Comparison of aminolysis of 2-pyridyl and 4-pyridyl x-substituted benzoates in acetonitrile: evidence for a concerted mechanism involving a cyclic transition state.
Comparison of aminolysis of 2-pyridyl and 4-pyridyl x-substituted benzoates in acetonitrile: evidence for a concerted mechanism involving a cyclic transition state.
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乙腈中 2-吡啶基和 4-吡啶基 x-取代苯甲酸酯的氨解比较：涉及循环过渡态的协同机制的证据。
DOI：
10.1021/jo402629e
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发表时间：
2014
期刊：
影响因子：
--
通讯作者：
T. Um
中科院分区：
文献类型：
--
作者：
I. Um;A. Bae;T. Um
本文报道了2-吡啶X-取代苯甲酸酯（6a-i）与一系列环仲胺在乙腈中反应的动力学研究。6a-i与哌啶反应的Hammett图由两条相交的直线组成，而Yukawa-Tsuno图显示出与ρX = 1.28和r = 0.63的良好线性相关性，表明非线性Hammett图不是由速率决定步骤的变化引起的，而是由在苯甲酰基部分中具有给电子基团（EDG）的底物的共振稳定引起的。Brønsted型图与βnuc = 0.59 ± 0.02呈线性，这是报道的通过协调机制进行的典型反应。提出了一种环状过渡态（TS），它迫使反应通过协调机制进行。氘动力学同位素效应为1.3 ± 0.1，与所提出的机制一致。活化参数的分析表明，ΔH（H）随取代基X从吸电子基团（EWG）到EDG的变化而线性增加，而TΔS（H）几乎保持不变，并有一个较大的负值.常数TΔS（ε）值进一步支持了该反应通过与环状TS的协同机制进行的提议。
A kinetic study on reactions of 2-pyridyl X-substituted benzoates (6a-i) with a series of cyclic secondary amines in MeCN is reported. The Hammett plot for the reaction of 6a-i with piperidine consists of two intersecting straight lines while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρX = 1.28 and r = 0.63, indicating that the nonlinear Hammett plot is not caused by a change in the rate-determining step but rather by resonance stabilization of substrates possessing an electron-donating group (EDG) in the benzoyl moiety. The Brønsted-type plots are linear with βnuc = 0.59 ± 0.02, which is typical of reactions reported to proceed through a concerted mechanism. A cyclic transition state (TS), which forces the reaction to proceed through a concerted mechanism, is proposed. The deuterium kinetic isotope effect of 1.3 ± 0.1 is consistent with the proposed mechanism. Analysis of activation parameters reveals that ΔH(‡) increases linearly as the substituent X changes from an electron-withdrawing group (EWG) to an EDG, while TΔS(‡) remains nearly constant with a large negative value. The constant TΔS(‡) value further supports the proposal that the reaction proceeds through a concerted mechanism with a cyclic TS.