2 : 5-Dimethyl thiophene

Additional examples of the reactivity of the 4-position in thiophenes with —I-j-M-substituent in the 3-position are provided by the bromination of 3-thiophenecarboxylic acid to 2,4,5-tribromothio-phenecarboxylic acid and the recent diacetylation of 2,5-dimethyl-thiophene by Goldfarb et who used the swamping catalyst... 

Dimethyl-thiophen 3-Phenyl-thiophen 2-Formyl-thiophen 2-Benzoyl-thiophen... 

The reactions of a number of ozonides and alkenes in the presence of BF3 Et20 resulted in the formation of E- and Z-l,2-dioxolanes in 12-70% yields (Scheme 30) <83JA6279>. 2,5-Dimethyl-thiophene endoperoxide can act as a sulfur- or oxygen-atom transfer agent in analogous reactions to those shown by furan endoperoxides <86JA2775>. 

Cyclopropanated products from thiophene can undergo further transformations. For instance, irradiation of tetraphenyldiazocyclopentadiene in the presence of 2,5-dimethyl-thiophene gives the product (248) by rearrangement of the cyclopropane (247) (72CC1257). With thiophene as the substrate the ylide (249) was also obtained. Likewise, ylide (15) is formed by photolysis of diazomalonic ester in the presence of thiophene (77JOC3365). 

Finally, a Diels-Alder reaction with inverse electron demand, in which the thiophene acts as the 27r-component, has been reported (78AP728). Tetrazinedicarboxylic ester adds to thiophenes to give the adduct (290) loss of nitrogen from this is followed by oxidation, yielding the thieno[2,3-rf]pyridazines (291) in 10-15% yield. With 2,5-dimethyl-thiophene as the substrate, aromatization is blocked, and the product (292) is obtained in 57% yield. 

Triplet sensitized addition of several unsaturated compounds with thiophenes has been carried out. Citraconic and 2,3-dimethylmaleic anhydrides add thus to thiophene, 2-methyl-, 3-methyl- and 2,5-dimethyl-thiophenes (73JCS(P1)2322, 63CB498). The structure and stereochemistry of the products have been confirmed by application of NOE. With 2-methylthiophene, addition occurred at the unsubstituted double bond whereas with 3-methylthiophene, addition took place on the substituted double bond (Scheme 83). Benzo[6]thiophene also adds to the above anhydrides under triplet sensitization. With citraconic anhydride, there is no regiospecificity, both isomers being formed. 

Increasing the reaction time lead to the formation of the dithione in only very low yield (5 %). Also, the synthesis of 40 has been reported to have been accomplished using the same procedure starting from phthaloyl chloride and 2,5-dimethyl thiophene. Conversion of this to the mono- and di-thiones has been reported to be unsuccessful. 

The reaction of ( , )-2,4-hexadiene with sulfur dioxide at room temperature affords a single dia-stereomeric m-2,5-dihydro-2,5-dimethyl thiophene 1,1-dioxide (m-13) in 70 % yield123,124. The addition of sulfur dioxide to ( ,Z)-2,4-hexadiene is a much slower reaction and requires elevated temperatures which results in epimerization of the trans-adduct trans-13 to its d.v-isomer d.v-13. Thus, after 12 hours at 96 °C both diastereomers of 13 are present in roughly equal amounts124. 

Bis dimethylthieno l,4,5,8-tetrateUurafulvalene 3,4-Dibromo-2,5-dimethyl thiophene in tetrahydrofuran is treated at — 78° with 2 equivalents of tert.-butyl lithium. After 2 h one equivalent of tellurium powder is added. The mixture is slowly warmed to 0 and kept at 0 until all the tellurium has dissolved. The mixture is cooled again to — 78°, treated with tert.-butyl lithium and then with tellurium at 0°. The ditellurolate solution is cooled to — 78°, mixed with 0.5 equivalent of tetrachloroethene, stirred for 18 h, and allowed to warm to 20°. The brown solid is isolated by filtration and extracted with carbon disulfide. The extract is evaporated and the residue recrystallized from 1,1,2-trichloroethane to give bronze-colored crystals yield 75% m.p. 295-298°. 

