Formation and stability of N-heterocyclic carbenes in water:: The carbon acid pKa of imidazollum cations in aqueous solution
Formation and stability of N-heterocyclic carbenes in water:: The carbon acid pKa of imidazollum cations in aqueous solution
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DOI：
10.1021/ja039890j
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发表时间：
2004-04-07
影响因子：
15
通讯作者：
Toth, K
中科院分区：
文献类型：
--
作者：
Amyes, TL;Diver, ST;Toth, K
本文报道了在25 ℃和I = 1.0（KCl）的D_2O溶液中，一系列简单咪唑阳离子交换氘的二级速率常数k（DO）（M ~（-1）s ~（-1）），得到相应的单重态咪唑-2-基卡宾。证明了咪唑-2-基卡宾被溶剂水逆质子化反应受溶剂重组的限制，反应速率常数为k（HOH）= k（reorg）= 10（11）s（-1）.数据用于计算可靠的碳酸pK（a）s，用于在C（2）处电离咪唑鎓阳离子，以在水中得到相应的单线态咪唑-2-基卡宾：对于咪唑鎓阳离子，pK（a）= 23.8，对于1，3-二甲基咪唑鎓阳离子，pK（a）= 23.0，对于1，3-二甲基苯并咪唑鎓阳离子，pK（a）= 21.6，1，3-双-（（S）-1-苯乙基）苯并咪唑鎓阳离子的pK（a）= 21.2。这些数据还提供了在单线态卡宾处的1，2-氢位移的热力学驱动力：对于母体咪唑-2-基卡宾在298 K下重排以在水中得到中性咪唑，K-12 = 5 × 10（16），这对应于23 kcal/mol的有利吉布斯自由能变化。我们提出了一个简单的理由所观察到的取代基对N-杂环卡宾相对于各种中性和阳离子衍生物的热力学稳定性的影响，强调了在评估N-杂环卡宾的稳定性时选择参考反应的重要性。
We report second-order rate constants k(DO) (M-1 s(-1)) for exchange for deuterium of the C(2)-proton of a series of simple imidazolium cations to give the corresponding singlet imidazol-2-yl carbenes in D2O at 25 degreesC and I = 1.0 (KCl). Evidence is presented that the reverse protonation of imidazol-2-yl carbenes by solvent water is limited by solvent reorganization and occurs with a rate constant of k(HOH) = k(reorg) = 10(11) s(-1). The data were used to calculate reliable carbon acid pK(a)s for ionization of imidazolium cations at C(2) to give the corresponding singlet imidazol-2-yl carbenes in water: pK(a) = 23.8 for the imidazolium cation, pK(a) = 23.0 for the 1,3-dimethylimidazolium cation, pK(a) = 21.6 for the 1,3-dimethylbenzimidazolium cation, and pK(a) = 21.2 for the 1,3-bis-((S)-1-phenylethyl)benzimidazolium cation. The data also provide the thermodynamic driving force for a 1,2-hydrogen shift at a singlet carbene: K-12 = 5 x 10(16) for rearrangement of the parent imidazol-2-yl carbene to give neutral imidazole in water at 298 K, which corresponds to a favorable Gibbs free energy change of 23 kcal/mol. We present a simple rationale for the observed substituent effects on the thermodynamic stability of N-heterocyclic carbenes relative to a variety of neutral and cationic derivatives that emphasizes the importance of the choice of reference reaction when assessing the stability of N-heterocyclic carbenes.