Capturing Reactive Carbanions by Microdroplets

Carbanions appear in many organic or biological reactions as fleeting intermediates, prohibiting direct observation orspectroscopic measurement. An aqueous environment is known to rapidly annihilate a carbanion species, reducing its lifetime to asshort as picoseconds. We report that aqueous microdroplets can capture and stabilize reactive carbanion intermediates isolated fromfour classic organic reactions, aldol and Knoevenagel condensations, alkyne alkylation, and the Reimer-Tiemann reaction, enablingthe detection of their carbanion intermediates by desorption electrospray ionization mass spectrometry. This is accomplished in realtime of the reaction, allowing new insights into reaction mechanisms to be obtained. The efficacy of microdroplets in capturing suchelusive species was examined by varying the solvent and the microdroplet negative charge density. We observed that microdropletscomposed of water-methanol outperform other solvents, such as pure water, in capturing carbanions, which is in contrast to theearlier 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

Automated summary

Carbanions, which are transient intermediates in organic or biological reactions, are difficult to observe directly. This study demonstrates that aqueous microdroplets can capture and stabilize carbanion intermediates from various reactions, allowing for their detection and providing insights into reaction mechanisms. The composition of the microdroplets, specifically water-methanol, plays a crucial role in the efficiency of carbanion capture.