The α-Effect in Hydrazinolysis of 4-Chloro-2-Nitrophenyl X-Substituted-Benzoates: Effect of Substituent X on Reaction Mechanism and the α-Effect
作者:Min-Young Kim、Tae-Eun Kim、Jieun Lee、Ik-Hwan Um
DOI:10.5012/bkcs.2014.35.8.2271
日期:2014.8.20
Second-order rate constants ($k_N$) have been measured spectrophotometrically for the reaction of 4-chloro-2-nitrophenyl X-substituted-benzoates (6a-6h) with a series of primary amines including hydrazine in 80 mol % $H_2O$/20 mol % DMSO at $25.0^\circ}C$. The Br$\o}$nsted-type plot for the reaction of 4-chloro-2-nitrophenyl benzoate (6d) is linear with $\beta}_nuc}$ = 0.74 when hydrazine is excluded from the correlation. Such a linear Br$\o}$nsted-type plot is typical for reactions reported previously to proceed through a stepwise mechanism in which expulsion of the leaving group occurs in the rate-determining step (RDS). The Hammett plots for the reactions of 6a-6h with hydrazine and glycylglycine are nonlinear. In contrast, the Yukawa-Tsuno plots exhibit excellent linear correlations with $\rho}_X$ = 1.29-1.45 and r = 0.53-0.56, indicating that the nonlinear Hammett plots are not due to a change in RDS but are caused by resonance stabilization of the substrates possessing an electron-donating group (EDG). Hydrazine is ca. 47-93 times more reactive than similarly basic glycylglycine toward 6a-6h (e.g., the $\alpha}$-effect). The $\alpha}$-effect increases as the substituent X in the benzoyl moiety becomes a stronger electron-withdrawing group (EWG), indicating that destabilization of the ground state (GS) of hydrazine through the repulsion between the nonbonding electron pairs on the two N atoms is not solely responsible for the substituent-dependent $\alpha}$-effect. Stabilization of transition state (TS) through five-membered cyclic TSs, which would increase the electrophilicity of the reaction center or the nucleofugality of the leaving group, contributes to the $\alpha}$-effect observed in this study.
二次速率常数($k_N$)已通过分光光度法测定,用于4-氯-2-硝基苯基X取代苯甲酸酯(6a-6h)与一系列伯胺(包括80摩尔% $H_2O$/20摩尔% DMSO中的25.0°C下的联氨)的反应。当联氨被排除在相关性之外时,4-氯-2-硝基苯基苯甲酸酯(6d)反应的Br$\o}$nsted型图是线性的,$\beta}_nuc}$ = 0.74。这种线性Br$\o}$nsted型图是典型的反应,先前报道这些反应通过逐步机制进行,其中离去基团的排出发生在速率决定步骤(RDS)中。6a-6h与联氨和甘氨酰甘氨酸反应的Hammett图是非线性的。相比之下,Yukawa-Tsuno图显示出极佳的线性相关性,$\rho}_X$ = 1.29-1.45,r = 0.53-0.56,表明非线性Hammett图并非由于RDS的变化,而是由于具有供电子基团(EDG)的底物的共振稳定化所导致。联氨对6a-6h的反应性大约是同样碱性的甘氨酰甘氨酸的47-93倍(例如,$\alpha}$效应)。随着苯甲酰基中取代基X成为更强的吸电子基团(EWG),$\alpha}$效应增加,表明通过两个N原子上的非键电子对之间的排斥来破坏联氨的基态(GS)并不是唯一导致取代基依赖的$\alpha}$效应的原因。通过五元环过渡态(TS)的稳定化,这将增加反应中心的亲电性或离去基团的离核性,有助于在本研究中观察到的$\alpha}$效应。