Second-order rate constants have been measured spectrophotometrically for the reactions of phenyl Y-substituted-phenyl carbonates 7a-g with butane-2,3-dione monoximate ($Ox^-$) in 80 mol % $H_2O$/20 mol % DMSO at $25.0\pm}0.1^\circ}C$. The $\alpha}$-nucleophile $Ox^-$ is 53-95 times more reactive than the corresponding normal-nucleophile 4-$ClPhO^-$ toward 7a-g, indicating that the $\alpha}$-effect is operative. The magnitude of the $\alpha}$-effect (e.g., the $k_Ox^-}/k_4-ClPhO^-}$ ratio) is independent of the electronic nature of the substituent Y. The cause of the $\alpha}$-effect for the reactions of 7a-g has been suggested to be ground-state (GS) effect rather than transition-state (TS) stabilization through a six-membered cyclic TS, in which $Ox^-$ behaves a general acid/base catalyst. This idea is further supported by the result that $OH^-$ exhibits negative deviation from the linear Br$\o}$nsted-type plot composed of a series of aryloxides, while $Ox^-$ deviates positively from the linearity. Differential solvation of the GS of $Ox^-$ and 4-$ClPhO^-$ has been suggested to be responsible for the $\alpha}$-effect exerted by $Ox^-$.
通过分光光度法测定了苯基 Y-取代
苯基碳酸酯 7a-g 与
丁二酮单氧
亚胺 (
$Ox^-$) 在 80 mol %
$H_2O$/20 mol %
DMSO 中于
$25.0\pm}0.1^\circ}C$ 下的二级速率常数。
$\alpha}$-亲核试剂
$Ox^-$ 对于 7a-g 的反应活性比相应的常规亲核试剂 4-
$ClPhO^-$ 高出 53-95 倍,这表明
$\alpha}$-效应在起作用。
$\alpha}$-效应的大小(例如
$k_Ox^-}/k_4-ClPhO^-}$ 比率)与取代基 Y 的电子性质无关。对于 7a-g 反应,
$\alpha}$-效应的原因被认为是基态(GS)效应,而不是通过六元环过渡态(
TS)的过渡态稳定化,其中
$Ox^-$ 起到了广义酸碱催化剂的作用。这一观点得到了以下结果的支持:
$OH^-$ 显示了从一列芳氧基化合物构成的线性 Br
$\o}$nsted 型图中负偏离,而
$Ox^-$ 则正偏离线性关系。
$Ox^-$ 和 4-
$ClPhO^-$ 的基态的溶剂化差异被认为是
$Ox^-$ 所施加的
$\alpha}$-效应的原因。