Synthesis and reactivity of new 1,4-bis(alkylthio)-3,6-diarylthieno[3,4-c]thiophene derivatives

1,4-Bis(tert-butylthio)-3,6-diphenyl- and 3,6-di(2-thienyl)thieno[3,4-c]thiophenes (2a,b) were synthesized from 2-(tert-butylthio)-3-phenyl- and 3-(2-thienyl)cyclopropenethiones (1a,b) and triphenylphosphine in dry benzene at 50 degrees C, although similar treatment of 2,4,6-triisopropylphenyl, N,N-diethylamino, pyrrolidino, and diphenyl-substituted cyclopropenethiones (1c-h) did not result in the production of the corresponding thieno[3,4-c]thiophene derivatives. The possible reaction pathway for the formation of 2a,b is described. The protonation of 2a with trifluoroacetic acid (TFA) gave 4-(tert-butylthio)-3,6-diphenylthieno[3,4-c]thiophene-1(3H)-thione (13a), the treatment of which with sodium hydride and then isopropyl iodide led to 4-(tert-butylthio)-3,6-diphenyl-1-(isopropylthio)thieno[3,4-c]thiophene (16) by the regeneration of the thieno[3,4-c]thiophene ring system, this making possible the synthesis of other alkylthio-substituted thieno[3,4-c]thiophene derivatives. The reactions of 2a,b with N-phenylmaleimide (NPM) gave predominantly the endo-cycloadducts (17a,b) at the 1- and 3-positions, and that of 2a with dimethyl acetylenedicarboxylate (DMAD) led to the benzo[c]thiophene derivative (19) by desulfurization.