Thiophene dimer formation

The mechanism is quite complex however, a largely accepted mechanism is shown in Scheme 3. Thiophene dimer formation is promoted by an electrochemical oxidation and coupling, followed by the loss of two protons. The neutral dimer is then oxidized and coupled with another unit producing a trimer the mechanism is repeated and a polymer with n thiophene rings is formed [39]. Several studies have... 

Katritzky and co-workers illustrated the reactivity of the C3 position with the formation of a thiophene dimer via an intramolecular cyclization. The cyclization was performed first by lithiation followed by the addition of zinc bromide under refluxing conditions to produce the dimerized product. 

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). 

In the desulfurization of 3-substituted thiophenes several stereoisomers may be formed in certain cases. Both meso and racemic compounds have been obtained from the desulfurization of 3,4-diaryl-substituted thiophenes. It is claimed, however, that only meso, -diphenyladipic acid is obtained upon desulfurization of 3,4-di-phenyl-2,5-thiophenedicarboxylic acid and only di-isoleucin from 3-thienylglycine. The formation of small amounts of dimeric products in the desulfurization has been discussed with reference to the mechanism of this reaction. ... 

The [2 + 2] photodimerization of a, j8-unsaturated sulfones is correctly viewed as a photoreaction of alkenes, rather than the sulfone group, and this aspect has been reviewed recently by Reid, as part of a wider survey of the photoreaction of O- and S-heterocycles. The topic continues to attract considerable interest and a few recent examples, as well as some synthetic applications, will be discussed here. Much of the photodimerization work has been carried out on the benzo[fc]thiophene (thianaphthene) 1,1-dioxide system. For example. Porter and coworkers have shown that both 3-carboxybenzo[i]thiophene 1,1-dioxide (65) and its methyl ester give only the head-to-head (hth), anti dimer (66) on irradiation in ethanol. In a rather unusual finding for such systems, the same dimer was obtained on thermal dimerization of 65. Similar findings for a much wider variety of 3-substituted benzo[fi]thiophene 1,1-dioxides have been reported more recently by Geneste and coworkers . In the 2-substituted analogs, the hth dimer is accompanied by some of the head-to-tail (htt), anti dimer. The formation of the major dimer appears to proceed by way of an excited triplet and the regiochemistry observed is in accord with frontier MO theory. 

Rhodium(II) acetate was found to be much more superior to copper catalysts in catalyzing reactions between thiophenes and diazoesters or diazoketones 246 K The outcome of the reaction depends on the particular diazo compound 246> With /-butyl diazoacetate, high-yield cydopropanation takes place, yielding 6-eco-substituted thiabicyclohexene 262. Dimethyl or diethyl diazomalonate, upon Rh2(OAc)4-catalysis at room temperature, furnish stable thiophenium bis(alkoxycarbonyl)methanides 263, but exclusively the corresponding carbene dimer upon heating. In contrast, only 2-thienylmalonate (36 %) and carbene dimer were obtained upon heating the reactants for 8 days in the presence of Cul P(OEt)3. The Rh(II)-promoted ylide 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). 

Condensation of copper salts of acetylenes (240) with o -bromobenzenethiol (239) by slow addition of a pyridine solution of (239) to a dilute pyridine solution of (240) under nitrogen at 110 °C for 24 hours gave the corresponding benzo[6]thiophenes (241) with the following yields R = H, 90% R = Bun or Prn, 80% R = C02Et, 35%. Formation of thianthrene by dimerization of the copper salt of (239) was a competing reaction which became the major pathway in more concentrated solutions. 

A [5 - 2 + 2 + 1] fragmentation followed by cyclization forming a new five-membered ring was observed by FVP studies of 2-propenyl-l,3-dithiolan 1,1-dioxide (79) (95H1967). The reaction mixture consists of four products thiophene (26%), 2,5-dihydrothiophene (80,34%), 4-methyl-2-propenyI-4//-l,3-dithiine (20%), and 2,6-dimethyl-2//,6//-l,5-dithiocine (20%). The last two compounds are formed by [4 + 2] or [4 + 4] dimerization of the intermediate 2-butenethial. Formation of 80 involves a 1,5-H shift of the as-butenethial, followed by cyclization. 

