Thiophene radical cations

The radical cations of thiophene 113a, and its derivatives have been made in Freon by y-radiolysis [243,244] and in solution by UV irradiation [245]. 

2 -Bithienyl 116a exists in two conformations in the gas phase as shown by electron diffraction studies [254]. 

The hyperfine splitting constants aH are 2.24 (4H,H3),0.16 (4H,H4),and 0.62 G (12H, Me) showing that the spin is delocalized over four equivalent thiophene rings. Anodic oxidation also produces the dication of 126c isolated as its perchlorate salt. The X-ray crystallographic structure shows that there are two delocalized dithienyl methylene units perpendicular to each other as shown in 127. Cyclic voltammetric studies of 128 in CH2C12 have been reported [273]. This compound can be viewed as a dibenzo TTF derivative connected by an a-terthienyl moiety. It shows three quasi-reversible oxidations in CH2C12 with peak potentials of 0.54, 0.70, and 0.82 V vs SCE. These may be compared with the first oxidation potentials of dibenzo TTF and 3 -(3,6-dioxaheptyl)ter-thienyl of 0.72 V and 1.05 V, respectively. 

The radical cation of dibenzothiophene 129 has been prepared in 1,1,1,3,3,3-hexafluoro-2-propanol solution by oxidation with Tl(OAc)3 and its EPR spectrum measured [274]. 

Unlike thiophene radical cation the SOMO for this species is 2bl with considerable spin density on sulfur. The reversible electrochemical oxidation potentials for 129 and some of its derivatives in l,l,l,3,3,3-hexafluoro-2-propanol are listed in Table 8. The reactions of 129 with radicals and with nucleophiles has been studied [275]. The position of attack by radicals on 129 should reflect the spin density at that position as found by EPR spectroscopic analysis. Indeed reaction with N02 occurs predominantly at S, C(2) and C(4) as expected. The valence bond configuration mixing model leads to the prediction that nucleophiles should preferentially attack 129 at C(l) and C(3) with little attack at S, C(2) and C(4). This is partly but not completely validated experimentally. Radi- 

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Cyclic voltametric analysis has been utilized to determine material properties of this class of heterocyclic compounds. All the DTPs 23 <2003JOC2921 > exhibited a well-defined irreversible oxidation presumably corresponding to the formation of the radical cation. When scanned to higher positive potentials, it resulted in two consecutive broad oxidations for most of the DTPs. The second oxidation is quite weak, followed by a more intense and well-defined third oxidation. Coupling of thiophene radical cation is usually rapid (r <10-5 s) <1995SM(75)95>. These additional broad waves most likely correspond to the oxidation of coupled products rather than further DTP oxidations. The electrochemical data of the DTP S 23 are given in the Table 10. 

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). 

Thiopyran-4-one fragments by way of a retro Diels-Alder (rDA) reaction followed by loss of CO. An alternative decomposition involves initial loss of CO which leads to the thiophene radical cation (Scheme 13) . 

A double role was proposed for iodine oxidant of the monomer to form thiophene radical cations which couple to produce short oligothiophenes, and glue by means of an adlayer on the Au(lll) surface such that the short oligothiophenes adhere on the surface, acting as nuclei for polymerization. 

Figure 4.5. Coupling of thiophene radical cation to polythiophene.

Fig. 6.5 Cut through the lowest PES of the furati, pyrrole, and thiophene radical cations along the coordinate of an effective mode. The latter is given by a straight line connecting the minimum of the A2 ionic ground state (taken to be the zero of energy in all cases) to the minimum of the Coin between the ground and first excited ( Si) ionic states. The dotted lines refer to thiophene, the dashed ones to pyrrole and the full lines to the furan radical cation...

The most widely accepted mechanism for the anodic polymerization of pyrroles and thiophenes involves the coupling of radical cations produced at the electrode (Scheme l).5 The oligomers so produced, which are more easily oxidized than the monomer, are rapidly oxidized and couple with each other and with monomer radical cations. Coupling occurs predominantly at the a-positions (i.e., 2- and 5-position),5 and so pyrroles and thiophenes with substituents in either of these positions do not undergo anodic polymerization. The reaction is stoichiometric in that two... 

