Synthesis of Thienoacenes by Electrochemical Double C–S Cyclization Using a Halogen Mediator: Utility of an S-MOM Group for Halogen-Mediated Electrolysis

Electrochemical synthesis of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives from S-methoxymethyl (MOM)-protected bis(o-sulfanylphenyl)acetylene derivatives is described. In the presence of Bu4NBr as a halogen mediator, electrochemical double C–S cyclization proceeded smoothly. MOM protection of thiols was essential for the reaction. While S-Me or S-p-methoxybenzyl (PMB)-protected bis(o-sulfanylphenyl)acetylenes did not afford BTBT and bromocyclized products were predominantly obtained, BTBT was selectively obtained when a similar compound protected with an S-MOM group was used in the reaction. Addition of H2O was significant for the reaction as both a sacrificial agent and a trapping agent for the eliminated MOM group. A variety of symmetrical and asymmetrical BTBT derivatives were obtained under the optimal conditions. Control experiments and DFT calculations suggest that the double-cyclization did not proceed concertedly, but rather in a stepwise fashion. The S-MOM protection strategy is also effective for the electrochemical synthesis of a more π-expanded thienoacene such as dibenzo[d,d’]thieno[3,2-b;4,5-b’]dithiophene (DBTDT).