The reaction of [Rh(C2H4)2(T75-G Me5)] with an excess of thiophene in benzene solution led to dimerization of thiophene and to formation of the unusual binuclear complex 28. The bridging ligand can be considered as an co-mercapto a,/3-unsaturated thioaldehyde.79,152 When [Co(C2H4)2... 

CsMes)] was substituted for [Rh(C2H4)2(r -CsMes)] instead of the dimerization of the thiophene, only the cleavage of the thiophene and the formation of a binuclear complex were observed, which, on treatment with H2S, afforded 29. Complex 29 contains a coordinated bisthioaldehyde (Scheme 10).153... 

Thiophene reacts with phthalimide (78TL125). With halogenated phthalimide, the main reaction is the formation of the dimer 78 of the starting material, while a substitution product 79 was obtained in very low yields (5.8% when X = Cl, 4.5% when X = Br, 15% when X = I). 

The radical anion Cw, can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines, fer-rocenes, tetrathiafulvalenes, thiophenes, etc. In homogeneous systems back-electron transfer to the reactant pair plays a dominant role resulting in a extremely short lifetime of Qo. In these cases no net formation of Qo is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors [154,155], In these cases selective one-electron reduction of C6o to Qo takes place by the irradiation of C6o with a Xe lamp (X > 540 nm) in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicoti-namide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 15) [154], The formation of C60 is confirmed by the observation of the absorption band at 1080 nm in the near infrared (NIR) spectrum assigned to the fullerene radical cation. 

Thienyldihydrothiophene is the thiophene analogue of the phenyl-cyclohexadiene intermediate deduced by Fields and Meyerson (1966c) from scrambling of protium and deuterium in pyrolysis of deuteriated benzene. Both intermediates account for the formation of dimeric species unaccompanied by highly energetic free hydrogen atoms. 

When A.A-dimethylisoindolinium bromide is treated with phenyl-lithium, it gives V-methylisoindole via the ylid (54).2 59,60 An attempt to prepare benzo[c]thiophene via the analogous ylid (55) failed. Thus, when l,3-dihydrobenzo[c]thiophene methylsulfonium iodide was treated with phenyllithium, it gave a mixture of methyl phenyl sulfide, spiro[5.5]-l-methylthio-2,3-benzo-6-methylthio-methyleneundeca-7,9-diene (56), and 3,4-bis(methylthio)-l,2 5,6-dibenzo-l,5-cyclooctadiene (57).59,60 The formation of methyl phenyl sulfide may be explained by the formation and ring cleavage of compound 58, and compounds 56 and 57 arise by Diels-Alder dimerization of the o-quinodimethane (59) formed by ring cleavage of the ylid (55). 

The reaction of thieno[3,4-e]thiophene (37) with dicyanoethyne (53) (Equation (1)) was examined <83JA1705> utilizing the MINDO/3 method. Extrusion of elemental sulfur as an atomic species from the initial 1 1 cycloadduct was shown to be energetically unfavorable, whereas formation of molecular sulfur (S6 or S8 species) resulted in a favorable energy change. A satisfactory mechanism for the extrusion of sulfur involves a thiirane species that dimerizes to a disulfide which decomposes to give the expected product, 5,6-dicyanobenzo[c]thiophene (54), and an intermediate that ultimately allows for the extrusion of S6 or S8 (Figure 6). 

All attempts to isolate the nonclassical 147r-electron tricyclic system (73) resulted only in the formation of dimer (73a), condensed at the benzene ring (Scheme 14). Reaction of (73) with iV-phenylmaleimide, on the other hand, takes place at both the benzene and thiophene rings (69JA6891). 

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