Radical-cation coupling

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... 

Strong evidence for the radical cation-radical cation coupling mechanism shown in Scheme 1 has been obtained from double-step chronoam-perometry studies.66,67 However, an alternative polymerization mechanism, involving the coupling of radical cations with unoxidized molecules, has been claimed by a number of authors.55,68,69 Some of the... 

The anodic polymerization of aniline can occur by a radical cation coupling mechanism analogous to that shown in Scheme 1, with coupling occurring between the N of one molecule and the para-position of another (Structure 4)21,22 However, a variety of other mechanisms have also been proposed,21 and it is likely that their relative rates depend upon the conditions (solvent, potential, pH, etc.) employed. The links between monomers are therefore not exclusively between the N and para-position (head-to-tail coupling). Head-head (-N=N-) and tail-tail (para-para) coupling occur more often as the pH is increased.71... 

RBSctions of Radical Anions With Radicals. The coupling of arene or alkene radical anions with radicals is an important reaction, and one that has significant synthetic potential. For example, radicals formed by nucleophilic capture of radical cations couple with the acceptor radical anion, resulting in (net) aromatic substitution. Thus, the l-methoxy-3-phenylpropyl radical (113 R = H) couples with dicyanobenzene radical anion loss of cyanide ion then generates the substitution product 132.2 + ... 

The substituent perturbations can be fully parametrized For (R3Si) CH3 -substituted radical cations, coupling constants are rationalized and predicted by the angularly... 

Allyl)Fp complexes are also subject to attack, at C-3, by radicals. The mechanism of allylic transposition of ()] -allyl)Fp complexes, as well as the mechanism of phosphite substitution for CO, has been ascribed to attack by Cp(CO)(L)Fe- on the original Fp-aUyl. The reaction of (12) with CCI4 proceeds by a radical chain mechanism, ultimately between CCI3 and the Fp-allyl. The substitution of a-halo ketones and esters most likely proceeds similarly. A radical cation coupling mechanism has been proposed for the dimerization of (jj -allyl)Fp and (j7 -propargyl)Fp complexes. ... 

Radical cation-radical cation coupling [308] is often observed during anodic oxidations when their concentration is sufficiently high the capacity of n radical cations to undergo atom abstraction reactions, e.g. of fullerene + [309], has little significance only. 

A typical example in heteroaromatic chemistry is the preparative electrolysis of some 2,5-diaryl-1,4-dithiins 13, which give two well resolved quasi-reversible one-electron transfer waves to the radical cation and the di-cation, respectively, in cyclic voltammetry. The electrolysis gives low yields of the 2,2 dimers (see Scheme 12) via the radical cation coupling mechanism [46]. 

In the case of the [2.2]- nerother substituents than methoxy—for example, with CH3, Br, CN, or NO2—can also be cyclized [148]. 

Thus, 9 and 10 are the major coupling products formed upon oxidation of aniline in aqueous H2SO4 solution. Theoretically, it is possible to imagine two mechanisms of the reaction pathway (i) dimers are formed by two radical cations coupling, and (ii) an electrophilic attack of the radical cation on a neutral molecule of aniline produces the corresponding dimer. In the second case it is assumed that the coupling product loses another electron and therefore the total number of electrons transferred in the overall processes would be the same in both mechanisms. 

The mechanism of electro-oxidation of DMA in acetonitrile was studied by rapid scan cyclic voltammetry. The reduction peak for the radical cation DMA 1 was observed at scan rates above 500 Vs1. It was found that DMA 1 " underwent a second-order radical cation—radical cation coupling with deprotonation to form V,V,V, V -tctramethyI benzidine. A rate constant of 6.3 x 105 M 1 s 1 was calculated58. No evidence of polymerization was detected. 

There is a scant data on the radical behavior of thianthrene radical cation [58]. As pointed out above, thianthrene radical cation couples with radicals. It shows little reactivity toward oxygen but this may be ascribed to its positive charge which should render it an electrophilic radical. It initiates the polymerization of styrene [59,60], a-methyl styrene [59], and ethyl vinyl ether [59,60],but not that of vinyl pyridine [61], vinyltrimethylsilane [59], methyl acrylate [59, 61], or acrylonitrile [59,61 ]. These results can be explained by cationic rather than radical polymerization. 

Polypyrrole (PPy) was first prepared electrochemically at a platinum electrode in acetonitrile by Diaz et al, [20] which was followed by a few related works from the same group [21]. A stoichiometry similar to reaction (I) was presented for the preparation of neutral PPy in aqueous media by Pletcher et al [22]. These authors invoked an initial coupling mechanism in which an electrochemically generated radical cation couples with a neutral pyrrole molecule to produce a dimer radical cation, which is then further oxidized... 

Figure 11.6 Radical cation coupling mechanism for polythiophene.

In the initiation step, the oxidation of a pyrrole monomer yields a radical cation. Coupling of the two generated radical cations and deprotonation produces a bipyrrole, as confirmed by Andrieux et al. 

Radical cation-radical cation coupling at a position with dimer formation ... 

Heteroarenes have been photochemically functionalized by PET reactions forming new C—C bonds both in an inter- and intramolecular fashion via a similar mechanism [46]. The heteroarenes could serve both as electron donor (e.g. pyrroles or indoles) or electron acceptor (e.g. cyanopyridines or cyanopyrazines). Again, fragmentation of the radical cation, coupled with the radical anion and loss of the anion, led to overall ipso-substitution. In addition to the cyano group, halides could also function as leaving groups, such that in some cases an attack at an unsubstituted position took place [46],... 

In Eq. 1, a monomer, M is oxidized at the electrode surface and releases an electron to form a radical cation. In Eq. 2, two of the radical cations couple to produce a dimer. The dimer then immediately releases two protons. Street et al. [17] found a decrease in pH during electrochemical polymerization, which provides supporting evidence for the elimination of protons. Equations 1 and 2 continuously repeat to increase the length of the growing oligomer chain. Equation 3 represents the net reaction in which jc monomeric units link to form a polymer HMJH. Overall, (2x - 2) protons and an equal number of electrons are released. In Eq, 4, the polymer HMJH is oxidized to form the polycarbonium ion, and y electrons. The... 

Because oxidative electrochemical polymerizations proceed via radical cation coupling, aprotic solvents that are also poor nucleophiles are generally employed. Acetonitrile is an example of such a solvent. However, the use of nucleophilic aprotic solvents is not excluded. Solvents such as dimethyl formamide may be used if protic acids are added to the solvent to decrease the nucleophilicity of the resulting electrolyte. 

Infrared spectra confirm the loss of two protons after radical cation coupling [377]. Bruckenstein and Sharkey [378] provide a theoretical analysis of the electropolymerization of azulene at a rotating ring-disk electrode. Hydrogen ion has been identified and quantified as a product of the electrochemical polymerization. 

Pol5unerization is initiated by monomer oxidation to yield the radical cation. The mechanism (55) is believed then to involve either radical-cation/radical-cation coupling or reaction of a radical-cation with a neutral monomer. The mechanism of electropolymerization of five-membered heterocycles with radical-cation/radical-cation conpling is shown in Fig. 6. 

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