Brønsted Acid‐Catalyzed Markovnikov Hydroarylation of Arylcyclobutene: A Route to gem‐Diaryl‐Substituted Cyclobutanes

A regioselective Markovnikov hydroarylation of arylcyclobutene has been developed using Brønsted acid catalysis through a protonation/Friedel‐Crafts sequence. The metal‐free catalytic reaction is operationally simple and feasible compared to previous reports that utilized elaborate arylcyclobutene and specific polyfluorinated reagents, affording a variety of gem‐diaryl‐ substituted arylcyclobutene in good to excellent yields. The possible mechanism of the reaction has been explored through step‐by‐step control experiments. This protocol involving indole in gem‐diaryl‐substituted cyclobutene scaffolds not only enriches the applications of Brønsted acid catalyzed hydroarylation, but also paves the way for a more extensive access to potential bioactive cyclobutanes structures.