期刊
JOURNAL OF ORGANIC CHEMISTRY
卷 79, 期 11, 页码 5209-5218出版社
AMER CHEMICAL SOC
DOI: 10.1021/jo500789v
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资金
- Ministero dell'Istruzione, dell'Universita e della Ricerca (MIUR)
A kinetic study of the hydrogen atom transfer from activated phenols (2,6-dimethyl- and 2,6-di-tert-butyl-4-substituted phenols, 2,2,5,7,8-pentamethylchroman-6-ol, caffeic acid, and (+)-cathechin) to a series of N-oxyl radical (4-substituted phthalimide-N-oxyl radicals (4-X-PINO), 6-substituted benzotriazole-N-oxyl radicals (6-Y-BTNO), 3-quinazolin-4-one-N-oxyl radical (QONO), and 3-benzotriazin-4-one-N-oxyl radical (BONO)), was carried out by laser flash photolysis in CH3CN. A significant effect of the N-oxyl radical structure on the hydrogen transfer rate constants (k(H)) was observed with kH values that monotonically increase with increasing NO H bond dissociation energy (BDENO-H) of the N-hydroxylamines. The analysis of the kinetic data coupled to the results of theoretical calculations indicates that these reactions proceed by a hydrogen atom transfer (HAT) mechanism where the N-oxyl radical and the phenolic aromatic rings adopt a pi-stacked arrangement. Theoretical calculations also showed pronounced structural effects of the N-oxyl radicals on the charge transfer occurring in the pi-stacked conformation. Comparison of the k(H) values measured in this study with those previously reported for hydrogen atom transfer to the cumylperoxyl radical indicates that 6-CH3-BTNO is the best N-oxyl radical to be used as a model for evaluating the radical scavenging ability of phenolic antioxidants.
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