Unraveling innate substrate-controlled arylation and bicyclization of 1,5-enynes with α,β conjugates: synthesis of substituted benzo[a]fluorenes

Abstract

Herein, we describe a novel strategy for the aromatic C–H functionalization of electron-rich arenes with 1,5-enynes anchored by conjugates, catalyzed by a Brønsted acid under metal- and solvent-free conditions. A diverse range of benzo[a]fluorenes have been realized in moderate to good yields by using this protocol. This process includes intermolecular nucleophilic arene insertion or 6-exo-dig cyclization (depending on the nature of the substrates) followed by a Friedel–Crafts-type reaction and 1,5-H transfer or intermolecular nucleophilic arene insertion, respectively. In addition, reactions with heterocyclic compounds such as (E)-2-benzoyl-3-(2-(phenylethynyl)pyridin-3-yl)acrylonitrile proceeded smoothly to afford 11-phenyl-11H-indeno[1,2-h]quinoline-6-carbonitrile analogues. Moreover, we successfully demonstrated solvent recycling experiments for green pathways. These reactions feature readily accessible starting materials, transition metal-free approaches, simple one-pot operations, gram-scale syntheses and wide functional group tolerance.