N-Didmethylation 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)CH2o, is first 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 (r =-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 mechanismfrom theN,N-dimethylanilines to the BTNO radical followed by deprotonation of the anilinium radical cation (ET-PT mechanism). By applying theMarcus equation to the kinetic data for X=CH3, OC6H5, OCH3 (rate-determining ET), a reorganization energy for theETreaction was determined (?BTNO/DMA=32.1kcalmol-1). From the selfexchange reorganization energy for the BTNO/BTNO- couple, a self-exchange reorganization energy value of 31.9 kcal mol-1 was calculated for the DMAo+/DMA couple.