The nucleophilic substitution reactions of 1,2-phenylene phosphorochloridate (1c) with X-pyridines are investigated kinetically in acetonitrile at $-25.0^\circ}C$. The free energy correlations for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Ph. The pyridinolysis rate of 1c with a cyclic five-membered ring is $2.70\times}10^5$ times faster than its acyclic counterpart (1a: phenyl ethyl chlorophosphate) because of great positive value of the entropy of activation of 1c ($\Delta}S^\neq}$ = +26 eu) compared to negative value of 1a ($\Delta}S^\neq}$= -24 eu) over considerably unfavorable enthalpy of activation of 1c ($\Delta}H^\neq}=20.5kcal\;mol^-1}$) compared to 1a ($\Delta}H^\neq}=12.7kcal\;mol^-1}$). Great enthalpy and positive entropy of activation are ascribed to sterically congested transition state (TS) and solvent structure breaking in the TS. A concerted mechanism involving a change of nucleophilic attacking direction from a frontside attack with the strongly basic pyridines to a backside attack with the weakly basic pyridines is proposed on the basis of greater selectivity parameters ($\rho}_X$ = -1.99 and $\beta}_X$ = 0.41) with the strongly basic pyridines compared to those ($\rho}_X$ = -0.42 and $\beta}_X$ = 0.07) with the weakly basic pyridines.
在
乙腈中,以动力学方法研究了1,2-苯二基膦
氯化物(1c)与X-
吡啶的亲核取代反应,反应温度为-25.0°C。对于亲核试剂中取代基X的变化,自由能相关性呈现出双相向上凹,且在X=3-Ph处有一个断裂点。1c的
吡啶解离速率是它的非环状类似物(1a:苯基乙基
氯膦酸酯)的2.70×10^5倍,这是由于与1a相比,1c的活化熵(ΔS≠=+26 eu)为正值,而1a的活化熵为负值(ΔS≠=-24 eu),尽管1c的活化焓(ΔH≠=20.5 kcal mol^-1)比1a的(ΔH≠=12.7 kcal mol^-1)更不利。巨大的活化焓和正值的活化熵归因于过渡态(
TS)的立体拥挤以及
TS中溶剂结构的破坏。基于与弱碱性
吡啶相比,强碱性
吡啶的选择性参数(ρX=-1.99和βX=0.41)大于弱碱性
吡啶的(ρX=-0.42和βX=0.07),提出了一个协同机制,涉及亲核攻击方向从强碱性
吡啶的前侧攻击到弱碱性
吡啶的后侧攻击的改变。