Pyrroles anodic oxidation

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. 

Pyrrole derivatives substituted in positions 1-, 3-, or 4- have also been electrochemically polymerized (positions 2- and 5- must be free for polymerization). Besides homopolymers, copolymers can also be prepared in this way. Other nitrogen heterocycles that have been polymerized by anodic oxidation include carbazole, pyridazine, indole, and their various substitution derivatives. 

Anodic oxidations of heteroaromatic cycles (furans, pyrroles, benzofurans) in the presence of methanol have been extensively studied [148-165]. The electromethoxyla-tion of differently substituted furans gives 2,5-dimethoxy-2,5-dihydrofurans in moderate to good yields (Scheme 96) [148-159, 166-170]. 

In addition, anodic oxidations, at a platinum or glassy carbon electrode, of heterocycles such as pyrrole or thiophene derivatives in organic solvents (acetonitrile, propylene carbonate, dichloromethane) were widely used to... 

Electrochemistry is one of the most promising areas in the research of conducting polymers. Thus, the method of choice for preparing conducting polymers, with the exception of PA, is the anodic oxidation of suitable monomeric species such as pyrrole [3], thiophene [4], or aniline [5]. Several aspects of electrosynthesis are of relevance for electrochemists. First, there is the deposition process of the polymers at the electrode surface, which involves nucleation-and-growth steps [6]. Second, to analyze these phenomena correctly, one has to know the mechanism of electropolymerization [7, 8]. And thirdly, there is the problem of the optimization of the mechanical, electrical, and optical material properties produced by the special parameters of electropolymerization. 

Studies by Heinze etal. on donor-substituted thiophenes or pyrroles [33] such as methylthio (= methylsulfonyl) or methoxy-substituted derivatives provide further clear evidence for this reaction pathway. They found, for instance, that 3-methylthiothiophene or 3-methoxythio-phene (2) undergo a fast coupling reaction. However, deposition processes or insoluble film formation could not be detected in usual experiments with these compounds, even at high concentrations. Similarly, the corresponding 3,3 -disubstituted bithiophenes (2a) do not polymerize, but the anodic oxidation of 4,4 -disubstituted bithiophenes (2c) produces excellent yields of conducting polymers. 

Finally, the intramolecular coupling reaction between an olefin and a pyrrole ring has been examined (Scheme 40). In this example, a 66% isolated yield of the six-membered ring product was obtained. A vinyl sulfide moiety was used as the olefin participant and the nitrogen protected as the pivaloyl amide in order to minimize the competition between substrate and product oxidation. Unlike the furan cyclizations, the anodic oxidation of the pyrrole-based substrate led mainly to the desired aromatic product without the need for subsequent treatment with acid. 

The use of nitroxyls that are amenable to immobilisation in a polymer layer around the anode would be an ideal way of constraining the catalyst where it can be regenerated electrochemically. Attempts to utilise a poly(pyrrole) film formed by anodic oxidation of the monomer 5 led to a system that will oxidise alkanols but which is unstable in continuous use [40], A more satisfactory polymer layer is... 

Anodic oxidation of enamine ketones or esters in CH30H-NaC104 at a graphite anode gives substituted pyrroles in 15-45% yield.101 Formation of the symmetrically substituted pyrroles 47 indicated radical dimerization of radical-cations formed as primary products from 46. This process leads to dications from which the pyrroles can be formed by cyclization and elimination of an amine [Eq. (44)]. 

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... 

In methanolic cyanide, N-substituted pyrroles193 are substituted in the 2-position by a cyano group on anodic oxidation. Methoxylation, which is often observed as a side reaction in the anodic oxidation in methanolic cyanide, was suppressed completely. When N-substituted pyrroles carry a methyl group in the 2- and 5-positions, a side-chain cyanation occurs.193,194... 

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... 

Fig. 11.11. Evolution of the visible spectrum during anodic oxidation of 36 mM pyrrole in aqueous 1 M KN03 solution on stainless steel grid electrode at 0.7 V (SCE). Spectra were obtained at 60-s intervals. (Reprinted with permission from D. J. Fermin, M. Mostany, and B. Scharifker, Electronically Conducting Polymers Synthesis and Electrochemical Properties of Polypyrrole, Curr. Topics Electro-chem. 2 132-136,1993.)...

Although the reactivity of enaminones is not always the same as that of typical enamines due to the additional conjugative interaction with the carbonyl group, the anodic oxidation of enaminones seems useful in organic synthesis since they yield dimerized or cyclized products upon anodic oxidation. In anodic oxidation of the enaminones or enaminoesters in methanol containing sodium perchlorate, for instance, derivatives of pyrrole are formed via initial dimerization (equation 4)5. 

