Scheme 1. C-S coupling reactions for the synthesis of thienoacenes and this work MOM-protected bis(o-sulfanylphenyl)acetylene ( ) was chosen as a model compound, and the electrochemical cyclization of 1a was examined in a LiClO4 solution of CH3CN (Table ). After simple electrolysis without a mediator (12 mA, 2 F mol -1 ), 1a was consumed completely but the desired double-cyclized product 2a was obtained in only 9% yield (entry 1). In the presence of n Bu4NCl (3 equiv) as a mediator, 2a was obtained in 35% yield along with a mixture of mono-cyclized product 3a and chlorinated mono-cyclized product 4a (10% yield, 3a/4a = 1:0.19). In contrast, n Bu4NI was not effective (2% yield, entry 3). With n Bu4NBr, the yield of 2a drastically increased to 50%, but the yields of by-products 3a and brominated mono-cyclized product 4b also increased (entry 4). When we reduced the electrical current to 2 mA, the yield of 2a slightly increased (57% yield, entry 5). With 4 F mol -1 of charge, the desired product 2a was obtained in 83% yield, but 3a and 4b were still obtained (entry 6). When we decreased the amount of n Bu4NBr to 1 equiv, the yield of 2a decreased to 66%, but the generation of 3a and 4b was suppressed (entry 7). When the electric current was increased to 5 mA, 2a was obtained in a similar yield, but the reproducibility of the reaction was unsatisfactory (66-74% yields in three runs).