Photocatalyzed Dearomative [2 + 2] Cycloaddition of Indoles with Alkynes to Access Cyclobutene-Fused Indolines

Recently, energy-transfer-based photocatalytic cycloaddition has developed as an elegant strategy to overcome aromatic stabilization and achieve dearomatization of indole. However, these procedures mainly focus on the [2 + 2] cycloadditions of indoles with alkenes to build cyclobutane-fused indolines. The intermolecular [2 + 2] cycloaddition between indoles and internal alkynes remains less explored due to their potentially higher ring strain and thermodynamic instability. Herein, a general and efficient strategy has been developed via thioxanthone-catalyzed energy-transfer-driven [2 + 2] cycloaddition of indoles and alkynes. Various cyclobutene-fused indolines were provided in up to 95% yield for 25 examples with excellent stereoselectivity (>20:1 dr) under the mild reaction conditions. This protocol demonstrates remarkable features with simplicity of operation, broad substrate compatibility, high efficiency, excellent stereoselectivity, and exceptional atom and step economy. This study expands a previously underexplored energy-transfer-catalyzed intermolecular dearomative [2 + 2] cycloaddition of indoles with internal alkynes, enriching the compound library of strained-ring molecules.