Destabilized Carbocations Caged in Water Microdroplets: Isolation and Real-Time Detection of α-Carbonyl Cation Intermediates

Over the last 50 years, proposals of alpha-carbonyl cation intermediates have been driven by chemical intuition and indirect evidence. Recently, wide interest in alpha-carbonyl cation chemistry has opened new gates to prepare alpha-functionalized carbonyl compounds. Though these intrinsically unstable carbocations are formed under forcing conditions (e.g., in a strong acid medium), their fleeting existence prohibits direct observation or spectroscopic measurement. We report that high-speed aqueous microdroplets can directly capture alpha-carbonyl cation intermediates from various reactions (Friedel-Crafts arylation, deoxygenation, and azidation) upon bombarding with the corresponding reaction aliquots. The alpha-carbonyl cations caged in water droplets are then desorbed to the gas phase, allowing their successful measurement by mass spectrometry. This has also enabled us to simultaneously monitor the relative abundance of the associated precursor, alpha-carbonyl cation intermediate, and product during the progress of the reaction.

Automated summary

In recent years, there has been increasing interest in α-carbonyl cation chemistry. By directly capturing these unstable intermediates in high-speed aqueous microdroplets, their measurement and monitoring during the reaction process have become possible.