [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 don ating to electron withdrawing substituents, a trend exactly contrary t o that found for the hydrogen atom transfer reactions of some of the s ame substrates with tert-butoxyl radicals. (b) The intermolecular kine tic deuterium isotope effects, k(H)/k(D)(inter), determined by competi tive experiments with 4-X-substituted N,N-dimethyl- and N,N-bis(trideu teriomethyl)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 correspo nding k(H)/k(D)(intra) values. (c) The relative reactivities of 4-X-su bstituted N,N-dimethylanilines, determined by competitive kinetics, sp anned a reactivity range of 25 (from X = NO2 to X = MeO) and were nice ly correlated by the substituent constants sigma(+). A rho value of -0 .88 (r(2) = 0.98) was determined by this correlation. The relative rea ctivity can also be fitted to the Rehm-Welter equation for electron-tr ansfer reactions. A value of 47 kcal mol(-1) for the reorganization en ergy was calculated. Altogether, the above results, and particularly p oints (a) and (b), allow us to dismiss the operation of a hydrogen ato m transfer mechanism. A one electron transfer mechanism is instead con sistent with these results and appears therefore the most likely pathw ay for the oxidative N-demethylation of N,N-dimethylanilines catalyzed by iron porphyrins. The intramolecular kinetic deuterium isotope effe ct profile is a useful tool for distinguishing electron transfer from hydrogen atom transfer mechanisms.