Department of Inorganic and Analytical Chemistry, Hungarian Academy of Sciences, Budapest University of Technology and Economics, Szt. Gellért tér 4., Budapest, H-1111 Hungary.
J Am Chem Soc. 2011 Feb 2;133(4):780-9. doi: 10.1021/ja103578y. Epub 2010 Dec 21.
The direct reaction of an imidazole-2-ylidene in a predominantly aqueous environment [about 0.1 M solution in a H(2)O (>60%)/THF solvent system] was investigated for the first time. The reaction yielded a stable solution of the corresponding imidazolium-hydroxide of pH 13, which is in agreement with results from an ab initio molecular dynamics simulation. In contrast, hydrolysis of the carbene in a mainly aprotic environment (>80% THF) gives a hydrogen-bridged carbene-water complex which could be detected by NMR and IR spectroscopies for the first time. This complex converts slowly to two isomeric ring opened products and is at higher water concentration in dynamic equilibrium with the imidazolium hydroxide. A computational mechanistic study of the carbene hydrolysis with a gradually increasing number of water molecules revealed that the imidazolium-hydroxide structure can only be optimized with three or more water molecules as reactants, and with the increasing number of water molecules its stability is increasing with respect to the carbene-water complex. In agreement with the experimental results, these findings point out that solvent stabilization and basicity of the hydroxide ion plays a crucial role in the reaction. With increasing number of water molecules the barriers connecting the reaction intermediates are getting smaller, and the ring opened hydrolysis products can be derived from imidazolium-hydroxide type intermediates. Computational studies on the hydrolysis of a nonaromatic imidazolidine-2-ylidene analogue clearly indicated the analogous ring-opened product to be by 10-12 kcal/mol more stable than the appropriate ion pair and the carbene-water complex, in agreement with the known aromatic stabilization of imidazol-2-ylidenes. Accordingly, these molecules hydrolyze with exclusive formation of the ring-opened product.
首次研究了在主要为水的环境中（在 H(2)O (>60%)/THF 溶剂体系中约 0.1 M 的溶液）咪唑-2-亚基的直接反应。该反应生成了相应的咪唑翁-氢氧化物的稳定溶液，其 pH 值为 13，这与从头算分子动力学模拟的结果一致。相比之下，在主要为非质子的环境（>80% THF）中，碳烯的水解生成了可通过 NMR 和 IR 光谱首次检测到的氢键桥接碳烯-水络合物。该络合物缓慢转化为两种异构的开环产物，并在较高的水浓度下与咪唑翁-氢氧化物处于动态平衡。随着水分子数量的逐渐增加，对碳烯水解的计算机理研究表明，只有当反应物中有三个或更多的水分子时，才能对咪唑翁-氢氧化物结构进行优化，并且随着水分子数量的增加，它相对于碳烯-水络合物的稳定性也在增加。与实验结果一致，这些发现表明溶剂稳定化和氢氧根离子的碱性在反应中起着至关重要的作用。随着水分子数量的增加，连接反应中间体的势垒变得更小，并且可以从咪唑翁-氢氧化物型中间体衍生出开环水解产物。对非芳香咪唑烷-2-亚基类似物水解的计算研究清楚地表明，开环水解产物比相应的离子对和碳烯-水络合物稳定 10-12 kcal/mol，这与已知的咪唑-2-亚基的芳香稳定化一致。因此，这些分子水解时仅形成开环产物。
