2- Thienyl radical, formation

The phenylation of thiophene with benzoyl peroxide gave a considerable amount of 2,2 -dithienyl one suggested mechanism involved the formation of 2-thienyl radicals by oxidation, and their subsequent dimerization. More recent studies indicate that the 2,2 -dithienyl is formed through an initial addition of benzoyloxy radicals to the thiophene nucleus followed by dimerization of the resulting radical and loss of two molecules of benzoic acid (Scheme 15). 

The typical reactions of halogenothiophenes can be classified under the following headings nucleophilic displacement, halogen-metal exchange, reductive dehalogenation and formation of thienyl radicals. 

The mechanism of this reaction shows that excitation of the substrate gave an n,n triplet state, but this excited state was unable to dissociate the carbon-iodine bond. This was demonstrated by showing that the n,n triplet state, when sensitized by chrysene, did not produce coupling products. Probably, the reaction occurred in an excited a,a triplet state mainly localized on the carbon-iodine bond, and the interaction between this triplet state and aromatic compounds led to homolytic cleavage of the C-I bond with the formation of both a 5-thienyl radical and a complex between the aromatic compound and the halogen atom. The formation of this complex was demonstrated by the presence of a short-lived transient with Amax = 510 nm, showing a second-order decay kinetics and a half-life of ca. 0.4 (is in laser flash photolysis. The thienyl radical thus formed... 

The normal Ullmann reaction is the formation of an Ar-Ar bond by a thermally activated coupling of a thienyl halide in the presence of Cu, Ni, Pd, or their compounds, but this method is not suitable for the preparation of oligothio-phene and polythiophene films of nanoscale thickness. A photoactivated Ullmann coupling has now been described for the in situ synthesis of such polythiophene films <2006CC729>. The concept involves the selective photodissociation of the C-I bond in 2,5-diiodothiophene on a copper surface the resultant thienyl radicals react with the copper to produce thienyl-Cu intermediates in a thin monomer film at room temperature. These intermediates react with the monomer and produce polythiophene. 

The electropolymerization of these monomers at constant current under the same micellar conditions led to the formation of thin, electroactive polymer films. The electropolymerization of 0.05 M EDOT in 0.1 M SDS containing O.IM LiC104 in water at a Pt electrode began at very low current (j = 0.1 mA/cm ), compared to that found in acetonitrile without SDS (j = 0.5 mA/cm ). This phenomenon nuy be attributed to a specific effect of the SDS surfactant, which alters the oxidation potentials of EDOT. Thin, electroactive and conductive poly(EDOT) films can be synthesized in the above aqueous micellar solution at constant currents ranging firom j = 0.1 mA/cm to j = 5 mA/cm. For j > 5 mA/cm the resulting poly(EDOT) films were non-electroactive and extremely degraded owing to the reaction between water molecules and the thienyl radical-cations formed (8). 

Radical Reactions.—Thiazolyl, pyridyl, and other heteroaryl radicals formed by aprotic diazotization of the corresponding heterocyclic amines substitute homolytically on thiophen with the formation of 2-heteroaryl-thiophens as the main products in 20—50% yield. The results of competitive experiments indicate that the reactivity of thiophen in this reaction at 70—80 °C is slightly higher than that of benzene. The currently accepted mechanism of the decomposition of benzoyl peroxide in thiophen has been criticized on the basis of new experimental results. No free thienyl radicals are involved in the reaction, as demonstrated by scavenging experiments, and the bithienyls formed are probably derived from dimerization of a benzoyloxythiophen radical a-complex, with subsequent loss of benzoic acid. Nitrene insertion into the thiophen ring has been observed in the thermal decomposition of 2-(2-azidobenzyl)thiophen and similar compounds, leading to thieno[3,2-6]quinoline derivatives. ... 

The photopolymerization of thienyl derivatives in the presence of phe-nyliodonium hexaflurophosphate in dichloromethane has been reported (09JCS(CC)6300). The reaction probably occurs through the formation of thienyl derivative radical cation via an electron transfer process, and subsequent coupling of two radical cations. 

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