An efficient photo-SET-induced cleavage of dithiane-carbonyl adducts and its relevance to the development of photoremovable protecting groups for ketones and aldehydes

Irradiation of dithiane-aldehyde/ketone adducts in the presence of benzophenone leads to C-C bond cleavage regenerating the carbonyl compounds. It is established that the mechanism of this reaction involves photochemically induced single electron transfer from the dithiane moiety to the excited molecule of ET-photosensitizer, accompanied by mesolytic C-C cleavage in the generated cation-radical, which is assisted by the anion-radical of benzophenone. This mechanism is confirmed by a Hammett plot study of the cleavage in the dithiane adducts of substituted aromatic aldehydes and a deuterium kinetic isotope effect study. Ab initio computations at UHF/6-31G* and MP2/6-31G* levels of theory in conjunction with self-consistent reaction field (self-consistent isodensity-polarized continuum model), to account for the solvent effect, also support the experimental findings. The reaction is most efficient for protection of aromatic aldehydes and ketones and aliphatic ketones, and is a novel method for protecting carbonyl functionalities with a photoremovable group.