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The oxidation of a series of alkyl aryl sulfides promoted by the phthalimide N-oxyl radical (PINO) has been investigated by kinetic and product analysis. Sulfoxides are formed as major reaction products in the oxidation of thioanisoles and benzyl phenyl sulfides. The observation of fragmentation products in the oxidation of 2-phenyl-2-propyl phenyl sulfide and diphenylmethyl phenyl sulfide indicates that the reaction involves an initial electron transfer reaction from the sulfide to PINO with the formation of aryl sulfide radical cations and the anion PINO-. Combination of the species then leads to a radical adduct precursor of sulfoxides while the rapid C–S cleavage occurs with aryl sulfide radical cations that can form the stable 2-phenyl-2-propyl or diphenylmethyl carbocation.

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.