Thiophene, from diacetylene

The synthesis of thiophene from diacetylene was first performed by Schulte (62CB1943), who used sodium sulfide in aqueous alcohol (pH 8-10), the yield being no more than 20%. 

The formation of thiophenes from conjugated diacetylenes and HjS (or its equivalent) is fairly standard . Cyclization in the skipped diacetylene is also becoming familiar (equation 146) >... 

The synthesis of thieno[3,2- >]thiophene [17, 18] is shown in Scheme 3.3. The commercially available 3-bromothiophene undergoes formylation via lithiation at the 2-position and the addition of Al-formy[piperidine. Subsequent treatment of 3 with ethyl 2-sulfanylacetate affords the ester 4, which is converted to thieno[3,2- >]thiophene by hydrolysis and decarboxylation steps. The product is thus obtained in a very satisfactory overall yield of 60%. A similar method can be used to prepare thieno[2,3- >]thiophene from thiophene-3-carboxaldehyde via the carboxylic acid [19], but an attractive alternative route was published in full by Otsubo et al. [20] following a brief communication from de Jong and Brandsma [21], In this strategy, trimethylsilyl-l,3-pentadiyne is treated with potassium rerr-butoxide, butyllithium and carbon disulfide and then with ferr-butanol in HMPA, to obtain thieno[2,3-fi]thiophene in 46 % yield. The reaction sequence can be used to obtain the product in multigram quantities and the diacetylene derivative can be easily prepared from (Z)-l-methoxybuten-3-yne in 65 % yield. 

Wynberg and co-workers have prepared helicenes composed of fused thiophene and benzene units from the photocyclization of alkene precursors [134], It was shown that the effect of substituting thiophenes for benzene rings in these structures led to a blue shift of the absorption maxima. Compound 61 can be converted to the circulene structure 62 in two steps and the latter compound represents another interesting class of fused thiophene stmctures [135]. Trithiophenes with a benzene core (63) can be prepared from diacetylene-fimctionalized bithiophenes via a five-coordinate rhodium(I) intermediate by the addition of elemental sulfur [136]. A related structure (64) was reported by Pei et al. [137], along with the elaborate helicene 65. The compounds were constracted via oxidative (FeCb) cyclization reactions of 1,2-dithienyl benzene derivatives. 

A general synthesis of dibenzo-annelated dithieno[3,Z-b Z, 3 - thiophene 18 has been reported from bis(o-hydro-xyl)diacetylenes. This cyclization constructs a fused tricyclic skeleton in one pot. The subsequent dechalconization with copper powder produces a series of thiophene and selenophene-based heterocenes (Scheme 64) <20050L5301>. 

Compound 3.921 was also used as a precursor to obtain diacetylenic sulfide 3.925. Heating the latter (benzene, 200°C, 1,4-cyclohexadiene, 4 hours) leads to a mixture of ethynyl-2-naphthylsulfide 3.926 and naphtho[2,l-t ]thiophene 3.927 with dominance of the first (Scheme 3.134) [411]. This shows that the intramolecular cyclization of arene radical with a triple bond can occur through intermolecular capture of hydrogen from 1,4-cyclohexadiene. 

In connection with work on very strained fused thiophens, compound (42) has been obtained through a Wittig reaction between cyclobutane-1,2-dione (43) and the bis-ylide (44). From the cis-form of the diacetylenic thiiran (45), the long-elusive thienocyclobutadiene (46) has been obtained by flow pyrolysis at 250 °C. Compound (46) decomposes immediately on attempted chromatography or exposure to air. In benzene solution at room temperature it dimerizes to (47). 

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