Second-order rate constants for reactions of 2-chloro-4-nitrophenyl X-substituted-benzoates 1a–1j with a series of cyclic secondary amines in MeCN have been measured. Comparison of the kinetic results with those reported previously for the corresponding reactions carried out in H2O containing 20 mol % DMSO has revealed that amines are less reactive in MeCN. The Brønsted-type plot for the aminolysis of 2-chloro-4-nitrophenyl benzoate (1f) in MeCN is linear with βnuc = 0.64, which is in contrast to the curved Brønsted-type plot reported for the reaction performed in the H2O–DMSO misxture. The Hammett plot for the reactions of 1a–1j with piperidine consists of two intersecting straight lines while the Yukawa–Tsuno plot exhibits an excellent linear correlation with ρX = 1.22 and r = 0.60, indicating that the nonlinear Hammett plot is not due to a change in rate-determining step but is caused by stabilization of substrates possessing an electron-donating substituent through resonance interactions between the substituent and the carbonyl functionality. It has been concluded that medium change from the H2O–DMSO mixture to MeCN forces the aminolysis of 1a–1j to proceed through a concerted mechanism by destabilizing the zwitterionic tetrahedral intermediate (T±).
测量了2-
氯-4-
硝基苯基X取代
苯甲酸酯1a–1j与一系列环状二级胺在MeCN中的反应的二级反应速率常数。将这些动力学结果与先前在含20摩尔%
DMSO的
H2O中进行的相应反应进行比较,发现胺在MeCN中的反应性较低。
2-氯-4-硝基苯甲酸酯(1f)在MeCN中的胺解反应的布朗斯特型图呈线性,βnuc = 0.64,这与在 -
DMSO混合物中进行反应时报告的曲线布朗斯特型图相对比。1a–1j与
哌啶反应的哈密特图由两条相交的直线组成,而由Yukawa-Tsuno图显示出良好的线性相关,ρX = 1.22,r = 0.60,表明非线性哈密特图并不是由于速率决定步骤的变化,而是由于带有电子供给取代基的底物通过取代基与羰基功能之间的共振相互作用而得到稳定。结论是,从 -
DMSO混合物转变为MeCN的介质改变使得1a–1j的胺解反应通过协同机制进行,因为它使两性四面体中间体(T±)不稳定。