Pyrroles, electrochemical oxidation

The electrochemical oxidation of monomers such as pyrrole,2-5 thiophene,6-9 aniline,10-13 etc., or their derivatives, initiates a polymerization process at the electrode/electrolyte interface that promotes the formation of a polymeric film that adheres to the electrode. A similar homogeneous polymerization process can be initiated by chemical oxidation or chemical polymerization.14-21 Some monomers can be polymerized as well by electrochemical or chemical reduction. 

Most 2,5-unsubstituted pyrroles and thiophenes, and most anilines can be polymerized by electrochemical oxidation. For pyrroles, acetonitrile,54 or aqueous55 electrolyte solutions are normally used, while the polymerization of thiophenes is performed almost exclusively in nonaqueous solvents such as acetonitrile, propylene carbonate, and benzonitrile. 0 Polyanilines are generally prepared from a solution of aniline in aqueous acid.21 Platinum or carbon electrodes have been used in most work, although indium-tin oxide is routinely used for spectroelectrochemical experiments, and many other electrode materials have also been employed.20,21... 

In 1979, the formation of conductive polypyrrole films by the electrochemical oxidation of pyrrole was reported for the first time This work has stimulated intense and fruitful research in the field of organic conducting polymers. Further important conductive polymers are polythiophene, polyaniline and polyparaphenylene. The development and technological aspects of this expanding research area is covered... 

Due to its electronic conductivity, polypyrrole can be grown to considerable thickness. It also constitutes, by itself, as a film on platinum or gold, a new type of electrode surface that exhibits catalytic activity in the electrochemical oxidation of ascorbic acid and dopamine in the reversible redox reactions of hydroquinones and the reduction of molecular oxygen iV-substituted pyrroles are excellent... 

The Clauson-Kaas pyrrole synthesis was adapted to a soluble polyglycerol (PG) support <060L403>. Electrochemical oxidation of furan 33 in the presence of methanol followed by hydrogenation gave 2,5-dimethoxytetrahydrofuran 34. Cyclocondensation with primary arylamines gave A-arylpyrroles 35. Removal from the PG support was then accomplished by treatment of 35 with LiOH which gave 2-pyrrolepropanoic acids 36. 

Anodic oxidation of l,3-diaryl-5-methyl-A2-pyrazoline-5-carboxylic acids in CH3CN-Et4NBF4 proceeded with decarboxylation to the aromatized pyrazoles in high yield.414 Similarly, electrochemical oxidation of N-acetyl-2,3-substituted A4-pyrroline-2-carboxylic acids in water-tetrahydrofuran (3 1) containing KOH forms the corresponding pyrroles (80-98%).415... 

When considering the synthesis of phospholes, one has to forget most of the classical and powerful methods employed for the preparation of thiophenes and pyrroles. For example, Paal-Knorr condensation, direct ortho-lithiation, halogenation with NBS or I2/Hg2+ and Vilsmeier-Haack formylation are not operative in phosphole chemistry. Likewise, no chemical or electrochemical oxidative polymerization... 

The electrochemical process at the modified electrode can thus be separated into two steps. First is a chemical reaction between PQQ and thiol to produce the reduced form of PQQ, PQQFE. This is followed by electrochemical oxidation of PQQFE to produce PQQ, which completes the cycle. We observed an oxidation potential of PQQEI2 much lower than the 0.5 V vs Ag/AgCl reported earlier for the detection of endo- and exogenous thiols such as cysteine and glutathione17. This difference could be due to changes in the procedure (smaller amount of pyrrole used) as well as different thiol structures. 

The electrochemical oxidation of a wide range of conjugated monomers results in the deposition of an electrically conductive film on the working electrode (Waltman and Bargon, 1986). This method has mostly been used for the polymerisation of heteroatom-containing monomers, e.g. pyrrole, thiophenef aniline, indole, etc., and polycyclic monomers, e.g. azulene, fluorene, pyrene , etc. The example of pyrrole is illustrative of the inherent advantages and problems of electrochemical polymerisation. 

Polypyrrole (PPy) has been studied in the form of thin films deposited on electrode surfaces, by electrochemical oxidative polymerization of pyrrole with anions (e.g. C104, HSO4 ) present in solution, resulting in relatively air-stable, highly conducting films. Chemical oxidation of pyrrole with Cu(II) or Fe(III) salts in solution has also been reported.58,59,60,61... 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Electrochemical oxidation is usually performed in constant current mode an optimum current density is up to 10 mA cm" (usually a higher current density is used than for, say, the oxidation of pyrrole). 

Others have shown that molecular templates, such as a polyacrylate film predeposited on a carbon electrode, can be used effectively to create PPy nanowires via electrochemical oxidation of pyrrole.190191 Electropolymerization in the presence of... 

Preparations of isopyrroles by oxidation generally give rise to 2-hydroxy-2//-pyrrole derivatives and have been carried out almost exclusively using 2,3,4,5-tetraarylpyrroles. Exceptions have been the formation of 2,2 -bis-2H-pyrroles39 and a 3f/-pyrrole.40 Oxidation has been attempted using a wide range of chemical oxidants, by photooxidation, and by electrochemical oxidation. The oxidation of monocyclic pyrroles was reviewed in 1973.3... 

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