Oxidation of thiophene with Fenton-like reagents produces 2-hydroxythiophene of which the 2(570 One isomer is the most stable (Eq. 1) <96JCR(S)242>. In contrast, methyltrioxorhenium (Vn) catalyzed hydrogen peroxide oxidation of thiophene and its derivatives forms first the sulfoxide and ultimately the sulfone derivatives <96107211>. Anodic oxidation of aminated dibenzothiophene produces stable radical cation salts <96BSF597>. Reduction of dihalothiophene at carbon cathodes produces the first example of an electrochemical halogen dance reaction (Eq. 2) <96JOC8074>. 

ESR experiments employing in situ photolytic decomposition of the peroxydisulfate anion (S20g ) have been carried out to study the reaction of S04 with aliphatic sulfoxides. In the case of dimethyl sulfoxide three radicals are detected ( CHj, CH3 S02, CH2 S(0)CH3), the proportion being pH-dependent. The reaction is assumed to proceed via an initially formed radical cation (not detected) which would be rapidly hydrated to give an intermediate identical with that generated by OH addition on the sulfoxide. Such a process parallels the rapid hydration of radical cations formed from thiophene in their reactions with SO/ and... 

Figure 6. Electrophilic polymerization of thiophene. Initial 71-bonding, formation of radical cation, electrophilic addition.

Aromatic molecules can be polymerized catalytically on clean metal surfaces, or electrochemically to produce oriented polymer films. Initial adsorption of aromatic molecules occurs by electron donation from the aromatic molecule to the surface. This electron donation creates radical cations that can polymerize. Molecular orientation in the films depends on the stable bonding configuration of the radical cation. Thiophene, pyridines, and pyrrole all polymerize with the ring substantially perpendicular to the surface, whereas aniline polymerizes with the phenyl rings parallel to the surface. The catalytically... 

A careful analysis based on these experimental results excluded a chain-propagation process [33a]. On account of the 3-position of the methylthio or methoxy substituent in the thiophene or pyrrole rings, three isomeric dimers may be formed. The main reaction path can be deduced from the mesomeric forms of the radical cations (2)". The two most important mesomeric structures are those with the unpaired electron in... 

In the case of protonated pyrroles, the p Ta value lies in the range between 4 and —4, whereas the pATa value of acetonitrile is about —10. Therefore, the oligomerization of pyrrole in pure acetonitrile may already stop at the level of a-intermediates of hi- or more likely of tetrapyrrole. Acetonitrile is a weaker base than the a-intermediates. Consequently, a stronger base must be used to initiate the elimination of protons. Water fulfills this condition. Pyrrole can be polymerized in acetonitrile in the presence of 1% water [6, 37]. A similar effect results from the application of a sterically hindered base such as 2,6-di-tert-butylpyridine [38]. However, the concentration should be kept low because, at high concentrations proton, abstraction from the monomeric radical cation may occur, thus forming a neutral radical [28d]. The base effect can be also observed in the case of thiophenes. 

Careful quantitative kinetic studies of the coupling steps of oKgomeric pyrroles and thiophenes have confirmed this mechanistic pattern [49]. In addition, quantum chemical studies reveal that the dimerization of two radical cations becomes perfectly feasible when solvent effects are included [50]. 

Technically important electrochemical reactions of pyrrole and thiophene involve oxidation in non-nucleophilic solvents when the radical-cation intermediates react with the neutral molecule causing polymer growth [169, 191], Under controlled conditions polymer films can be grown on the anode surface from acetonitrile. Tliese films exhibit redox properties and in the oxidised, or cation doped state, are electrically conducting. They can form the positive pole of a rechargeable battery system. Pyrroles with N-substituents are also polymerizable to form coherent films [192], Films have been constructed to support electroactive transition metal centres adjacent to the electrode surface fomiing a modified electrode,... 

In methanol, the radical-cation intermediates from oxidation of thiophenes and N-methylpyrroles can be trapped to give low molecular weight products. Reactivity resembles that of furan but with additional consequences because of the properties of thioethers and amines. 

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