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) has been carried out 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 carried out 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 enabled by the pi-stacking that occurs in the transition state structures between the toluene and the N-oxyl aromatic rings. An increase of the HAT reactivity of QINO was observed in the presence of HClO4 0.15 M and Mg(ClO4)2 0.15 M due to the protonation or complexation with the Lewis acid of the pyridine nitrogen that leads to a further decrease of the electron density in the N-oxyl radical. These results fully support the use of N-hydroxyquinolinimide (NHQI) as a convenient substitute of N-hydroxyphthalimide (NHPI) in the catalytic aerobic oxidations of aliphatic hydrocarbons characterized by relatively high C-H bond dissociation energies.