Journal of Organic Chemistry p. 5144 - 5149 (2005)
Update date:2022-08-11
Topics:
Baciocchi, Enrico
Bietti, Massimo
Gerini, Maria Francesca
Lanzalunga, Osvaldo
The reactivity of the phthalimide N-oxyl radical (PINO) toward the N-methyl C-H bond of a number of 4-X-substituted N,N-dimethylanilines (X = OMe, OPh, CF3, CO2Et, CN) has been investigated by product and kinetic analysis. PINO was generated in CH3CN by reaction of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 or, for the kinetic study of the most reactive substrates (X = OMe, OPh), with tert-butoxyl radical produced by 266 nm laser flash photolysis of di-tert-butyl peroxide. The reaction was found to lead to the N-demethylation of the N,N-dimethylaniline with a rate very sensitive to the electron donating power of the substituent (ρ+ = -2.5) as well as to the oxidation potential of the substrates. With appropriately deuterated N,N-dimethylanilines the intermolecular and intramolecular deuterium kinetic isotope effects (DKIEs) were measured for some substrates (X = OMe, CO2Et, CN) with the following results. First, intramolecular DKIE [(kH/kD) intra] was found to be always different and higher than intermolecular DKIE [(kH/kD)inter]; second, no intermolecular DKIE [(kH/kD)inter = 1] was observed for X = OMe, whereas substantial values of (kH/k D)inter were exhibited by X = CO2Et (4.8) and X = CN (5.8). These results, while are incompatible with a single step hydrogen atom transfer from the N-C-H bond to the N-oxyl radical, as proposed for the reaction of PINO with benzylic C-H bonds, can be nicely interpreted on the basis of a two-step mechanism involving a reversible electron transfer from the aniline to PINO leading to an anilinium radical cation, followed by a proton-transfer step that produces an α-amino carbon radical. In line with this conclusion the reactivity data exhibited a good fit with the Marcus equation and a λ value of 37.6 kcal mol-1 was calculated for the reorganization energy required in this electron-transfer process. From this value, a quite high reorganization energy (> 60 kcal mol-1) is estimated for the PINO/NHPI(-H)- self-exchange reaction. It is suggested that the N-demethylated product derives from the reaction of the α-amino carbon radical with PINO to form either a cross-coupling product or an α-amino carbocation. Both species may react with the small amounts of H2O present in the medium to form a carbinolamine that, again by hydrolysis, can be eventually converted into the N-demethylated product.
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