Effects of ion-pairing and hydration on the SNAr reaction of the F- with p-chlorobenzonitrile in aprotic solvents
Effects of ion-pairing and hydration on the SNAr reaction of the F- with p-chlorobenzonitrile in aprotic solvents
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DOI：
10.1039/b716159j
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发表时间：
2008-01-01
影响因子：
3.3
通讯作者：
Pilo-Veloso, Dorila
中科院分区：
文献类型：
--
作者：
Pliego, Josefredo R., Jr.;Pilo-Veloso, Dorila
采用包括液相优化的从头算方法研究了氟离子与对氯苯腈在二甲基亚砜溶液中的SNAr反应。分析了反离子和氟离子与一个水分子的水合作用的影响。计算结果表明，气相SNAr反应比相应的涉及氟离子和2-氯丁烷的S（N）2反应更有利。然而，在SNAr反应上的显著更高的溶剂效应使得芳环上的亲核取代比液相中的脂族反应更不利。对于无水四丁基氟化铵体系，理论自由能垒为22.1 kcal mol（-1），与实验自由能垒24.4 kcal mol（-1）接近。较小的四甲基铵阳离子与氟离子强烈缔合，使势垒增加5 kcal mol（-1）。同样地，只有一个水分子水合氟离子具有相同的效果。对涉及离子对和自由阴离子在不同极性介质中的反应的分析预测了意想不到的行为，并表明每个反离子都有一个理想的溶剂极性。
Theoretical ab initio calculations including liquid phase optimizations were used to investigate the SNAr reaction of the fluoride ion with p-chlorobenzonitrile in dimethyl sulfoxide solution. The effect of the counter ion and hydration of the fluoride ion with one water molecule was analyzed. The calculations indicate that the gas-phase SNAr reaction is more favorable than the corresponding S(N)2 reactions involving fluoride ion and 2-chlorobutane. However, the substantially higher solvent effect on the SNAr reaction makes the nucleophilic substitution on the aromatic ring less favorable than the aliphatic reaction in the liquid phase. For the anhydrous tetrabutylammonium fluoride system, the theoretical free energy barrier of 22.1 kcal mol(-1) is close to the experimental one of 24.4 kcal mol(-1). The smaller tetramethylammonium cation strongly associates with the fluoride ion and increases the barrier by 5 kcal mol(-1). Similarly, just one water molecule hydrating the fluoride ion has the same effect. An analysis of the reaction involving the ion pair and the free anion in different polarity media predicts an unexpected behavior and indicates there is an ideal solvent polarity for each counter ion.