Microdroplet Stabilization Enabled Direct Mass Spectrometric Identification of Electrogenerated Transient Carbocation Intermediates

The electrochemically generated carbocations are pivotal yet elusive intermediates in electrosynthesis. By coupling nanoelectrospray ionization with a tip-confined electrochemical microreactor, the ultrafast transfer of the electrogenerated species into the charged microdroplets is achieved. Interestingly, the highly reactive carbocations become abnormally inert in the electrosprayed microdroplets, and made a survival throughout the entire lifespans of the microdroplets. By taking full advantage of the newfangled stabilizing effect of the microdroplets, the direct mass spectrometric identification of the electrogenerated transient benzylic and phenylic carbocations, whose half-lives in bulk solutions are on the nanosecond level is achieved. The detailed paths of the carbocation-driven C(sp³)-H and C(sp²)-H aminations are also revealed by the stepwise evaluation of their initial electrochemical generation and their ultrafast succedent chemical transformations. With its high sensitivity, excellent specificity, and minimal time delay, the wide applications of the in situ electrochemical mass spectrometry is anticipated as a powerful tool for not only the identification of previously unseen intermediates but also the in-depth exploration of the intricate electrochemical transformations.