Formation of a trithia[5]helicene in an unexpected photoreaction of a methyl-substituted bis(dithienylethenyl)thiophene through a double sequence of 6π-electrocyclization/aromatization (dehydrogenation/demethylation)

Photochemical reactions of 2,5-bis[2,2-di(thien-2-yl)ethenyl]thiophene (1a) and its tetramethyl derivative 1b, under direct photoexcitation, and photoinduced electron-transfer conditions, were studied. Our initial prediction was that 1b would undergo a photoreaction as part of a reversible photochromic system, while a reaction of its non-methyl substituted analog 1a would undergo a typical double sequence of 6π-electrocyclization (6π-EC)/aromatization (AR) (double dehydrogenation, [sbnd]2H) to give 7,11-di(thien-2-yl)trithia[5]helicene (2a). In fact, we observed that photoirradiation of 1b leads to formation of the 7,11-bis(3-methylthien-2-yl)trithia[5]helicene (2b). In this process, 2b is formed via a double sequence of 6π-EC/AR (dehydrogenation/demethylation, [sbnd]H/[sbnd]Me). Moreover, the yield of formation of 2b is much higher than that of 2a, which reacts through a double sequence of 6π-EC/AR ([sbnd]2H). Thus, this photochemically-induced paradoxical cascade reaction based on the newly uncovered reactivity serves as an efficient method to construct the trithia[5]helicene framework.