Photochemical reaction of some aromatic thiones with aryl‐ and alkyl‐substituted (methylthio)acetylenes. The (2+2)‐cycloaddition products and the occurrence of the equilibrium thiete ⇋ α, β‐unsaturated thioxo compound

Abstract

AbstractUpon irradiation (λ > 525 nm), xanthenethione 2o and thioxanthenethione 2s react in a non‐regiospecific manner with the triple bond of the acetylenes 6a (Ph‐C≡C‐SCH3) and 6b [(2‐thienyl)–C≡C‐SCH3] to produce in each case the two possible (2 + 2)‐cycloaddition products, which were isolated as thietes or α,β‐unsaturated thioxo compounds. In a solution of each of these reaction products, spectroscopic results confirmed the occurrence of the equilibrium thiete ⇋ α, β‐unsaturated thioxo compound. With acetylene 6c (t‐Bu‐C≡C‐SCH3), the photochemically excited thiones 2o and 2s gave, in each case, only the (2 + 2)‐cycloaddition product 8. In solution these thietes 8 do not equilibrate with the corresponding α, β‐unsaturated thiones. Surprisingly, no products could be isolated or detected after irradiation of thiobenzophenone 2h in the presence of acetylenes 6a or 6b.