Intramolecular [2 + 2] Cycloaddition of Isocarbostyrils Catalyzed by an Iridium Visible-Light Photocatalyst

Irradiation on N-(ω-alkenyl)isocarbostyrils in the presence of an iridium photocatalyst by LEDs emitting 455 nm light gave the corresponding cyclobutane-fused benzo[b]quinolizine derivatives stereoselectively in high yields. Loading 1 mol % of the catalyst was enough to obtain high yields of the products in convenient reaction time in many cases. The reaction likely proceeds through stepwise [2 + 2] cycloaddition via a triplet biradical intermediate.