Electrochemical Three‐Atom Insertion via 3 + 2 Cycloaddition of Thionium Cations and Diazo Compounds

Atom insertion represents a powerful yet conceptually challenging mode of skeletal editing, as it requires controlled cleavage and reorganization of strong covalent bonds within an existing molecular framework. Consequently, most established atom‐insertion reactions rely on highly reactive reagents or preorganized cyclic frameworks, which limit their applicability, particularly for acyclic systems. Herein, we report a fundamentally new skeletal editing strategy that enables net three‐atom insertion into an acyclic framework through precise control over a highly reactive cationic intermediate. Selective anodic oxidation of a thioaryl group, acting as an electroauxiliary, generates a thionium cation that undergoes an unprecedented reaction with a diazo compound, resulting in the simultaneous incorporation of two nitrogen atoms and one carbon atom into a C(sp 3 )S bond. This process directly converts thioaryl‐substituted chain compounds into azines via net three‐atom insertion in a single electrochemical operation. Electrochemical experiments, cation pool studies, and density functional theory calculations collectively support a mechanism involving a formal 3 + 2 cycloaddition between the thionium cation and the diazo compound, followed by CS bond cleavage and skeletal reorganization.