Dipartimento di Chimica, Sapienza Università di Roma and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, P.le A. Moro, 5 I-00185 Rome, Italy.
J Org Chem. 2013 Feb 1;78(3):1026-37. doi: 10.1021/jo302483s. Epub 2013 Jan 18.
The kinetics of reactions of the phthalimide N-oxyl radical (PINO) with a series of activated phenols (2,2,5,7,8-pentamethylchroman-6-ol (PMC), 2,6-dimethyl- and 2,6-di-tert-butyl-4-substituted phenols) were investigated by laser flash photolysis in CH(3)CN and PhCl in order to establish if the reactions with PINO can provide a useful tool for evaluating the radical scavenging ability of phenolic antioxidants. On the basis of the small values of deuterium kinetic isotope effects, the relatively high and negative ρ values in the Hammett correlations and the results of theoretical calculations, we suggest that these reactions proceed by a hydrogen atom transfer (HAT) mechanism having a significant degree of charge transfer resulting from a π-stacked conformation between PINO and the aromatic ring of the phenols. Kinetic solvent effects were analyzed in detail for the hydrogen transfer from 2,4,6-trimethylphenol to PINO and the data obtained are in accordance with the Snelgrove-Ingold equation for HAT. Experimental rate constants for the reactions of PINO with activated phenols are in accordance with those predicted by applying the Marcus cross relation.
通过在 CH(3)CN 和 PhCl 中使用激光闪光光解研究了邻苯二甲酰亚胺 N-氧自由基（PINO）与一系列活化酚（2,2,5,7,8-五甲基色满-6-醇（PMC）、2,6-二甲基和 2,6-二叔丁基-4-取代酚）的反应动力学，以确定与 PINO 的反应是否可以为评估酚类抗氧化剂的清除自由基能力提供有用的工具。基于氘动力学同位素效应的小值、哈米特相关性中相对较高且负的 ρ 值以及理论计算的结果，我们建议这些反应通过氢原子转移（HAT）机制进行，该机制具有来自 PINO 和酚的芳环之间π堆积构象的显著电荷转移。详细分析了从 2,4,6-三甲基苯酚到 PINO 的氢转移的溶剂动力学效应，获得的数据符合 HAT 的斯内尔格罗夫-英戈尔德方程。通过应用马库斯交叉关系预测的 PINO 与活化酚的反应实验速率常数与预测值一致。
