<i>In Situ</i> Generation of 1‐Acetylpyrene as a Visible‐Light Photocatalyst for the Thia‐Paternò‐Büchi Reaction

Abstract The thia‐Paternò‐Büchi reaction represents a straightforward approach to build thietane cores. Unfortunately, the significant instability of thiocarbonyls, particularly thioketones and thioaldehydes, has hitherto rendered this photochemical [2+2]‐cycloaddition underexploited. To address this limitation, we report herein a visible‐light photochemical domino reaction including: the in situ generation of thiocarbonyls though a Norrish type II fragmentation of pyrenacyl sulfides, and the aforementioned thia‐Paternò‐Büchi reaction with various non‐volatile electron‐rich alkenes. The highly efficient synthesis of a wide range of unprecedented thietanes from intrinsically highly unstable thiocarbonyls, such as thioaldehydes and aliphatic thioketones, was made possible by the multitasking capability of pyrenacyl sulfides as a source of thiocarbonyl substrates and as precursors of 1‐acetylpyrene, which acts as the photocatalyst for the thia‐Paternò‐Büchi reaction. The photosensitizer properties of the latter have been experimentally established and a triplet‐triplet Dexter energy transfer based mechanism is proposed.