The α-Effect in Nucleophilic Substitution Reactions of Y-Substituted-Phenyl Diphenylphosphinates with HOO<sup>-</sup>and OH<sup>-</sup>
作者:Hyo-Jeong Hong、Ae Ri Bae、Ik-Hwan Um
DOI:10.5012/bkcs.2013.34.8.2251
日期:2013.8.20
Second-order rate constants ($k_HOO^-}$) for the nucleophilic substitution reactions of Y-substituted-phenyl diphenylphosphinates (4a-4i) with $HOO^-$ in $H_2O$ have been measured spectrophotometrically. The $\alpha}$-nucleophile $HOO^-$ is 10-70 times more reactive than the reference nucleophile $OH^-$ although the former is ca. $4pK_a$ units less basic than the latter, indicating the $\alpha}$-effect is operative. The Bronsted-type plot for the reactions of 4a-4i with $HOO^-$ is linear with $\beta}_lg}=-0.51$, a typical $\beta}_lg}$ value for reactions which were reported to proceed through a concerted mechanism. The Yukawa-Tsuno plot is also linear with $\rho}=1.40$ and r = 0.47, indicating that a negative charge develops partially on the O atom of the leaving group, which can be delocalized to the substituent Y through resonance interactions. Thus, the reactions have been proposed to proceed through a concerted mechanism. The magnitude of the $\alpha}$-effect (i.e., the $k_HOO^-}/k_HO^-}$ ratio) decreases linearly as the leaving-group basicity increases. It has been concluded that solvation effect is not solely responsible for the $\alpha}$-effect found in this study but the transition-state stabilization through an intramolecular H-bonding interaction is also responsible for the $\alpha}$-effect.
用分光光度法测量了 Y-取代苯基二苯基膦酸盐 (4a-4i) 与 $HOO^-$ 在 $H_2O$ 中发生亲核取代反应的二阶速率常数 ($k_HOO^-}$)。$alpha}$亲核物 $HOO^-$的反应性比参考亲核物 $OH^-$ 高 10-70 倍,尽管前者的反应性约为 $4pK。$4pK_a$单位的碱性比后者低,这表明$α}$效应在起作用。4a-4i 与 $HOO^-$ 反应的勃朗斯特型曲线图是线性的,$\beta}_lg}=-0.51$,这是一个典型的$\beta}_lg}$值,据报道反应是通过协同机制进行的。Yukawa-Tsuno 图也是线性的,$\rho}=1.40$,r = 0.47,这表明在离去基团的 O 原子上产生了部分负电荷,该负电荷可通过共振作用分散到取代基 Y 上。因此,这些反应被认为是通过协同机制进行的。随着离去基团碱性的增加,$\alpha}$效应的大小(即$k_HOO^-}/k_HO^-}$比率)呈线性下降。研究得出的结论是,溶解效应并不是本研究中发现的 $\alpha}$ 效应的唯一原因,通过分子内 H 键相互作用实现的过渡态稳定也是 $\alpha}$ 效应的原因。