Iron-catalyzed Intramolecular Hydroarylation Of Alkynyl Arenes

Recently, the thermal hydroarylation of conjugated dienynes has long been an area of intense interest among physical organic and computational chemists, which provides a reliable way to construct highly substituted aromatic systems. While there are still some challenges, for example, the catalysts used always are noble metal like palladium, and the reaction conditions are not moderate. Meanwhile, due to the different ways of generation, the details of mechanism are not clear. Thus, we synthesized the standard alkyne arenes and used the Lewis acid Fe(OTf)2 as catalyst to design a Friedel–Crafts type hydroarylation reaction. Then we conducted the optimization of the reaction conditions, and prepared more than thirty kinds of substrates to investigate the electron effect, steric effect and the tolerance of different functional groups, producing phenanthrenes as products. Finally, we proposed a plausible reaction mechanism generated by a cationic iron-arene intermediate through the results of the kinetic isotope effect for the cyclization of the standard substrate-d1. The cationic iron-arene intermediate efficiently produce the vinyl cation due to π-electron delocalization between the arene and the conjugated alkyne, followed by the electrophilic attack, deprotonation and reprotonation giving the target product.