Synthesis of thienoacenes by electrochemical double C-S cyclization using a halogen mediator

Thienoacenes are significant compounds as organic materials. One of the most efficient ways to synthesize thienoacenes is to form multiple C-S bonds in a single step. Because unprotected S-H bonds are easily oxidized to S-S bonds, S-Me protected substrates are commonly used for the purpose. However, their reactivity is insufficient, and one-step construction of multiple C-S bonds is still challenging. We herein report the electrochemical synthesis of thienoacenes from S-methoxymethyl (MOM)-protected diarylacetylenes. In the presence of Bu₄NBr as a halogen mediator, electrochemical double C-S cyclization of diarylacetylenes bearing two MOM groups proceeded to afford [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives. While S-Me or S-p-methoxybenzyl (PMB)-protected diarylacetylenes did not afford BTBT, BTBT was selectively obtained when a substrate protected with S-MOM groups was used. 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).