Aromatic rings electrochemical oxidations

Trisarylamines Trisarylamines have been successfully used as redox catalysts in many indirect electrochemical oxidations. Their advantage is the possibility to adjust their oxidation potential by selection of the substituents on the aromatic rings. 

The electrochemical properties of another series of Cu(I) complexes, based on substituted bipyridine and quinoline derivatives, have been also investigated68 (Fig. 17.31). To stabilize the Cu(I) oxidation state of Cu(I) polypyridine complexes, electron-withdrawing substitutents like esters have been considered. The same effect was also obtained with pyridyl-quinoline and biquinoline complexes, thanks to the increased 7i-accepting properties of the quinoline condensed aromatic ring. 

All molecules with nonbonding electron pairs (e.g., H20, / OH, ROR, / NH2, / SH, RSR) are, by definition, Lewis bases with a degree of nucleophilicity. Their electrochemical oxidation potential is a measure of (1) the ease of removal for one of the electron pair of electrons and (2) relative nucleophilicity (the less positive the potential, the more nucleophilic). Aromatic molecules with Lewis base substituents are easier to oxidize than the aliphatic forms of the substituents (e.g., PhOMe, +1.75 V vs. SCE MeOH, +2.5 V vs. SCE) because the aromatic ring provides a means for delocalizing the positive chaige and electron spin that would result from electron removal (in the case of PhOMe, there are five additional hydrogen atoms to share the positive chaige and six... 

According to this model, the first stage in the treatment of nitrophenols aqueous wastes was the release of the nitro group from the aromatic ring. As a consequence, phenols or quinones were formed. These organic compounds were oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. Also the cathodic reaction steps were considered in the global process when the electrochemical cell was undivided at the cathode, the reduction of the nitro to the amine group and the transformation of nitrate into ammonia were observed. In alkaline media, aminophe-nols were polymerised and transformed into a dark brown solid. 

Amidines 1170 substituted with electron-rich aromatic rings undergo an oxidative intramolecular cyclization process to give A-substituted benzimidazoles 1172. The reaction proceeds well in MeCN with CAN or electrochemical oxidation and probably involves the cationic intermediate 1171 (Scheme 285) <1996JOC3902>. 

Pyrrole offers two positions that can be substituted to obtain soluble polymers, the N- and the 3-/or 4-positions on the aromatic ring. In general, N-substitution lowers the conductivity caused by the loss of conjugation due to steric interactions, which leads to a torsion of adjacent pyrrole units the advantage of the N-substitution is the improved environmental stability and the more straightforward structural characterization [155]. For the preparation of N-substituted and ring-substituted PPys oxidative chemical and electrochemical... 

The base unit of polypyrrole is pyrrole, the structure of which is a five member hetero-aromatic ring containing a nitrogen atom, as shown in Fig. 13a. Polypyrrole is synthesized from the pyrrole monomer by mild oxidation, using chemical or electrochemical technique. After the oxidation of the monomer, a black solid polymer is precipitated from the solution. The polypyrrole structure in its oxidized form is shown in Fig. 13b. Film thicknesses on the order of 1-1.5 pm, using in situ deposition, were obtained for our application [19]. 

Jurva et al. performed comprehensive studies on the comparability of EC-MS and cytochrome P 450 catalyzed reactions [48,49] and found that EC-MS successfully mimicked those CYP reactions that are initiated by a one-electron oxidation, such as /V-dealkylation, 5-oxidation, / -oxidation, alcohol oxidation, and dehydrogenation, whereas the reactions that are initiated via direct hydrogen atom abstraction, for example, O-dealkylation and hydroxylation of unsubstituted aromatic rings, were not mimicked. In the latter cases the oxidation potentials were too high for electrochemical oxidation in aqueous solutions. A further disadvantage is that EC lacks the stereospecifity of the reactions, in contrast to CYP catalyzed reactions [50],... 

Cuadrado and co-workers have reported the synthesis of chromium-containing organosilicon dendrimers.334 These dendrimers, 282, have chromium tricarbonyl units incorporated pendent to the terminal aromatic rings.334 Synthesized via the reaction of the silane dendrimer precursor with chromium hexacarbonyl, complete complexation was not possible due to steric hindrance at higher generations. Electrochemical studies showed that the oxidation of the chromium atoms occurred reversibly in the absence of a nucleophilic species and that the chromium tricarbonyl units behaved as isolated redox centers. 

The improved electrochemical synthesis (7) of poly pyrrole has led to its use as coating for the protection of n-type semiconductors against photocorrosion in photoelectrochemical cells. (8,9) Recently, it was announced that pyrrole was not the only five-membered heterocyclic aromatic ring compound to undergo simultaneous oxidation and polymerization. Thiophene, furan, indole, and azulene all undergo electrochemical polymerization and oxidation to yield oxidized polymers of varying conductivities (5 x 10 3 to 102 cm- ). (10-13) The purpose... 

The electrochemical oxidation of 2-furyl-2-thienylmethane in methanol resulted in loss of aromaticity of the furan ring and gave 70 (R = 2-thienyl). The reaction took a different course with 2,2 -dithienylmethane, which oxidized at the methylene group to give methoxybis(2-thienyl)methane (71) and di-2-thienyl ketone.106... 

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