作者:Tomoyuki Nakamura、Yasumasa Watanabe、Shuji Suyama、Hiroshi Tezuka
DOI:10.1039/b106937n
日期:2002.6.27
The reaction of a series of 2-alkyl-2-propoxyl radicals 2 in cumene has been studied. Alkoxyl radicals 2 were generated from the thermolysis of the corresponding tert-alkyl peroxypivalates, and underwent several modes of unimolecular reactions, that is, β-scission to give methyl radical, β-scission to give alkyl radicals and 1,5-H shift, which competed with hydrogen abstraction from cumene. The ratios of the rate constant for alkyl radicals elimination to that for methyl radical elimination, kβ(R)/kβ(Me), were determined by using the radical trapping method employing TEMIO as the scavenger. The logarithm of the relative rate was satisfactorily correlated with the heat of formation of the leaving alkyl radicals containing the steric parameters of neopentyl radical 2d. Then the plot
of ln [kβ(R)/kβ(Me)] vs. the SOMO energies of the corresponding alkyl radicals showed a linear relationship with a slope of −7.6 eV−1, which is comparable to that of formolysis of alkyl bromides. The mechanism involving a fairly polar transition state of the alkyl radicals fragmentation from tert-alkoxyl radicals 2 is discussed in terms of the MO diagrams and the solvent effects.
研究了一系列 2-烷基-2-丙氧基自由基 2 在积雪烯中的反应。烷氧基自由基 2 是由相应的叔烷基过氧化新戊酸酯热分解生成的,并经历了几种单分子反应模式,即通过 β 裂解生成甲基自由基、通过 β 裂解生成烷基自由基和 1,5-H 迁移,这与从积雪烯中抽取氢气发生了竞争。通过使用 TEMIO 作为清除剂的自由基捕获法,确定了烷基自由基消除的速率常数与甲基自由基消除的速率常数(kβ(R)/kβ(Me))之比。相对速率的对数与含新戊基自由基 2d 立体参数的离去烷基自由基的形成热呈满意的相关性。ln[kβ(R)/kβ(Me)]与相应烷基自由基的 SOMO 能的关系图显示出线性关系,斜率为-7.6 eV-1,与溴化烷基的甲解关系相当。从 MO 图和溶剂效应的角度讨论了从叔烷氧基自由基 2 分裂出烷基自由基的极性过渡态的机理。