Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
J Phys Chem A. 2009 Dec 10;113(49):13663-74. doi: 10.1021/jp901380y.
To explore the effects of the electronic nature of charged phenyl radicals on their reactivity, reactions of the three distonic isomers of n-dehydropyridinium cation (n = 2, 3, or 4) have been investigated in the gas phase by using Fourier-transform ion cyclotron resonance mass spectrometry. All three isomers react with cyclohexane, methanol, ethanol, and 1-pentanol exclusively via hydrogen atom abstraction and with allyl iodide mainly via iodine atom abstraction, with a reaction efficiency ordering of 2 > 3 > 4. The observed reactivity ordering correlates well with the calculated vertical electron affinities of the charged radicals (i.e., the higher the vertical electron affinity, the faster the reaction). Charged radicals 2 and 3 also react with tetrahydrofuran exclusively via hydrogen atom abstraction, but the reaction of 4 with tetrahydrofuran yields products arising from nonradical reactivity. The unusual reactivity of 4 is likely to result from the contribution of an ionized carbene-type resonance structure that facilitates nucleophilic addition to the most electrophilic carbon atom (C-4) in this charged radical. The influence of such a resonance structure on the reactivity of 2 is not obvious, and this may be due to stabilizing hydrogen-bonding interactions in the transition states for this molecule. Charged radicals 2 and 3 abstract a hydrogen atom from the substituent in both phenol and toluene, but 4 abstracts a hydrogen atom from the phenyl ring, a reaction that is unprecedented for phenyl radicals. Charged radical 4 reacts with tert-butyl isocyanide mainly by hydrogen cyanide (HCN) abstraction, whereas CN abstraction is the principal reaction for 2 and 3. The different reactivity observed for 4 (as compared to 2 and 3) is likely to result from different charge and spin distributions of the reaction intermediates for these charged radicals.
为了探究带电荷苯基自由基的电子特性对其反应活性的影响，通过傅里叶变换离子回旋共振质谱法，在气相条件下研究了脱氢吡啶鎓阳离子（n = 2、3 或 4）的三种离域型异构体的反应。三种异构体均仅通过氢原子提取与环己烷、甲醇、乙醇和 1-戊醇反应，与烯丙基碘主要通过碘原子提取反应，反应效率顺序为 2 > 3 > 4。观察到的反应活性顺序与计算得到的带电荷自由基的垂直电子亲和能很好地相关（即，垂直电子亲和能越高，反应越快）。带电荷自由基 2 和 3 也仅通过氢原子提取与四氢呋喃反应，但 4 与四氢呋喃的反应生成了非自由基反应产物。4 的异常反应活性可能源于其离域的卡宾型共振结构的贡献，该结构有利于亲核加成到带电荷自由基中最具亲电性的碳原子（C-4）。这种共振结构对 2 反应活性的影响不明显，这可能是由于在该分子的过渡态中存在稳定的氢键相互作用。带电荷自由基 2 和 3 从苯酚和甲苯的取代基上提取氢原子，但 4 从苯基环上提取氢原子，这是苯基自由基前所未有的反应。带电荷自由基 4 主要通过氢氰酸（HCN）提取与叔丁基异氰化物反应，而 2 和 3 的主要反应是 CN 提取。4 的不同反应活性（与 2 和 3 相比）可能源于这些带电荷自由基的反应中间体的不同电荷和自旋分布。
