Recently, synthetic organic electrochemistry has achieved a dramatic uptick in popularity. One of the more challenging and thus elusive electrosyntheses is net redox neutral reactions, in which both oxidation and reduction steps are involved to achieve overall redox neutrality. Two possible conditions can trigger the occurrence of redox neutral electrochemical reactions. One involves the product radical cation or anion being stable enough to migrate to the cathode or anode so that a SET reduction or oxidation occurs to yield the final neutral product. The other requires the product radical cation or anion to undergo a SET oxidation or reduction with the starting material in a chain mechanism. The former is more arduous to meet as a tandem oxidation/ reduction has to occur on a macroscopically separated anode and cathode. Most of the reported redox-neutral electrochemical syntheses are centered on the latter condition. For examples invoking a chain mechanism, catalytic current efficiency was usually used as the primary evidence, and limited kinetic studies were sometimes reported to strengthen the argument. Ambiguity about the chain event often remains. In this work, a home-built electrochemistry/mass spectrometry (EC/MS) platform was used to generate an N-cyclopropylaniline radical cation electrochemically and to monitor its reactivity toward alkenes by mass spectrometry (MS), which led to the discovery of a new redox neutral reaction of intermolecular [3 + 2] annulation of N-cyclopropylanilines and alkenes to provide an aniline-substituted 5-membered carbocycle via direct electrolysis (yield up to 81%). A chain mechanism, involving the regeneration of the substrate radical cation and the formation of the neutral product, is shown to be responsible for promoting such a redox neutral annulation reaction, as supported by experimental evidence of EC/MS, divided cell electrosynthesis, and catalytic current calculation.

Electrocatalytic redox neutral [3+2] annulation of N-cyclopropylanilines and alkenes

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.