Hydrogen Atom Transfer (HAT) Processes Promoted by the Quinolinimide-N-oxyl Radical. A Kinetic and Theoretical Study

A kinetic study of the hydrogen atom transfer (HAT) reactions from a series of organic compounds to the quinolinimide-N-oxyl radical (QINO) was performed in CH₃CN. The HAT rate constants are significantly higher than those observed with the phthalimide-N-oxyl radical (PINO) as a result of enthalpic and polar effects due to the presence of the N-heteroaromatic ring in QINO. The relevance of polar effects is supported by theoretical calculations conducted for the reactions of the two N-oxyl radicals with toluene, which indicate that the HAT process is characterized by a significant degree of charge transfer permitted by the π-stacking that occurs between the toluene and the N-oxyl aromatic rings in the transition state structures. An increase in the HAT reactivity of QINO was observed in the presence of 0.15 M HClO₄ and 0.15 M Mg(ClO₄)₂ due to the protonation or complexation with the Lewis acid of the pyridine nitrogen that leads to a further decrease in the electron density in the N-oxyl radical. These results fully support the use of N-hydroxyquinolinimide as a convenient substitute for N-hydroxyphthalimide in the catalytic aerobic oxidations of aliphatic hydrocarbons characterized by relatively high C-H bond dissociation energies.