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Hydroxylamines as Oxidation Catalysts: Thermochemical and Kinetic Studies

2003; American Chemical Society; Volume: 68; Issue: 5 Linguagem: Inglês

10.1021/jo026660z

1520-6904

Riccardo Amorati, Massimo Lucarini, Verónica Mugnaini, Gian Franco Pedulli, F. Minisci, Francesco Recupero, Francesca Fontana, Paola Astolfi, Lucedio Greci,

Catalysis and Oxidation Reactions

Bond dissociation enthalpies (BDE) of hydroxylamines containing alkyl, aryl, vinyl, and carbonyl substituents at the nitrogen atom have been determined by using the EPR radical equilibration technique in order to study the effect of the substituents on the O-H bond strength of these compounds. It has been found that substitution of an alkyl group directly bonded to the nitrogen atom with vinyl or aryl groups has a small effect, while substitution with acyl groups induces a large increase of the O-H BDE value. Thus, dialkyl hydroxylamines have O-H bond strengths of only ca. 70 kcal/mol, while acylhydroxylamines and N-hydroxyphthalimide (NHPI), containing two acyl substituents at nitrogen, are characterized by BDE values of ca. 80 and 88 kcal/mol, respectively. Since the phthalimide N-oxyl radical (PINO) has been recently proposed as an efficient oxidation catalyst of hydrocarbons or other substrates, the large BDE value found for the parent hydroxylamine (NHPI) justifies this proposal. Kinetic studies, carried out in order to better understand the mechanism of the NHPI-catalyzed aerobic oxidation of cumene, are consistent with a simple kinetic model where the rate-determining step is the hydrogen atom abstraction from the hydroxylamine by cumylperoxyl radicals.