N-Demethylation of N,N-Dimethylanilines by the Benzotriazole N-Oxyl Radical: Evidence for a Two-Step Electron Transfer-Proton Transfer Mechanism

The reaction of the benzotriazole N-oxyl radical (BTNO) with a series of 4-X-N,N-dimethylanilines (X = CN, CF3, CO2CH2CH3, CH3, OC6H5, OCH3) has been investigated in CH3CN. Product analysis shows that the radical, 4-X-C6H4N(CH3)CH2 center dot, is formed, which can lead to the N-demethylated product or the product of coupling with BTNO. Reaction rates were found to increase significantly by increasing the electron-donating power of the aryl substituents (rho(+) = -3.8). With electron-donating substituents (X = CH3, OC6H5, OCH3), no intermolecular deuterium kinetic isotope effect (DKIE) and a substantial intramolecular DKIE are observed. With electron-withdrawing substituents (X = CN, CF3, CO2CH2CH3), substantial values of both intermolecular and intramolecular DKIEs are observed. These results can be interpreted on the basis of an electron-transfer mechanism from the N,N-dimethylanilines to the BTNO radical followed by deprotonation of the anilinium radical cation (ET-PT mechanism). By applying the Marcus equation to the kinetic data for X = CH3, OC6H5, OCH3 (rate-determining ET), a reorganization energy for the ET reaction was determined (lambda(BTNO/DMA) = 32.1 kcal mol(-1)). From the self-exchange reorganization energy for the BTNO/BTNO- couple, a self-exchange reorganization energy value of 31.9 kcal mol(-1) was calculated for the DMA(center dot 1)/DMA couple.