A redox potential controlled intermolecular [2 + 2] cross-cycloaddition has been developed in the presence of a thioxanthylium photoredox catalyst. Electron-rich styrenes such as β-bromostyrene (E p/2 = +1.61 V vs SCE) were selectively oxidized by a thioxanthylium photoredox catalyst (E 1/2 (C*/C•–) = +1.76 V vs SCE) to styryl radical cations and reacted with styrene (E p/2 = +1.97 V vs SCE) to furnish polysubstituted cyclobutanes in high yields. The present reaction can be successfully applied to intermolecular [2 + 2] cross-cycloaddition of β-halogenostyrenes, which cannot be effectively achieved by the hitherto reported representative organophotoredox catalysts.

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Moderately oxidizing thioxanthylium photoredox catalysts that operate under irradiation with green light have been developed. These catalysts exhibit relatively moderate excited-state reduction potentials [E 1/2 (C*/C •− ) = 1.75−1.94 V vs saturated calomel electrode (SCE)] and can efficiently promote radicalcation Diels−Alder reactions under irradiation with green light. Interestingly, β-halogenostyrenes (E p/2 = 1.57−1.61 V vs SCE) are well tolerated, affording synthetically useful halocyclohexenes.