On the Basicity of Conjugated Nitrogen Heterocycles in Different Media
On the Basicity of Conjugated Nitrogen Heterocycles in Different Media
复制标题
DOI：
10.1002/ejoc.201700749
复制
发表时间：
2017-08-17
影响因子：
2.8
通讯作者：
Leito, Ivo
中科院分区：
文献类型：
--
作者：
Lokov, Mart;Tshepelevitsh, Sofja;Leito, Ivo
在这项工作中，我们探讨了结构和溶剂对不同介质（极性质子溶剂乙腈、极性质子溶剂水和气相）中大量选择的共轭N-杂环氮碱基的碱度之间的关系。总共提供了 39 种化合物在不同介质中的 58 个先前未发表的碱度值，其中包括 30 个在乙腈中通过实验确定的 pK(a) 值。我们给出了喹啉[7,8-h]喹啉的 pK(a) 和气相碱度值，它是最基本的共轭氮杂环化合物之一，没有增强碱度的取代基。通过比较不同溶剂中相关化合物的碱度数据以及使用等阻反应，碱度趋势合理化。气相碱度主要取决于分子将过量正电荷分散到大量原子上的能力。在溶液中，情况不太清楚，由于溶剂效应，具有局部电荷的较小系统通常会导致更高的碱度。特别是，发现额外苯环的稠合并不总是导致溶液碱度增加：其效果可以是碱度增加或减少，具体取决于环的大小、氮原子的数量和位置以及介质。 MeCN 和水中测得的 pK(a) 值之间的相关性表明，这两种不同的溶剂对所研究的杂环的碱性具有相似的影响。
In this work we explored the relationship between the structure and solvent effects on the basicity of a large selection of conjugated N-heterocyclic nitrogen bases in different media: the polar aprotic solvent acetonitrile, the polar protic solvent water and the gas phase. Altogether, 58 previously unpublished basicity values in different media for 39 compounds are presented, including 30 experimentally determined pK(a) values in acetonitrile. We present the pK(a) and gas-phase basicity values for quino[7,8-h]quinoline, which is one of the most basic conjugated nitrogen heterocyclic compounds without basicity-enhancing substituents. The trends in basicity are rationalized by comparing the basicity data of related compounds in different solvents, as well as by using isodesmic reactions. The gas-phase basicity is predominantly determined by the ability of a molecule to disperse the excess positive charge over a large number of atoms. In solution the situation is less clear and smaller systems with localized charge often lead to higher basicities because of solvent effects. In particular, it was found that the fusion of an additional benzene ring does not always lead to an increase in basicity in solution: its effect can be either basicity-increasing or -decreasing, depending on the ring size, number and position of nitrogen atoms and medium. A correlation between the measured pK(a) values in MeCN and in water suggests that these two different solvents exert a similar effect on the basicity of the studied heterocycles.