Oxidation of pyrroles

Stability. Diesel fuel can undergo unwanted oxidation reactions leading to insoluble gums and also to highly colored by-products. Discoloration is beheved to be caused by oxidation of pyrroles, phenols, and thiophenols to form quiaoid stmctures (75). Eventually, these colored bodies may increase in molecular weight to form insoluble sludge. 

The a-hydioxypyiioles, which exist piimadly in the tautomeric pyiiolin-2-one form, can be synthesized either by oxidation of pyrroles that ate unsubstituted in the a-position or by ting synthesis. P-Hydtoxypyttoles also exist primarily in the keto form but do not display the ordinary reactions of ketones because of the contributions of the polar form (25). They can be teaddy O-alkylated and -acylated (41). 

In 1968 DairOlio et al. published the first report of analogous electrosyntheses in other systems. They had observed the formation of brittle, filmlike pyrrole black on a Pt-electrode during the anodic oxidation of pyrrole in dilute sulphuric acid. Conductivity measurements carried out on the isolated solid state materials gave a value of 8 Scm . In addition, a strong ESR signal was evidence of a high number of unpaired spins. Earlier, in 1961, H. Lund had reported — in a virtually unobtainable publication — that PPy can be produced by electrochemical polymerization. 

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... 

Pyrrole was first polymerised in 1916 [239, 240] by the oxidation of pyrrole with H202 to give an amorphous, powdery product known as pyrrole black, which was... 

Pyrrole-2-carboxylic acid esters have been prepared from ethyl chloroformate and pyrrolylmagnesium bromide1 2 or pyrrolyllithium,3 by hydrolysis and decarboxylation of dimethyl pyrrole-1,2-dicarboxylate followed by re-esterification of the 2-acid4 and by oxidation of pyrrole-2-carboxaldehyde followed by esterification with diazomethane.4... 

Anodic oxidation of pyrrole and N-substituted pyrroles results in the formation of polypyrroles in an oxidized state, which can be useful for the preparation of conducting organic polymers.185-188 Oxidation of 2,5-di-substituted pyrroles produces soluble products and no layer of polymers.187 One of the proposed applications of such a layer of conducting polymer is the protection of semiconductor electrodes from photocorrosion.189-191... 

Oxidation of pyrrole with hydrogen peroxide is known to yield a mixture of 3- (219) and... 

The polymeric pyrrolic autoxidation products probably result from the oxidized monomeric systems, which are analogous in structure to those isolated from photooxidation and peroxide oxidation reactions. Thus, for example, analysis of the products of the autoxidation of 1-methylpyrrole (Scheme 47) would suggest that 1 -methyl-A3-pyrrolin-2-one (153) is initially formed from a radical reaction of the pyrrole with triplet oxygen. This reaction sequence should be compared with that proposed for the oxidation of pyrroles with hydrogen peroxide (Scheme 50), which yields (181), (182) and (183) as the major isolable products. The acid-catalyzed reaction of a pyrrole with its oxidation product e.g. 153) also results in the formation of polymeric material and the formation of pyrrole black is probably a combination of oxidation and acid-catalyzed polymerization processes. 

The oxidation of indoles and pyrroles by Fe(III) ions is less predictable than other chemical oxidations. 2-Methyl- and 3-methyl-indoles, respectively, yield (187) and (188), and whilst pyrroles may form pyrrole black , the rate of oxidation of pyrrole and of 1-methylpyrrole appears to be relatively slow. C-Alkyl and electron-donating substituents enhance the formation of oligimers, e.g. (189) -> (190) and (191) -> (192), and although electron-withdrawing substituents reduce the susceptibility of the pyrrole ring to oxidation, acyl- and alkoxycarbonyl-pyrroles of the type (193) are readily oxidized to the thermochromic dimer (194), which is in equilibrium with the dimer (195) via the monomeric pyrrolyl radical (72BCJ3584). 

Another interesting formation of 6H-1,3-oxazin-6-one derivatives by the oxidation of pyrrole derivatives was described by Sprio215 [Eq. (57)] (see also Yee et a/.216). 

Quite surprisingly, the behavior of this nucleus toward oxidizing agents has not yet been studied extensively, and the published literature is sometimes incorrect. Moreover, heterocyclic texts pay little attention to this problem. Since tars, gums, and black compounds were often formed during the oxidation of pyrroles, this reaction was commonly believed almost unapproachable this opinion does not appear justified according to recent results. 

Hydrogen peroxide has been widely used in the oxidation of pyrroles. Along with ill-defined polymeric compounds, simple oxidation products were often isolated, with the best yields in neutral or weakly basic media. 

The only products obtained by carrying out oxidation of pyrroles with the above reagents were pyrrole blacks (see Section V). 

Another ESR study118 was concerned with the pyrrole blacks obtained by anodic oxidation of pyrrole in dilute sulfuric acid.119 This substance showed a high conductivity, and an ESR signal, again due to trapped electrons and free from hyperfine structure, but with an exceptionally low g factor (2.0026 + 0.0001). 

The literature of the oxidation of pyrroles does not yet provide a clear general picture of the behavior of the pyrrole nucleus toward oxygen and other oxidizing agents. 

Although some mechanistic details are still controversial, it has been established that the oxidative polymerization (chemically or electrochemically) of pyrrole and pyrrole derivatives proceeds via an E(CE) mechanism which involves cation-radical propagating species. The most commonly accepted mechanism of polypyrrole formation is illustrated in Fig. 57 [237,242]. The polymerization begins with the one-electron oxidation of pyrrole to produce cation radical 399. This cation radical has been... 

The first chemical oxidation of pyrrole 106 (scheme 26) was achieved as early as 1916 by using hydrogen peroxide to obtain an amorphous powder known as pyrrole black [158]. The room temperature conductivity of PPy 20 prepared with acid or peroxide are in the range of 10"to 10 " S/cm, which can be inereased by halogen doping to 10 S/cm [159]. This low conductivity is due to the high degree of saturation of the pyrrole monomer units caused by defects. In the last... 

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... 

Suspensions of polypyrrole were prepared by the FeCls oxidation of pyrrole in an aqueous solution of methylcellulose (49). The product was dried and yielded films with conductivities of 0.2 S/cm. Scanning electron microscopy revealed globular polypyrrole embedded in the methylcellulose matrix. After several months, these suspensions remained stable with no detectable precipitation. Similarly, the electropolymerization of 3-methyl-thiophene in solutions of poly(methyl methacrylate) and poly(vinyl chloride) was reported (50, 51). 

Pyrrole is not polarographically reducible but can be reduced at a lead cathode in dilute sulfuric acid to pyrroline and further to pyrrolidine [203]. Under similar conditions 1,2-dimethylpyrroline [204] is also reduced to the pyrrolidine and indoles [205-207] to indo-lines or dimerized products [208]. 1-Methylindole can be reduced to 2,3-dihydro-1-methyl-indole in aqeuous THE at very negative potentials using TBAOH as electrolyte [209]. Pyrrole may also be oxidized anodically oxidation of pyrrole may result in the formation of polypyrrole useful for preparation of conducting polymers (Chapter 32). 

---