The synthesis of precursors to conjugated polymers

Poly(benzo[c]thiophene) (PBcT) was prepared by Wudl et al in 1984 from the monomer benzo[c]thiophene (BcT) and was found to have a bandgap of 1.0 eV, the lowest bandgap polymer reported at that time. Since this finding, very few substituted BcTs have been prepared. -- Two substituted BcTs, 5,6-dioxymethlenebenzo[c]thiophene 56 and 5,6-bis(methylcarboxylate)benzo[c]thiophene 57), were prepared using adapted literature procedures in four and six steps, respectively, in good yields. In the case of 57 a novel monomer, several approaches were taken, one of which led to the discovery of NBS and silica as a rather unique benzylic brominating agent. -- Other compounds of interest included terheterocycles, 1,3-dithienylbenzo[c]thiophenes 85. The following compounds, 5,6-dimethoxythiophthalic anhydride 105, 5,6-methylenedioxyphthalaldehyde 112 and 5,6-ethylenedioxyphthalic acid 171, were prepared as potentially useful precursors for the synthesis of substituted terheterocycles. Compound 105 was prepared by the hydroxymethylation of 3,4-dimethoxybenzoic acid 117 followed by the reaction with sulfur. Upon its isolation, it was reacted with two equivalents of 2-thienyllithium and also with one equivalent of 2-thienyllithium followed by methyliodide to form 1-thienoyl-2-methylthiacaronyl-5,6-dimethoxybenzene 108. Compund 112 was prepared in four steps starting from piperonal and 171 was prepared by a Diels-Alder reaction of a 3,4-disubsituted thiophene, EDT 157, and dimethylacetylene dicarboxylate. The literature synthesis of 157 gave disappointing results, but modifications to this procedure led to a significant improvement in the yield of this important thiophene