Department of Chemistry, Indian Institute of Science Education and Research Tirupati, Tirupati 517507, India.
Department of Chemistry, Stanford University, Stanford, California 94305, United States.
J Am Chem Soc. 2022 May 4;144(17):7573-7577. doi: 10.1021/jacs.2c01577. Epub 2022 Apr 22.
Carbanions appear in many organic or biological reactions as fleeting intermediates, prohibiting direct observation or spectroscopic measurement. An aqueous environment is known to rapidly annihilate a carbanion species, reducing its lifetime to as short as picoseconds. We report that aqueous microdroplets can capture and stabilize reactive carbanion intermediates isolated from four classic organic reactions, aldol and Knoevenagel condensations, alkyne alkylation, and the Reimer-Tiemann reaction, enabling the detection of their carbanion intermediates by desorption electrospray ionization mass spectrometry. This is accomplished in real time of the reaction, allowing new insights into reaction mechanisms to be obtained. The efficacy of microdroplets in capturing such elusive species was examined by varying the solvent and the microdroplet negative charge density. We observed that microdroplets composed of water-methanol outperform other solvents, such as pure water, in capturing carbanions, which is in contrast to the earlier report that presented the highest performance of pure water microdroplets in capturing carbocations. We offer some mechanistic insights to explain the discriminatory behavior of these two oppositely charged species in microdroplets.
碳负离子在许多有机或生物反应中作为短暂的中间体出现，禁止直接观察或光谱测量。众所周知，水相环境会迅速消除碳负离子物种，将其寿命缩短至皮秒级。我们报告说，水微滴可以捕获和稳定从四个经典有机反应（Aldol 和 Knoevenagel 缩合、炔烃烷基化和 Reimer-Tiemann 反应）中分离出来的反应性碳负离子中间体，通过解吸电喷雾电离质谱法检测它们的碳负离子中间体。这可以在反应的实时进行，从而获得对反应机制的新见解。通过改变溶剂和微滴负电荷密度来检查微滴捕获这种难以捉摸的物种的效果。我们观察到，由水-甲醇组成的微滴在捕获碳负离子方面优于其他溶剂，如纯水，这与早期报道的纯水微滴在捕获碳正离子方面表现出最高性能的情况相反。我们提供了一些机制上的见解，以解释这两种带相反电荷的物质在微滴中的歧视性行为。
