Copper‐Catalyzed Heterocyclic Recombination of Aziridine and Diazetidine for the Synthesis of Imidazolidine**

The discovery of new catalytic applications for metals remains an important goal in organic synthesis. If a catalyst has multiple functions, such as inducing bond cleavage and formation, it can streamline multi-step transformations. Herein, the Cu-catalyzed synthesis of imidazolidine through heterocyclic recombination between aziridine and diazetidine is reported. Mechanistically, Cu catalyzes the conversion of diazetidine into the corresponding imine, which then reacts with aziridine to form imidazolidine. The scope is sufficiently wide to form various imidazolidines, as many functional groups are compatible with the reaction conditions.