Pyrrole radical cations

Quantitative investigations of the kinetics of these a-coupling steps suffered because rate constants were beyond the timescale of normal voltammetric experiments until ultramicroelectrodes and improved electrochemical equipment made possible a new transient method calledjhst scan voltammetry [27]. With this technique, cyclic voltammetric experiments up to scan rates of 1 MV s are possible, and species with lifetimes in the nanosecond scale can be observed. Using this technique, P. Hapiot et al. [28] were the first to obtain data on the lifetimes of the electrogenerated pyrrole radical cation and substituted derivatives. The resulting rate constants for the dimerization of such monomers lie in the order of 10 s . The same... 

Pyridone undergoes fragmentation by loss of CO and formation of the pyrrole radical cation. 3-Hydroxypyridine, on the other hand, loses HCN to give the furan radical cation while 4-pyridone... 

FIGURE 1 The electrochemical polymerization mechanism for the formation of polypyrrole begins when a pyrrole radical cation forms from the oxidation of the monomer. Then two radical cations react to create a dimer and split off two protons. The dimer undergoes further oxidation to generate a radical, which reacts with another radical to propagate chain growth and eventually produce polypyrrole. 

Therefore, coupling is believed to occur between oligomer radical cations and pyrrole radical cations at the electrode surface. The growth of polypyrrole chains probably terminates when the ends of the growing chains become sterically blocked... 

Figure 4.6. Coupling of pyrrole radical cation to polypyrrole (spin densities at radical cation are given).

Conducting polymer-based immobilization or wired enzymes is a global enzyme immobilization method that differs in many respects from those just described. In one example, a redox polymer is formed on the surface by the oxidation of pyrrole molecules to pyrrole radical cations, which then polymerize on the surface to form conductive polypyrrole [60,68]. Other conducting polymers include polyvinylpyridine, polythiophene, polyaniline, and polyindole. If enzymes are present in the solution as polymerization takes place, they are entrapped within the polymer. When these polymers are cross-linked with redox mediators such as [Os(bpy)2Cl]+ 2 the resulting amperometric (or potentiomet-ric) biosensors are referred to as wired enzyme electrodes [5-7]. The distance between the redox centers of the polymer and the FADH2 centers of the reduced enzyme is reduced sufficiently for electrons to be transferred and, therefore, for the mediated electro-oxidation of glucose on conventional electrodes. These electrodes do not require diffusing redox mediators or membranes to contain the enzyme and the redox polymer. 

At high copper(II) chloride indole ratios the pyrrole ring of 2-methylindole was chlorinated in yields approaching 92%. This reaction is believed to involve radical cations of indoles formed in an electron-transfer process. At low copper(ll) chloride indole ratios dimers were formed [86JCS(P 1)2305]. 

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... 

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... 

Although several examples of anodic dimerizations involving the coupling of cation radicals have been described,34 the most systematic mechanistic analyses have concerned electropolymerizations leading to conducting polymers.35 They involve a series of successive cation radical-cation radical coupling steps as represented in Scheme 2.33 for the case of pyrrole. Deprotonation of the first... 

Examples of the coupling of enam-ines are rare [60]. In most cases, the enamines undergo methanolysis to form electroinactive aminoacetals prior to the coupling reaction. Enamino ketones or enamino esters, however, yield via dimerization of the radical cations and subsequent ring closure pyrrole derivatives... 

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]. 

The initial removal of electrons (following the oxidation, p-doping process) leads to the formation of a positive charge localised in the polymer chain (radical cation), accompanied by a lattice distortion which is associated with a relaxation of the aromatic structural geometry of the polymer chain towards a quinoid form. This form extends over four pyrrolic rings ... 

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,... 

Pentaphenyl- and 2,3,4,5-tetraphenyl-pyrrole show reversible one-electron oxidation to the radical-cation in acetonitrile with E° = 0.86 and 1,008 V vy, see re-... 

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