Thiophene, 2-chloro-, formation

The use of free-radical reactions for this mode of ring formation has received rather more attention. The preparation of benzo[Z)]thiophenes by pyrolysis of styryl sulfoxides or styryl sulfides undoubtedly proceeds via formation of styrylthiyl radicals and their subsequent intramolecular substitution (Scheme 18a) (75CC704). An analogous example involving an amino radical is provided by the conversion of iV-chloro-iV-methylphenylethylamine to iV-methylindoline on treatment with iron(II) sulfate in concentrated sulfuric acid (Scheme 18b)(66TL2531). 

Such eliminations do not occur with chloro compounds. 2,5-Dichlorothiophene undergoes AlCU-catalyzed sulfonylation with ben-zenesulfonyl chloride. Attempts to extend this reaction to thiophene, 2,5-dimethylthiophene, and 2-chloro-5-iodothiophene resulted in the formation of intractable tars even with other catalysts. Sul-fones of this type are often prepared in much better yields by using thiophenesulfonyl chlorides and benzenes in the Friedel-Crafts reaction. ... 

Typical examples are the formation of 3-chloro-2-phenylbenzo[b]-thiophene (21) in near quantitative yield from diphenylacetylene at room temperature,27 and the synthesis of the dichlorothianonane (22) from 1,5-cyclooctadiene.28... 

Further research on the substitution of the thiophene 3-position with phenyl groups containing electron-withdrawing or electron-donating groups (such as methyl, methoxy, fluoro, chloro, bromo, trifluoromethyl, sulfoxy) in the para position have lead to polymers with unique features [57]. The electron-withdraw-ing groups allow the formation of a radical anion and thus stabilize the n-doped state. As a result, such conducting polymers can be reversibly oxidized and reduced and electrochromic devices can be built with identical anode and cathode materials [58]. 

Numerous syntheses involving formation of the thiophene ring have been reported. Nucleophilic substitution by ethyl mercaptoacetate of 2-chloro-3-cyanopyridine with subsequent base-promoted cyclization yields ethyl 3-aminothieno[2,3-i]pyridine-2-carboxylic acid, which is easily transformed by standard methods into the parent compound (258 Scheme 63) (74JHC975, 76JHC273). 

A photoinduced substitution reaction initiated by electron transfer has also been proposed to account for the formation of 2-chloro-3-(2-thienyl)-1,4-naphthoquinones (217) from 2,3-dichloro-l, 4-naphthoquinone (215) and thiophenes (216)623 (equation 160). 

Nucleophilic displacement of the chlorine atom of 3-chloro-1,2-benzisothiazole has proved to be a popular procedure. Boeshagen and Geiger34 have continued their earlier work on nitrogen nucleophiles, and now include carbon, oxygen, and sulfur nucleophiles.35 In some cases, rearrangements occur, as in the formation of 3-amino-2-acylbenzo[6]thiophenes (20) from reaction of 21 with methyl ketones. Similar results are obtained from the reaction of other carbon nucleophiles, and it has been suggested that attack may be either at the 3-carbon or the sulfur atom.36 The reaction of 3-chloro-1,2-benzisothiazole (8) with the anion of ethyl cyanoacetate, for example,... 

The parent thiophene 1,1-dioxide 1 has been generated by dehydrobromination of the bromide 2 (Scheme 2) [55-57]. The reagents used for this conversion include piperidine [55],R2CuLi,RLi,RMgBr,BuNH2 [56], and K/ultrasound [57]. The use of K/ultrasound system in toluene allowed the formation of 3-methyl-, 3-chloro-, and 3,4-dimethylthiophene 1,1-dioxides [57]. Hofmann elimination of the ammonium salt 4 led to the parent thiophene dioxide [58]. Treatment of compound 3 with bases also generates the parent thiophene dioxide by elimination of two molecules of HBr [57,59,60]. 

The reaction of (2) with alkanesulphenyl chlorides led to a mixture of (3) and (4). Lower reaction temperatures favoured the formation of (3). The reaction of vinylidene chloride with hydrogen sulphide at 470-550°C gave 15% of 2-chloro- and 16% of 3-chloro-thiophen as the main products. ... 

The saturated vapour pressures of chloro and chlorosilyl derivatives of thiophen have been determined. Liquid vapour isothermal equilibrium for thiophen with benzene and with alcohols has been measured. The formation constants, extinction coefficients, and total absorption intensities for complexes between thiophens and iodine have been determined. ... 

A variety of thioketones have found applicability in the synthesis of heterocyclic compounds. Chloro-thioacetone reacted with trialkoxy-phosphines to give thiirans. Thiophens have been prepared from mono-thio-j3-diketones, j8-thioketo-esters, > and from a-thioketo-acids. a-Thioacyl-y-lactones and -thiolactones rearrange in acid alcoholic solution to give dihydrofurans and dihydrothiophens, respectively. The simultaneous action of carbon disulphide, sulphur, and triethylamine on aliphatic thioketones gave mixtures of l,2-dithiole-3-thiones and 1,3-dithiole-2-thiones. The formation of thio-ozonides (1,2,4-trithiolans) by... 

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