Although [2+2] and [4+2] cycloadditions of thiocarbonyl compounds with alkenes or dienes are well-known, only few examples involving a,b-unsaturated thiocarbonyl (thiadiene) systems have been reported. Thus, the photoreaction between thiobenzophenone (Tbz) and alkenes is known to give [2+2] Paterno-Büchi cycloadducts (thietanes) in high yields. In the present work, formation of new photoproducts due to [4+2] cycloaddition is observed from moderate to high yields when pyrylium salts are used as photosensitizers. Here, the thiadiene system is defined by the thiocarbonyl group and one of the double bonds of the phenyl groups. This reaction is stereoselective but not stereospecific, and its efficiency is strongly dependent on the nature of the olefin. The rate of the process decreases with the increasing oxidation potential of the alkenes. The mechanistic aspects of this [4+2] photocycloaddition of a thiadiene system have been investigated by using different types of alkenes (styrene, b-methylstyrene, cis- and trans-stilbene) supporting either electron-withdrawing or electron-releasing substituents. The reactivity of the intermediates has been evaluated by means of laser flash photolysis. Besides, quantum chemical calculations have been performed in order to explain the stereoselectivity observed in cycloadduct formation. Although the radical cations of both Tbz and alkenes are generated by PET, time-resolved studies reveal that only the former is involved in the process. The scope and limitations of this novel PET reaction are discussed.
