Synthesis of 1‑(2-Aminovinyl)indoles and 1,3′-Biindoles by Reaction of 2,2-Diaryl-Substituted 2H‑Azirines with α‑Imino Rh(II) Carbenoids

An effective and operationally simple method for the preparation of 1-(2-(sulfonamido)­vinyl)­indoles (SAV-indoles) by the Rh­(II)-catalyzed reaction of 2,2-diaryl-2H-azirines with 1-sulfonyl-1,2,3-triazoles has been developed. This method enables the stereoselective synthesis of a variety of 1,2,3-trisubstituted indoles having a Z configuration of the (1-aryl-2-(sulfonamido)­vinyl) substituent. The reaction mechanism, supported by DFT calculations, involves the formation of 1,4-diazahexa-1,3,5-trienes, which rapidly cyclize to 2,2-diaryl-1-sulfonyl-1,2-dihydropyrazines. These compounds can be isolated at an early stage of the reaction, but under prolonged heating they isomerize into 7aH-indolium ylides, followed by a barrierless 1,5-prototropic shift to SAV-indoles. The developed methodology was also applied to the preparation of 1,3′-biindoles from 2,2-diaryl-2H-azirines using 3-diazoindolin-2-imines instead of 1-sulfonyl-1,2,3-triazoles.