Synthesis of 3-Methylidene-2,3-dihydropyrroles via Formal 1,2-Enamine Migration/Cyclization Cascade of Rhodium Carbenes

3-Methylidene-2,3-dihydropyrroles with an ester or keto group at the C4 position were obtained in moderate to good yields from enamine-tethered triazoles in the presence of a rhodium(II) catalyst. A formal 1,2-enamine migration/cyclization cascade triggered by formation of an alpha-amino carbene is proposed to illustrate the mechanism. The valuable exocyclic double bond showed distinctive reactivity, which is very useful for further introduction of functional groups. The dihydropyrrole is stable under neutral, weakly acidic, or weakly basic conditions.