First-principle calculation of equilibrium cesium ion-pair acidities in tetrahydrofuran
First-principle calculation of equilibrium cesium ion-pair acidities in tetrahydrofuran
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四氢呋喃中铯离子对平衡酸度的第一性原理计算
DOI：
10.1021/ja073548f
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发表时间：
2007-11-07
影响因子：
15
通讯作者：
Guo, Qing-Xiang
中科院分区：
文献类型：
--
作者：
Fu, Yao;Shen, Kuang;Guo, Qing-Xiang
THF 中有机酸的离子对酸性是合成有机化学的基础。尽管 Streitwieser 和同事已经通过实验测量了许多碳酸的离子对酸度，但开发一种能够准确预测这些量的理论方法非常重要，因为并非所有有机酸（例如非常弱的酸或具有多个酸性位置的复杂合成中间体）都适合实验表征。在本研究中，报告了第一个预测 THF 中铯离子对酸度的理论方案，其可靠性已针对几乎所有可用的实验数据进行了测试。发现当前理论模型的均方根误差等于1.2 pK单位。利用新开发的理论方法，研究了不同类型碳酸的铯离子对的结构。还系统地比较了THF中的铯离子对酸度和DMSO中的绝对离子酸度，这证实了Streitwieser之前的发现，即两种酸度范围仅有微小差异。值得注意的是，通过详细的能量分析，发现了两种酸度“幸运”匹配的机制。也就是说，Cs 结合（“微”溶剂化）和离子对溶剂化的组合过程类似于碳负离子在 DMSO 中的一步溶剂化。最后，发现THF中含氮酸的铯离子对酸度与DMSO中的绝对离子酸度仅有微小差异。因此，人们可以根据 Bordwell 的数据轻松估计 THF 中几乎所有类型的有机含氮酸的铯离子对酸度。
The ion-pair acidities of organic acids in THF are fundamental to synthetic organic chemistry. Although the ion-pair acidities of a number of carbon acids have been experimentally measured by Streitwieser and co-workers, it is important to develop a theoretical method that can accurately predict these quantities because not all the organic acids (e.g., very weak acids or complex synthetic intermediates with multiple acidic positions) are amenable to experimental characterization. In the present study is reported the first theoretical protocol for predicting the cesium ion-pair acidities in THF whose reliability has been tested against almost all the available experimental data. It is found that the root-mean-square error of the current theoretical model equals 1.2 pK units. With the newly developed theoretical method in hand, the structures of cesium ion pairs of different types of carbon acids are then studied. The cesium ion-pair acidities in THF and absolute ionic acidities in DMSO are also systematically compared, which confirms Streitwieser's previous finding that the two scales of acidities have only minor difference. Significantly, from detailed energy analysis the mechanism for the "fortunate" match of the two scales of acidities is found. That is, the combined process of the Cs binding ("micro"-solvation) and the solvation of the ion pair resembles the one-step solvation of a carbanion in DMSO. Finally, it is found that the cesium ion-pair acidities of nitrogen acids in THF have only minor difference from the absolute ionic acidities in DMSO. Consequently, one can easily estimate the cesium ion-pair acidities of almost all types of organic nitrogen acids in THF on the basis of Bordwell's data.