Thiophenes, alkoxy-substituted

Kiryanov, A.A., Sampson, R and Seed, A.J., Synthesis of 2-alkoxy-substituted thiophenes, 1,3-thiazoles, and related S-heterocycles via LawessoiTs reagent-mediated cyclization under microwave irradiation applications for liquid crystal synthesis, /. Org. Chem., 2001, 66, 7925-7929. 

In this category, the monomers are based on 3-alkoxy-substituted thiophene with optional nonalkoxy substituent at 4-position as given in Chart 10.1. 

Electrically conductive PTs are prepared by polymerization in an electrolyte in the presence of polymeric compounds with sulfonic acid groups [692]. The electrochemical polymerization of thiophene and Nafion gives a self-supporting film showing a uniform distribution of sulfur across the film and a reversible voltammogram [693]. Alkoxy substituted thiophene derivatives are electro-chemically polymerized in a solvent in the presence of a proton source in the form of a Bronsted acid [694]. The electrochemical preparation of poly-(3-alkyloxythiophene) without a Brdnsted acid has also been studied [695]. 

Alkoxy Substituted Thiophenes and Alkoxy Substituted Bithiophenes. The... 

The synthesis of a number of dialkoxybenzo[6]thiophenes by ring closure is described in Section IV, D, and a number of reactions of alkoxy-substituted thioindoxyls has been discussed in Section VI,1,2. 

Alkoxy-substituted bithiophenes have also been successfully polymerized, using Cu(C104)2 as oxidant.50 As with related FeCl3 oxidations of alkyl-substituted bithiophenes,51 these dimeric substrates are easier to oxidize than the corresponding substituted thiophene monomers, due to their lower oxidation potentials. 

Despite the predominant 2,5 -coupling with both the 3-alkyl and 3-alkoxy thiophene monomer substrates, the presence of the ring substituents introduces a further structural complication. Coupling of the substituted thiophene units can now lead to four possible triad regioisomers for the resultant polymers, depending on whether the coupling occurs in a 2,5 -head-to-tail (HT), 2,2 -head-to-head (HH), or 5,5 -tail-to-tail (TT) manner. These structures 11-14 for the polythiophene products are depicted in Figure 6.4 (X = S). 

Diffraction data for the alkoxy-substituted materials are scarce. For PMEEMT x-ray diffraction patterns were recorded [44] at two temperatures, room temperature and 150°C, showing in both cases three to four rather broad peaks on a diffuse background. The peaks correspond to interplanar distance d= 17.8 A, 7.0 A, 3.8 A and 2.4 A, the latter two being close to two values also found for PATs as the 6-axis parameter and a diffuse feature at 0 = 2.6 A related to the main chain periodicity. It is remarkable that the diffraction pattern survives heating to 150"C, which is above the thermochromic transition for PMEEMT. A recent diffraction study of poly methanol-thiophene) prepared electrochemically showed only broad amorphous scattering around 0= 1.4 A [94],... 

Various monoalkoxy substituted polythiophenes are the earliest examples reported in this category. These are followed by the 3-alkoxy-4-alkyl substituted thiophenes, introduced by Hoechst AG group, that result in conductivity enhancement as compared to 3-alkoxythiophenes." ... 

Other 3-substituted thiophenes that have been polymerized include 3-methoxy [309-311], other 3-alkoxy [312], 3-phenyl [313,314], 3-(4-methoxyphenyl) [314], 3-(4-trifluoromethylphenyl) [314], 3-bromo [315], 3-alkylsulfonatethiophene [316-318], and others [319-323]. Poly-3-alkylsulfonate thiophenes are particularly interesting due to a striking property. Sodium poly-3-thiophene-j8-ethanesulfonate and sodium poly-3-thiophene-6-butanesulfonate and their conjugate acids are water soluble in both the doped and undoped states [317,318]. Ikenoue et al. [318] examined the conduction mechanism for this self-doped conducting polymer. 

What happens if one introduces substituted analogs of the pyrrole monomer unit, e.g. N-methyl pyrrole in place of pyrrole This is seen in Fig. 4-8 the voltammetric behavior of the substituted analogs is very similar to that of P(Py), but with an anodic shift of the redox peak potentials. This is explained by the likely participation of protons in the facilitation of the redox of P(Py), which is not possible in N-substituted analogs. Another effect of N-substitution in poly(pyrroles) is of course increased environmental stability, as oxidants invariably first attack the exposed, electron rich N-atom of unsubstituted pyrrole. That monomer structure can sometimes directly influence polymer behavior is also seen in the plot of oxidation potentials of monomer vs. polymer for poly(thiophene) analogs in Fig. 4-9 here the two oxidation potentials exactly parallel each other, i.e. a monomer more difficult to oxidize implies a polymer also more difficult to oxidize. On the other hand, alkoxy-substituted P(ANi) show voltammetric behavior very close to that of the unsubstituted P(ANi), with peaks shifted less than 90 mV anodically (Fig. 4-10). 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

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