Intermolecular [2 + 2] Photocycloaddition of α,β-Unsaturated Sulfones: Catalyst-Free Reaction and Catalytic Variants

2-Aryl-1-sulfonyl-substituted cyclobutanes were prepared in an intermolecular [2 + 2] photocycloaddition from various α,β-unsaturated sulfones and olefins upon irradiation at λ = 300 nm (26 examples, 60-99% yield). Lewis acids catalyzed the [2 + 2] photocycloaddition of 2-benzimidazolyl styryl sulfones. At short wavelengths, the latter substrates underwent C-S bond cleavage but AlBr3 (5 mol %) allowed for an intermolecular reaction with 2,3-dimethyl-2-butene at longer wavelengths. A chiral-at-metal Lewis acid (2 mol %) facilitated an enantioselective reaction (up to 77% ee).