RESUMO
The reaction of the benzotriazole N-oxyl radical (BTNO) with a series of 4-X-N,N-dimethylanilines (X = CN, CF(3), CO(2)CH(2)CH(3), CH(3), OC(6)H(5), OCH(3)) has been investigated in CH(3)CN. Product analysis shows that the radical, 4-X-C(6)H(4)N(CH(3))CH(2)(*), 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 (rho(+) = -3.8). With electron-donating substituents (X = CH(3), OC(6)H(5), OCH(3)), no intermolecular deuterium kinetic isotope effect (DKIE) and a substantial intramolecular DKIE are observed. With electron-withdrawing substituents (X = CN, CF(3), CO(2)CH(2)CH(3)), 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 = CH(3), OC(6)H(5), OCH(3) (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(*+)/DMA couple.
RESUMO
A kinetic study of the one-electron oxidation of a series of substituted ferrocenes (FcX: X = H, COCH3, CO2Et, CH2OH, Et, and Me2) by the benzotriazole-N-oxyl radical (BTNO) and of ferrocene (FcH) by a series of ring-substituted benzotriazole-N-oxyl radicals (Z-BTNO: Z = H, 6-CF(3), 6-Cl, 6-Me, 6-MeO) has been carried out in CH3CN. N-Oxyl radicals were produced by hydrogen abstraction from 1-hydroxybenzotriazoles (Z-HBT) by the cumyloxyl radical produced after 355 nm laser flash photolysis of a solution of dicumyl peroxide in CH3CN. In both systems, the rate constants exhibited a satisfactory fit with the Marcus equation allowing us to determine self-exchange reorganization energy values for the BTNO/BTNO- couple, which resulted in good agreement: 34.7 kcal mol(-1) from the oxidation of ferrocenes by BTNO and 30.5 kcal mol(-1) from the oxidation of ferrocene by aryl-substituted Z-BTNO. From the average value of 32.6 kcal mol(-1) it is possible to calculate a self-exchange rate for the BTNO/BTNO- couple of 7.6 x 10(5) M(-1 )s(-1), which is 3 orders of magnitude higher than that determined for the PINO/PINO- couple. The difference in the intrinsic barrier between the two oxidants is so large that it overcomes the thermodynamic factor and the oxidation of ferrocene by BTNO results significantly faster than that by PINO in spite of the higher reduction potential of the latter N-oxyl radical. The higher reactivity of BTNO with respect to PINO in an electron-transfer process contrasts with what is observed in hydrogen atom transfer processes where PINO is always more reactive than BTNO due to the higher NO-H bond dissociation energy in N-hydroxyphthalimide (HPI) than in HBT (88 vs. 85 kcal mol(-1), respectively). Thus, the relative reactivity of PINO and BTNO radicals might represent a criterium to help in the distinction of ET and HAT reactions promoted by these transient N-oxyl radicals.
RESUMO
A kinetic and product study of the reactions of chlorpromazine 1, N-methylphenothiazine 2, and N-ethylphenothiazine 3 with singlet oxygen was carried out in MeOH and MeCN. 1 undergoes exclusive side-chain cleavage, whereas the reactions of 2 and 3, in MeOH, afforded only the corresponding sulfoxides. A mechanism for the reaction of 1 is proposed where the first step involves an interaction between singlet oxygen and the side-chain dimethylamino nitrogen. This explains why no side-chain cleavage is observed for 2 and 3.