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

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.