Synthesis of 1-(2-Aminovinyl)indoles and 1,3'-Biindoles by Reaction of 2,2-Diaryl-Substituted 2 H-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-2 H-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 7a H-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-2 H-azirines using 3-diazoindolin-2-imines instead of 1-sulfonyl-1,2,3-triazoles.