Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
J Am Chem Soc. 2012 Feb 1;134(4):2085-93. doi: 10.1021/ja207899j. Epub 2012 Jan 18.
To directly compare the reactivity of positively charged carbon-centered aromatic σ-radicals toward methanol in solution and in the gas phase, the 2-, 3-, and 4-dehydropyridinium cations (distonic isomers of the pyridine radical cation) were generated by ultraviolet photolysis of the corresponding iodo precursors in a mixture of water and methanol at varying pH. The reaction mixtures were analyzed by using liquid chromatography/mass spectrometry. Hydrogen atom abstraction was the only reaction observed for the 3- and 4-dehydropyridinium cations (and pyridines) in solution. This also was the major reaction observed earlier in the gas phase. Depending on the pH, the hydrogen atom can be abstracted from different molecules (i.e., methanol or water) and from different sites (in methanol) by the 3- and 4-dehydropyridinium cations/pyridines in solution. In the pH range 1-4, the methyl group of methanol is the main hydrogen atom donor site for both 3- and 4-dehydropyridinium cations (just like in the gas phase). At higher pH, the hydroxyl groups of water and methanol also act as hydrogen atom donors. This finding is rationalized by a greater abundance of the unprotonated radicals that preferentially abstract hydrogen atoms from the polar hydroxyl groups. The percentage yield of hydrogen atom abstraction by these radicals was found to increase with lowering the pH in the pH range 1.0-3.2. This pH effect is rationalized by polar effects: the lower the pH, the greater the fraction of protonated (more polar) radicals in the solution. This finding is consistent with previous results obtained in the gas phase and suggests that gas-phase studies can be used to predict solution reactivity, but only as long as the same reactive species is studied in both experiments. This was found not to be the case for the 2-iodopyridinium cation. Photolysis of this precursor in solution resulted in the formation of two major addition products, 2-hydroxy- and 2-methoxypyridinium cations, in addition to the hydrogen atom abstraction product. These addition products were not observed in the earlier gas-phase studies on 2-dehydropyridinium cation. Their observation in solution is explained by the formation of another reactive intermediate, the 2-pyridylcation, upon photolysis of 2-iodopyridinium cation (and 2-iodopyridine). The same intermediate was observed in the gas phase but it was removed before examining the reactions of the desired radical, 2-dehydropyridinium cation (which cannot be done in solution).
为了直接比较正电荷碳中心芳香 σ-自由基在溶液中和气相中与甲醇的反应性，通过在水和甲醇的混合物中紫外线光解相应的碘前体，生成了 2-、3-和 4-脱水吡啶鎓阳离子（吡啶自由基阳离子的离域异构体）。通过使用液相色谱/质谱分析反应混合物。在溶液中，3-和 4-脱水吡啶鎓阳离子（和吡啶）仅观察到氢原子的提取反应。这也是早些时候在气相中观察到的主要反应。根据 pH 值的不同，氢原子可以由不同的分子（即甲醇或水）和不同的位置（在甲醇中）由 3-和 4-脱水吡啶鎓阳离子/吡啶提取。在 pH 值为 1-4 范围内，甲醇的甲基是 3-和 4-脱水吡啶鎓阳离子（就像在气相中一样）的主要氢原子供体部位。在较高的 pH 值下，水和甲醇的羟基也作为氢原子供体。通过优先从极性羟基中提取氢原子的未质子化自由基的丰度更大，这一发现得到了合理化。发现这些自由基的氢原子提取的产率随着 pH 值在 1.0-3.2 范围内的降低而增加。这种 pH 效应通过极性效应来合理化：pH 值越低，溶液中质子化（更极性）自由基的分数越大。这一发现与先前在气相中获得的结果一致，表明气相研究可用于预测溶液反应性，但前提是在两个实验中研究相同的反应性物质。对于 2-碘吡啶鎓阳离子，情况并非如此。在溶液中光解该前体除了生成氢原子提取产物外，还生成了两种主要的加成产物，2-羟基-和 2-甲氧基吡啶鎓阳离子。这些加成产物在以前关于 2-脱水吡啶鎓阳离子的气相研究中没有观察到。它们在溶液中的观察可以通过 2-碘吡啶鎓阳离子（和 2-碘吡啶）光解形成另一种反应性中间体 2-吡啶鎓阳离子来解释。在气相中观察到了相同的中间体，但在检查所需自由基 2-脱水吡啶鎓阳离子的反应之前就将其除去了（这在溶液中是无法进行的）。
