Thiophenes radicals from

Radical substitution reactions and their mechanisms and applications have been reviewed several times [189,190]. Thiophene participates well in radical reactions. There are reviews describing both unimolecular radical nucleophilic substitutions (SrnI) [191] and homolytic aromatic substitutions (HAS) of thiophenes [192]. The formation of thiophene radicals from peroxides, thienylamines and iodothiophenes has been discussed [192]. 

This, after 1,3,6,8-tetraazapyrene, was only the second example of obtaining both a negative and a positive radical from the same heterocyclic compound. Attempts to generate radical-ions from other condensed thiophenes succeeded only with dithieno[2,3-d 2, 3 -radical-cation [HFSC 0.42 (2H), 2.36 (IH), and 2.98 (IH) Gauss]. 

The mechanism of the reaction of thiophene with a variety of radicals as a function of pH has been studied using ESR (81JCS(P2)207). Attack by -OH at pH 6 proceeds by direct addition with a preference to add to the a-position the ratio of (226) to (227) is 4 1. At low pH the (3-adduct easily loses OH- to form the thiophene radical-cation, which may undergo rehydration. In the case of 2-methyIthiophene the radical-cation deprotonates to give the thenyl radical this is reminiscent of the electrochemical oxidation (Section 3.14.2.6). The radical-cations are also formed by direct electron abstraction from the thiophene substrates by chlorine anion-radicals. At pH >6, (226) starts disappearing with formation of ring-opened products (Scheme 61). 

Given the selection of an appropriate substrate, the generation of a carbon centred radical from such a substrate can initiate a series of bond-making and bond-breaking processes which are sometimes referred to as radical cascade reactions. These can be of great synthetic value. Thus, treatment of the ketene dithioacetal 1 with a five fold excess of tributyltin hydride in hot benzene under nitrogen and in the presence of a catalytic amount of AIBN gave the metallated benzo[ fc]thiophene 2, itself a valuable synthetic intermediate, in 70% yield. 

Figure 61 Polythiophene formation via mechanism involving electrophilic attack of cation radical upon neutral thiophene monomer. (From Ref. 247.)...

Anion-radicals from dithienothiophene dioxides liave been studied (e.g., 121) for which ESR and polarographic results have been presented. Similar results were obtained for certain isomers of 121 but not for others. During electrolytic reduction of deuterated dithienothiophene dioxides, it was found that H-D exchange occurred in the anion-radicals. The source of protons was either impurity from solvent decomposition (DMF) or an adventitious unknown present in acetonitrile. Janssen has also correlated electronic absorption spectra and polarographic reduction potentials with calculated molecular quantities for a wide range of thiophene S,S-diox-ides. ... 

The same Bologna group, who have contributed much to our knowledge of thiophene radicals, have also investigated the anion-radicals of isomeric rraw.s-dithienylethylenes, 1-(2-thienyl)-2-phenylacetylene, and trans- - -thienyl)-2-phenylethylene. The ESR spectra indicate that rotation of the aryl groups in these molecules is slow on the time scale of the ESR experiment, and separate rotamers have been detected for the radical from... 

The univalent radical from thiophene is called thienyl. The ring atoms of thiophene are copla-nar, as in furan (see Fig. 5.4). The greater atomic radius of sulfur causes the bond between the heteroatom and one of the or-C-atoms to be longer by 35.2 pm than in furan. 

Paulmier (01JCS(P1)37) described the synthesis of the thieno(3,2-fc) pyrrole skeleton 186a-f from thiophene radical precursors 185a-f, using BusSnH (Scheme 48). 

Oxidation and reduction introduce charges into the polymer material, and in order to maintain electroneutrality, counterions are incorporated. A considerable number of ions are incorporated in the polymer and their influence is not negligible. During the polymerization process, depending on the dielectric permittivity of the solvent, the cation radicals and anions form ion pairs with different readiness. The ion pairs have different reactivity from the free cation radicals. If anions have too much nucleophilicity, they may even form covalent compounds with the cation radicals, which prevents their polymerization reactions. For instance, halide ions cannot be used in connection with reactive cation radicals, such as thiophene radicals. 

These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398). 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

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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