Strain-Release-Driven <i>exo</i> -Cyclization of Bicyclo[1.1.0]butyl Ketones Promoted by Brønsted Acids to Access 2,3-Diazabicyclo[3.1.1]heptenes

A Brønsted acid-promoted <i>exo</i>-cyclization of bicyclo[1.1.0]butyl ketones with hydrazines has been established. This approach enables efficient ring expansion, leading to a diverse range of 2,3-diazabicyclo[3.1.1]heptenes. Mechanistic studies reveal that the carbonyl group is pivotal, fulfilling a dual role as both the reactive center and a latent activating group. Moreover, the unique dual nucleophilic character of hydrazines, featuring two nitrogen-based nucleophilic sites, is crucial to the success of this transformation.