Photochemistry of α,β-Unsaturated Thiones: Addition to Electron-Rich Olefins from T₁

1,1,3-Trimethyl-2-thioxo-l,2-dihydronaphthalene (1) adds to electron-rich olefins upon excitation to either S₂ (ππ*) or S₁ (nπ*) states. Excitation to S₂ level resulted in the same mixture of products, namely thietane and 1,4-dithiane, as on excitation to S₁ level. Addition occurs to the thiocarbonyl function and not to the carbon-carbon double bond. The addition is site-specific, and the formation of thietane is regiospecific. The ratio of thietane to 1,4-dithiane in the product mixture is dependent on the concentration of the thioenone. The addition is suggested to originate from the lowest triplet state (T₁) and involves diradical intermediates.