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

TL;DR

This paper introduces a method to identify short-lived carbocation intermediates in electrochemical reactions using microdroplet stabilization and mass spectrometry.

Contribution

A novel method for stabilizing and identifying transient carbocations using electrosprayed microdroplets and in situ mass spectrometry.

Findings

Electrogenerated carbocations are stabilized in microdroplets and survive long enough for mass spectrometric identification.

The method enables the direct observation of nanosecond-lived benzylic and phenylic carbocations.

The technique reveals detailed reaction pathways of carbocation-driven amination processes.

Abstract

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.

The in situ mass spectrometric identification of the electrogenerated benzylic and phenylic carbocations, whose half‐lives in bulk solutions are on the nanosecond scale, is achieved by taking advantage of the unique stabilization effect of the electrosprayed microdroplets.

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