A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)
A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)
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DOI：
10.1002/poc.2979
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发表时间：
2012-11-01
影响因子：
1.8
通讯作者：
Mayr, Herbert
中科院分区：
文献类型：
--
作者：
Horn, Markus;Mayr, Herbert
通过将传统的光度法和电导法与停流技术相结合，我们能够研究三苯甲基溴、氯化物和羧酸盐在乙腈水溶液中的电离速率常数，反应活性范围为10（-5）至10（3）s（-1）。通过这种方式，可以比较产生pK（R+）相差21个单位的三苯甲基鎓离子的电离过程，对应于120 kJ mol（-1）的自由能。对这种影响深远的相关性的分析为有机反应性的几个方面提供了新的见解。我们观察到（i）从S（N）1反应的溶剂分解机制的变化，没有和共同的离子返回，（ii）反应的形成和消耗的碳阳离子可以观察到视觉，（iii）溶剂分解的电离步骤几乎完成之前，与水的反应变得显着，和（iv）电离，产生持久的碳阳离子的解决方案。从具有碳阳离子样过渡态的反应到具有少量碳阳离子特征的过渡态的反应的连续变化引起Winstein-Grunwald m值的较大变化。电离过程的m = 0.2的值表明，m的低值不一定意味着S（N）2反应。虽然4，4 '，4“-三甲氧基三苯甲基离子与水的反应速度比4-二甲基氨基三苯甲基离子快1000倍以上，并且pK（R+）值比4-二甲基氨基三苯甲基离子负3个单位，但通过在乙腈水溶液中电离相应的乙酸盐，两种碳正离子以几乎相等的速率形成。这种比较再次表明，离电性不仅仅是亲电性的逆，并且对进一步研究固有屏障的重要性提出了挑战。版权所有（C）2012约翰威利父子有限公司
By combining conventional photometric and conductimetric methods with stopped-flow techniques, we were able to investigate ionization rate constants of trityl bromides, chlorides, and carboxylates in aqueous acetonitrile in a reactivity range from 10(-5) to 10(3) s(-1). In this way, it became possible to compare ionization processes yielding tritylium ions which differ by 21 units in pK(R+), corresponding to 120 kJ mol(-1) in free energy. Analysis of such far-reaching correlations provided new insights into several aspects of organic reactivity. We observed (i) the change of solvolysis mechanisms from S(N)1 reactions without and with common ion return, (ii) reactions where formation and consumption of the carbocations could be observed visually, (iii) solvolyses where the ionization step is almost complete before the reaction with water becomes significant, and (iv) ionizations that yield solutions of persistent carbocations. The continuous change from reactions with carbocation-like transition states to reactions with transition states of little carbocation character caused large variations of Winstein-Grunwald m values. Values of m = 0.2 for ionization processes show that low values of m do not necessarily imply S(N)2 reactions. Although the 4,4',4 ''-trimethoxytritylium ion reacts more than 1000 times faster with water than the 4-dimethylaminotritylium ion and has a pK(R+) value that is more negative by three units, both carbocations are formed with almost equal rates by ionization of the corresponding acetates in aqueous acetonitrile. This comparison again illustrates that electrofugality is not simply the inverse of electrophilicity and presents a challenge for further study of the importance of intrinsic barriers. Copyright (C) 2012 John Wiley & Sons, Ltd.