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

The thia‐Paternò‐Büchi reaction represents one of the straightforward approaches 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 here 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 thiocabonyls, 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.