Anodic oxidation in methanol of thiophene and substituted thiophenes at low temperatures (—20 to — 30°C) results in methoxylation and ring opening with loss of sulfur as SO2 [190]. From thiophene is isolated butenedialdehyde tetramethylacetal, some methoxysuc-cinic dialdehyde tetramethylacetal, and a small amount of methyl y6-formylpropionate. In general, oxidation of thiophenes results in the formation of derivatives of a, -unsaturated y-dicarbonyl compounds or y-keto esters. Cyanomethoxylation of 2,5-dimethyl thiophene yields mainly cis- and rm/i5-2-cyano-5-methoxy-2,5-dimethyldihydrothiophene and 3-cyano-2,5-dimethylthiophene [191]. 

Problem 20.5 Pyrroles, furans, and thiophenes are made by heating 1,4-dicarbonyl compounds with (NH4)2C03, P4O10, and P2S5, respectively. Which are used to prepare (a) 3,4-dimethylfuran (b) 2,5-dimethyl-thiophene (c) 2,3-dimethylpyrrole ... 

Substitution reactions may be effected on saturated analogues as well as on aromatic species. An interesting reaction of Tebbe s reagent with succinic thioanhydride derivatives 9 rapidly leads to monoolefination and. more slowly, to bisolefination. Acidic isomerization of the bis>adduct leads to 2.5-dimethyl thiophene derivatives (Eq. 3) <94JOC494>. 

Cross-linked PDMS/PEI composite membranes were used in PV separation of thiophene-n-heptane, 2-methylthiophene-n-heptane, 2,5-dimethylthiophene-n-heptane, n-butyl mercaptan-n-heptane, and n-butyl sulfide-n-heptane mixtures (Chen et al. 2008). PV results indicated that as the feed temperature increased from 50°C to 90°C, the partial flux increased and the enrichment factor decreased. The partial flux and the enrichment factor of sulfur species had the same order n-butyl sulfide < n-butyl mercaptan < 2,5-dimethyl thiophene < 2-methylthiophene < thiophene. As the organic S content increased from 50 ng/pl to 250 ng/pl, the partial flux of organic sulfur increased and the enrichment factor dcCTcased slightly but the total flux was scarcely influenced by the concentration of organic sulfur as very low content of organic sulfur led to minor contribution to the total flux. 

In an investigation on the benzophenone-sensitized photoaddition of citraconic anhydride and 2,3-dimethylmaleic anhydride to several thiophen and benzo[6]thiophen derivatives, it was found that, when 2,5-dimethyl-thiophen was used as substrate, only small amounts of the expected adduct (103) were formed, the main product being (104). Using only benzo-phenone in the photoaddition, the yield of (104) was 62%. The reaction... 

Write out a synthesis of 3-bromo-2,5-dimethyl-thiophene from 115. 

Oiniethyl-tluoplieii 17, 41,120, II41, 2.5 Dimethyl-thiophen 17, 41,120, II41. 3.4.Dimethyl.thiophen 17, 42,120, II42. 

Dieckmann cyclization of (21a) gave, depending upon conditions, either (21b) or (21c) predominantly. These compounds were transformed into the thiophens (21d) and (21e). The acid-catalysed reaction of (22) with hydrogen sulphide at -35 °C leads via the enethiol to ethyl 2,5-dimethyl thiophen-3-carboxylate as the main product. The monoacetals of substituted succinaldehyde (23), which are conveniently prepared through hydro-formylation of a -unsaturated aldehyde acetals with trans-bis(triphenyl-phosphine)carbonyl chlororhodium(i) as catalyst, have been used for the synthesis of optically active 3-substituted thiophens (from optically active a/3-unsaturated aldehyde acetals) through the acid-catalysed reaction with... 

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