Recently, evidence for the transient ex istence of cation-radicals from simple pyrroles and indoles has been furnished by the observation of anodic regiospecific cyanation of these heterocycles.455 Both heterocycles are preferentially cyanated at the 2-position. Methyl side chains at these positions are also activated to cyanation and deuteration. Indole cation-radicals have been generated by photoionization in an aqueous medium.456 Unsubstituted at N, their lifetime in neutral solution is 10-6sec before they lose the N-proton however, it is longer in more acidic conditions.456 The photophysical properties of indole, its cation-radical, and neutral radical have been the subject of a recent theoretical analysis.457 On anodic oxidation of 2,3-diphenyl indole in acetonitrile, the initially formed cation-radicals dimerize to a product identified, primarily on the basis of 13C NMR, as 3-(5-indolyl)-indolenine (141).458... 

The precursor 514 has been shown to undergo anodic oxidation to produce the 5-amino-l,2-thiazolium salt 515, which rearranged to the pyrrole 516 upon treatment with triethylamine (Scheme 68) <1995JPR310>. The mechanistic aspects of this approach have also been discussed <1996J(P1)2339>. Cycloaddition of nitrones with acetylenes has been used to generate A -isoxazolines, which underwent thermal rearrangement to pyrroles fused to the isoquinoline framework <1996T12049>. 

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 direct synthesis by anodic oxidation of a new series of electrically conducting poljnners is described.. Our polymers derive from sulfur and/or nitrogen containing hetero-cycles such as 2-(2-thienyl)pyrrole, thiazole, indole, and phthalazine. The anodic oxidation of these monomers is carried out in acetonitrile solutions containing tetrabu-tylammonium salts (TBA X ) ith X = BF, tetraethylammonium salt, TEA H C-C H -S0. Characterization of the materials by electrical conductivity, electron spin resonance, uv-visible spectroscopy, and cyclic voltammetry is discussed. 

Anodic oxidation in DMF of the open-chain tetrapyrrole, 1, 8 -dimethyl-a,c-bila-diene, yielded porphyrin one methyl group is lost from a cyclic intermediate with four conjugated pyrrole subunits by attack of a nucleophile [32]. 

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). 

In suspensions of carbon black in pyrrole, anodic polymerization takes advantage of the fact that carbon black particles are negatively charged on their surface which makes it possible for them to migrate to a positively charged anode where they become embedded within a growing polypyrrole matrix This production method is suitable for production of materials for sensors, supercapacitors, fuel cells, etc. The effect of carbon black on the chemical oxidation of pyrrole in carbon black suspensions is shown in Figure 6.26. ° ... 

V-Chloropyrrole, prepared from pyrrole and aqueous sodium hypochlorite, rearranges in methanol to a mixture of 2- and 3-chloropyrroles 2,5-dichloropyrrole is also formed/ Anodic oxidation of 1,2,5-trimethylpyrrole in the presence of cyanide ions yields the pyrroline (110) as the primary... 

PPy may be prepared by chemical or electrochemical polymerization. The latter consists of the anodic oxidation of a solution comprising pyrrole and an electrolyte salt dissolved in an appropriate solvent. Because the oxidation potential of PPy is lower than that of the monomer [5], the polymer is oxidized during polymerization and counter-anions from the electrolyte... 

The polymmzatiQii of pyrrole can be carried out dedrochemically by anodic oxidation, during which process simultaneous doping occnts. Typically, electrolysis of a solution of pyrrole (0.06 M) and (Et)4N Bp4 (0.1 in acetonitrile containing 1% water leads to deposition of an insoluble blue-black film of the polymer at the anode. The film contains BF4, has a conductivity of about 10 S cm, and the composition shown below ... 

The PPy films that were doped with PFg and p-toluenesulfonate (TsO) were obtained by anodic oxidation of pyrrole at —30°C under potentiostatic conditions. The electrolytic cell contained 0.06 mol/L of pyrrole and 0.06 mol/L of the appropriate salt in propylene carbonate (PC) containing 1 vol% of water. A glassy carbon plate and a platinum foil were used as the anode. The current was adjusted to 0.125 mA/cm. The reaction took place for 24 h under a stream of nitrogen gas. For PPy(PFg) and PPy(TsO), elemental analyses determined that there is approximately one unit of dopant per three pyrrole rings [28,30,41,42]. 

H. D. Tabba, K. M. Smith, Anodic oxidation potentials of substituted pyrroles derivation and analysis of substituent partial potentials, Journal of Organic Chemistry 1984, 49, 1870. 

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