The synthesis, thermochromism, and cycloadditive properties of an O-methylenethioquinone system

The irradiation of 4,5-benzo-1,2-dithiole-3-thione (1) in the presence of cyclopentene and of tetramethylethylene gives 1:1 adducts. In solution these adducts are in equilibrium with eight-membered-ring dimers. The thermodynamic parameters for this equilibrium have been determined. The available evidence suggests a head-to-head configuration. These same monomeric adducts give Diels–Alder addition products with normal dienophiles, but also react similarly with simple thiones (adamantanethione, norbornanethione, cyclohexanethione) to give thioorthoesters. The structures of the latter substances have been demonstrated both spectroscopically and, chemically, by hydrolysis to a thiosalicylic acid derivative. The thioorthoesters have been shown to undergo stereoisomerization on acid catalysis. Of the two mechanisms considered for this process (reversal to thiones and ion-pair formation) the former has been excluded.