The nucleophilic substitution reactions of bis(2,6-dimethylphenyl) chlorophosphate (5), containing the four ortho-methyl substituents, with X-pyridines are investigated kinetically in MeCN at $65.0^\circ}C$. The free energy correlations for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a breakpoint at a X = 3-Cl. Unusual positive $\rho}_X$ (= +5.40) and negative $\beta}_X$ (= -0.83) values are obtained with the weakly basic pyridines. The pyridinolysis rate of 5 is hundreds times slower compared to that of bis(phenyl) chlorophosphate because of the steric hindrance of the four ortho-methyl substituents in the two phenyl rings. Ion-pair mechanism is proposed and positive $\rho}_X$ and negative $\beta}_X$ values are substantiated by an imbalance of the transition state.
在 MeCN 中,在
$65.0^\circ}C$ 条件下,研究了含有四个正交甲基取代基的双(2,6-二甲基苯基)
氯磷酸酯 (5) 与 X-
吡啶的亲核取代反应。亲核物中取代基 X 变化的自由能相关性呈现出双相凹向上的趋势,在 X = 3-Cl 处出现断点。弱碱性
吡啶的
$\rho}_X$ (= +5.40)和
$\beta}_X$ (= -0.83)值不同寻常。由于两个苯基环中的四个正交甲基取代基的立体阻碍作用,5 的
吡啶分解速率比
氯磷酸二(苯基)酯慢数百倍。提出了离子对机制,并通过过渡态的不平衡证实了正
$\rho}_X$和负
$\beta}_X$值。