A kinetic study of the reaction of N,N-dimethylanilines with 2,2-diphenyl-1-picrylhydrazyl radical: A concerted proton-electron transfer?

The reactivity of the 2,2-diphenyl-1-picrylhydrazyl radical (dpph) toward the N-methyl C-H bond of a number of 4-X-substituted-N,N-dimethylanilines (X = OMe, OPh, CH3, H) has been investigated in MeCN, in the absence and in the presence of Mg(ClO4)(2), by product, and kinetic analysis. The reaction was found to lead to the N-demethylation of the N,N-dimethylaniline with a rate quite sensitive to the electron donating power of the substituent (rho(+) = -2.03). With appropriately deuterated N,N-dimethylanilines, the intermolecular and intramolecular deuterium kinetic isotope effects (DKIEs) were measured with the following results. Intramolecular DKIE [(k(H)/k(D))(intra)] was found to always be similar to intermolecular DKIE [(k(H)/k(D))(inter)]. These results suggest a single-step hydrogen transfer mechanism from the N-C-H bond to dpph which might take the form of a concerted proton-electron transfer (CPET). An electron transfer (ET) step from the aniline to dpph leading to an anilinium radical cation, followed by a proton transfer step that produces an alpha-amino carbon radical, appears very unlikely. Accordingly, a rate-determining ET step would require no DKIE or at least different inter and intramolecular isotope effects. On the other hand, an equilibrium-control led ET is not compatible with the small slope value (-0.22 kcal(-1) K-1) of the log k(H)/Delta G plot. Furthermore, the reactivity increases by changing the solvent to the less polar toluene whereas the reverse would be expected for an ET mechanism. In the presence of Mg2+, a strong rate acceleration was observed, but the pattern of the results remained substantially unchanged: inter and intramolecular DKIEs were again very similar as well as the substituent effects. This suggests that the same mechanism (CPET) is operating in the presence and in the absence of Mg2+. The significant rate accelerating effect by Mg2+ is likely due to a favorable interaction of the Mg2+ ion with the partial negatively charged alpha-methyl carbon in the polar transition state for the hydrogen transfer process.