Elucidation of the Vicarious Nucleophilic Substitution of Hydrogen Mechanism via Studies of Competition between Substitution of Hydrogen, Deuterium, and Fluorine
作者:Mieczysław Ma̧kosza、Tadeusz Lemek、Andrzej Kwast、François Terrier
DOI:10.1021/jo010590z
日期:2002.1.1
Relations of rates of the vicarious nucleophilic substitution of hydrogen (VNS) and S(N)Ar substitution of fluorine in 2-fluoronitrobenzenes with chloroalkyl aryl sulfone carbanions were determined from competitive experiments carried out at various concentrations of base. The observed dependence of the VNS/S(N)Ar rate ratio on the base concentration confirmed the two-step mechanism of the VNS, which
通过在各种浓度的碱下进行的竞争性实验确定了2-氟硝基苯中的氟的亲核取代度(VNS)和氟的S(N)Ar取代率与氯烷基芳基砜碳负离子的关系。观察到的VNS / S(N)Ar比率与碱浓度的相关性证实了VNS的两步机理,该过程由可逆形成的α-氯碳阴离子的σ(H)加合物形成为硝基芳烃,然后是碱-诱导的HCl的β消除。还证明这两个过程都是限速步骤:低碱浓度下的β-消除和高碱浓度下的亲核加成。与该结论一致的发现是,VNS反应中的动力学同位素效应从4降低。2(初级KIE的典型值)到0.8(次级KIE的典型值)随碱浓度的增加而增加。还报道了我们的发现,即在我们的竞争性实验中使用的某些实验条件下,这项工作中研究的2-氟硝基苯的S(N)Ar取代受到碱催化。