Intermolecular Zinc-Mediated Hydroamination of Alkynes to Indoles

Significance

Reported herein is a Zn-mediated intermolecular hydroamination of terminal alkynes with aryl hydrazines to give hydrazones which, in situ, undergo Fischer indolization to afford 2,3-disubstitued indoles. The one-pot procedure is highly Markovnikov-selective and uses simple zinc salts (see review below) to afford the indole derivatives in good to excellent yields. Other tested Lewis acids [FeCl₃, ZnBr₂, Zn(OAc)₂] and metal triflates/salts [Sc(OTf)₃, Yb(OTf)₃, HAuCl₄, H₂PtCl₆] gave only oligomers of alkynes. The scope of the reaction was well investigated. When applied to internal alkynes, the reaction failed to give indole products which indicate a limitation to the synthesis of only 2-methylindoles.