Redox-potential-controlled intermolecular [2 + 2] cycloaddition of styrenes for the regio- and diastereoselective synthesis of multisubstituted halogenocyclobutanes

The redox potential is an important factor for controlling the outcome of photoredox catalysis. Particularly, the selective oxidation of substrates and the control over the reactions are challenging when using photoredox catalysts that have high excited-state reduction potentials. In this study, a redox-potential-controlled intermolecular [2 + 2] cycloaddition of styrenes using a thioxanthylium organophotoredox (TXT) catalyst has been developed. This TXT catalyst selectively oxidizes β-halogenostyrenes and smoothly promotes the subsequent intermolecular [2 + 2] cycloadditions to give multisubstituted halogenocyclobutanes with excellent regio- and diastereoselectivity, which has not been effectively achieved by the hitherto reported representative photoredox catalysts. The synthesized halogenocyclobutanes exhibit interesting free radical scavenging activity. The present reaction contributes to the field of redox-potential-controlled electron transfer chemistry.