Photochemical Oxidation of Thioketones by Singlet Molecular Oxygen Revisited: Insights into Photoproducts, Kinetics, and Reaction Mechanism

Photosensitized oxidation of trimethyl[2.2.1]-bicycloheptane thioketones by O-1(2) can yield more photoproducts than exclusively ketones and sulfines. Moreover, the ketone/sulfine ratio can be reversed when protic conditions and high thioketone concentrations are used, conversely to earlier results reporting ketones as the main photoproducts. A new mechanistic proposal for sulfine formation is suggested following intermolecular oxygen transfer from a peroxythio-carbonyl intermediate to a second thioketone molecule. Reaction quantum yields (10(-5)-10(-2)) depend on the reaction conditions and time. Sulfine production reaches a maximum at short irradiation times, whereas decomposition to the corresponding ketone is observed at long reaction times. When the thioketone substrate has a hydrogen atom at the alpha position a peroxyvinylsulfenic acid intermediate can be formed by proton transfer. Reaction of this intermediate with another thioketone molecule can yield more sulfine and its tautomeric vinylsulfenic acid, which dimerizes in situ to the thiosulfinate. The hydroperoxyl group of the peroxyvinylsulfenic acid can also rearrange to the alpha position, and by reaction with the starting thioketone, alpha-hydroxy thioketone and additional sulfine can be formed, while dehydration yields the alpha-oxo thioketone. In situ [2 + 2] and [4 + 2] self-cycloaddition of the alpha-oxo thioketone yields significant amounts of the corresponding adducts at prolonged irradiation times.