Oxidative Addition to the N-C Bond Vs Formation of the Zwitterionic Intermediate in Platinum(II)-Catalyzed Intramolecular Annulation of Alkynes to Form Indoles: Mechanistic Studies and Reaction Scope.

Abstract

In this study, Pt(II)-catalyzed intramolecular translocation annulation of ortho-alkynylamides to the formation of indoles is presented, where a proposed intermediacy of zwitterionic intermediate has been substantiated over the oxidative addition. We focused our attention on Pt(II)-catalyzed aminoacylation of alkynes both theoretically and experimentally using low boiling solvent where the formation of deacylation product was suppressed simultaneously. One-step intramolecular [1,3]-acyl migration from the zwitterionic intermediate is highly unlikely, which imparts a high energy barrier of +99.0 kcal mol-1. Another possible approach involving oxidative addition to the N-C bond, migratory insertion to alkyne, and subsequent reductive elimination is also explored through DFT studies to justify the reaction consequence. However, based on the computational studies, it is suggested that initial zwitterion formation is highly favored over oxidative addition. We suggest the formation of an acylium intermediate, which can further react with indol-3-ylplatinum species in an intramolecular manner, albeit within the same solvent cage to form 3-acyl indoles.