首页> 外文期刊>Bulletin of the Korean Chemical Society >Aminolysis of Benzyl 2-Pyridyl Thionocarbonate and t-Butyl 2-Pyridyl Thionocarbonate: Effects of Nonleaving Groups on Reactivity and Reaction Mechanism
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Aminolysis of Benzyl 2-Pyridyl Thionocarbonate and t-Butyl 2-Pyridyl Thionocarbonate: Effects of Nonleaving Groups on Reactivity and Reaction Mechanism

机译:苄基2-吡啶基硫代碳酸酯和叔丁基2-吡啶基硫代碳酸酯的氨解:非离去基团对反应性和反应机理的影响

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摘要

A kinetic study is reported for nucleophilic substitution reactions of benzyl 2-pyridyl thionocarbonate (5b) and t-butyl 2-pyridyl thionocarbonate (6b) with a series of alicyclic secondary amines in H2O at 25.0 °C. General-base catalysis, which has often been reported to occur for aminolysis of esters possessing a C=S electrophilic center, is absent for the reactions of 5b and 6b. The Bronsted-type plots for the reactions of 5b and 6b are linear with β_(nuc) = 0.29 and 0.43, respectively, indicating that the reactions of 5b proceed through a stepwise mechanism with formation of a zwitterionic tetrahedral intermediate (T~±) being the rate-determining step while thoseof 6b proceed through a concerted mechanism. The reactivity of 5b and 6b is similar to that of their oxygen analogues (i.e., benzyl 2-pyridyl carbonate 5a and t-butyl 2-pyridyl carbonate 6a, respectively), indicating that the effect of modification of the electrophilic center from C=O to C=S (z. e., from 5a to 5b and from 6a to 6b) on reactivity is insignificant. In contrast, 6b is much less reactive than 5b, indicating that the replacement of the PhCH2 in 5b by the t-Bu in 6b results in a significant decrease in reactivity as well as a change in the reaction mechanism (z. e., from a stepwise mechanism to a concerted pathway). It has been concluded that the contrasting reactivity and reaction mechanism for the reactions of 5b and 6b are not due to the electronic effects of PhCH2 and t-Bu but are caused by the large steric hindrance exerted by the bulky t-Bu in 6b.
机译:据报道,在25.0°C的条件下,H2O中的2-吡啶基硫代碳酸苄酯(5b)和2-吡啶硫代碳酸叔丁基酯(6b)与一系列脂环族仲胺发生亲核取代反应的动力学研究。 5b和6b的反应不存在通用碱催化,该反应常报道发生在具有C = S亲电中心的酯的氨解反应中。 5b和6b反应的布朗斯德型图分别与β_(nuc)= 0.29和0.43呈线性关系,表明5b反应通过逐步机理进行,两性离子四面体中间体(T〜±)的形成为速率确定步骤,而图6b中的那些则通过协调机制进行。 5b和6b的反应活性类似于它们的氧类似物(即分别为碳酸苄基2-吡啶基酯5a和碳酸叔丁基2-吡啶酯6a),表明从C = O修饰亲电子中心的作用对C = S(ze,从5a至5b和从6a至6b)的反应性微不足道。相反,6b的反应活性远低于5b,这表明6b中的t-Bu取代5b中的PhCH2会导致反应性显着降低以及反应机理的改变(ze,从逐步机理看协调一致的途径)。已经得出结论,5b和6b反应的对比反应性和反应机理不是由于PhCH2和t-Bu的电子效应,而是由6b中庞大的t-Bu施加的大位阻引起的。

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