离子液体中芳环亲核取代机理及选择性研究

Solvents are ubiquitous and have played a very important role in the physical organic chemistry history. It is necessary to understand the solvent effects on organic reactions, which would acts as a guide to find suitable medium systems for the chemical industry. Ionic Liquids are labelled as a green solvent with relative low vapour pressures and high boiling points, and have been widely studied and applied among the academic and industry. However, compared with the rapid developments of synthetic applications of ILs, fundamental aspects of chemical transformations in ILs are severely lagged behind, especially in solvation behaviour and solvent effects of ionic liquids on the organic reactions. These issues significantly hampers a rational development of chemistry in ionic liquids. In this application proposal, a systematical study of kinetics for aromatic nucleophilic substitution (SNAr) in ionic liquids will be performed by using a series of classical activated arenes as potential substrates and various type of O, S and N compounds as nucleophiles. By combining the thermodynamic parameter, pKa of nucleophiles and kinetic results, the possible mechanism of SNAr reactions in ionic liquids will be elucidated by the construction of Linear Free Energy Relationship (LFER), such as, Hammett and Bronsted plots. The regio-selectivity for the SNAr in ILs will also be studied in detail, the general effects of cations and anion on the selectivity will be discussed.

溶剂效应在物理有机化学研究中占据重要地位，理解溶剂对有机化学反应的影响，对化学工业具有指导意义。离子液体是一种具有较低蒸汽压，较高沸点的绿色溶剂，在科研工业上近些年得到广泛研究和应用，但相对于在其中的合成应用研究，离子液体对有机物的溶剂化以及对有机反应的溶剂效应研究较为滞后, 严重的阻碍了离子液体中的理性化学的发展。本课题拟在系统研究在若干常见室温离子液体中一系列经典芳环化合物为底物，采用在这些离子液体中已知pKa的若干不同结构的有机氧酸，氮酸及苯硫酚对应的共轭碱作为亲核试剂，利用Hammett和Brönsted 方程等线性自由能相关（LFER）参数来描述在离子液体中的SNAr的反应机理，同时与其他分子溶剂进行对比从而描述离子液体对此类反应的溶剂效应。此外，本研究还将涉及离子液体中芳环亲核取代的区域选择项研究，并尝试得出阴阳离子对选择性的一般规律。

项目的背景主要研究内容已经在申请书中详细介绍。 在本项目的支持下，探索结果表明，离子液体中胺类化合物作为芳环亲核试剂，其反应速率和一些个常见的非质子型分子溶剂如DMSO、DMF中相当。选择性研究结果表明离子液体的阴离子对2，4-二氟硝基苯的取代反应选择性有着决定性作用。此外质子型离子液体阳离子结构对芳环取代的选择性也有一定的影响。我们还对一些列吡咯类质子型离子液体中的溶剂化行为做了必要探索，采用改变阴阳离子的结构的方式来研究不同结构电性的底物的平衡酸度，利用大量线性自由能相关和回归分析对质子型离子液体和分子溶剂，非质子型离子液体的溶剂化区别进行了研究。同时，我们对离子液体中的一类碳酸，硝基烷烃的平衡酸度进行了系统测量，延伸了离子液体中酸度标尺的范围。此外，我们还采用理论计算的方法，对大约500个氮杂环卡宾，85个氮杂环烯烃在DMSO中的碱性进行了计算，并发现它们和二氧化碳加和物的稳定性之间存在线性相关，为未来理性设计和应用这些氮杂环化合物提供了热力学理论支持。