Construction of Cyclobutane‐Fused Quinolizine Frameworks Through Intramolecular [2 + 2] Cycloaddition of Reissert Compounds from Isoquinolines Catalyzed by an Iridium Visible‐Light Photocatalyst

Irradiation of 2‐(ω‐alkenoyl)‐1,2‐dihydroisoquinoline‐1‐carbonitriles (Reissert compounds) with blue light‐emitting diodes ( λ = 455 nm) in the presence of an iridium‐based photocatalyst efficiently afforded cyclobutane‐fused benzo[ b ]quinolizinone derivatives in high yields and with moderate to high stereoselectivity. A catalyst loading as low as 0.5 mol% was sufficient to drive the reaction to completion within 1 h in many cases. Mechanistic studies suggest that the transformation proceeds via a stepwise [2 + 2] cycloaddition pathway involving a triplet biradical intermediate.