Organophotocatalytic [2+2] Cycloaddition of Electron‐Deficient Styrenes**.

A visible‐light organophotocatalytic [2+2] cycloaddition of electron‐deficient styrenes is described. Photocatalytic [2+2] cycloadditions are typically performed with electron‐rich styrene derivatives or α,β‐unsaturated carbonyl compounds, and with transition‐metal‐based catalysts. We have discovered that an organic cyanoarene photocatalyst is able to deliver high‐value cyclobutane products bearing electron‐deficient aryl substituents in good yields. A range of electron‐deficient substituents are tolerated, and both homodimerisations and intramolecular [2+2] cycloadditions to fused bicyclic systems are available by using this methodology. [ABSTRACT FROM AUTHOR]