UV ‐Light‐Induced Ring Expansion of Chlorodiazirines with 3‐Alkylideneazetidines towards Pyrroles

Comprehensive Summary Pyrroles are privileged heterocycles in both natural systems and industrial applications, serving as key structural motifs in bioactive molecules, functional materials, pharmaceuticals, agrochemicals, and catalysts, which has driven continuous efforts to develop efficient synthetic methodologies for structurally diverse five‐membered azacyclic derivatives. Ring expansion with carbynyl cation equivalents enables the selective reshaping of pre‐existing molecular scaffolds with remarkable step economy, given that the capacity of carbynyl cations to form three new covalent bonds on a single carbon atom in one step. Carbynyl cation equivalents have been widely employed in ring expansion reactions to afford aromatic products, predominantly via [2+1] cycloaddition with unsaturated cyclic scaffolds or ylide formation with aromatic heteroatom. However, ring expansion reactions that proceed via ylide formation from carbynyl cation equivalents remain limited, and the use of saturated rings to construct aromatic skeletons still remains unexplored. Ring expansion of 3‐alkylideneazetidines provides a direct route to five‐membered azacycles, but this transformation remains greatly confined to noble metal catalysts and the synthesis of non‐aromatic azacycles. Herein, we report a UV‐light‐induced ring expansion of chlorodiazirines as carbynyl cation equivalents with 3‐alkylideneazetidines, proceeding via ammonium ylide formation to afford 1,2,4‐trisubstituted pyrroles in moderate to good yields. The reaction exhibits broad compatibility with substrates bearing diverse electronic properties. Control experiments suggested that both singlet and triplet α‐chlorocarbenes could be involved in this reaction. Possible mechanisms for this reaction involve ammonium ylide formation and ring expansion through direct or stepwise diradical/ion‐pair pathways, followed by elimination and aromatization to afford the product. The utility of this protocol was substantiated by scalable synthesis and subsequent versatile diversification of the product into benzoyl‐, formyl‐, bromo‐, and fully functionalized pyrrole derivatives. This transformation represents a unique example of aromatic skeleton construction through saturated azacycle expansion utilizing carbynyl cation equivalents. The catalyst‐ and base‐free protocol features the advantages of mild reaction conditions, high step economy, and environmental friendliness.