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Phenols electron transfer

Iron, Fe2+ (d6) Iron, Fe2+ (d6) 4, tetrahedral 6, octahedral N-Thiolate O-Carboxylate, alkoxide, oxide, phenolate Electron transfer, nitrogen fixation in nitrogenases, electron transfer in oxidases... [Pg.4]

A reaction mechanism for the above reactions was proposed which consists of initial formation of the copper precursor complexes of Fig. 3 (without coordinated phenolate), coordination of phenolate, electron transfer from phenolate to Cu2+ and subsequent reduction to Cu1+ with formation of a phenoxy radical, and reoxidation of Cu1+ to Cu2+ with oxygen. Various copper(II) catalysts having different stereochemistries (octahedral or tetrahedral coordination) due to coordination of amines like pyridine (Py) or acetate (OAc) groups in different ligand sites were observed by NMR and electron paramagnetic resonance techniques. [Pg.21]

However with 2,4,6-trisubstituted phenols electron-transfer occurs followed by nucleophilic attack by nitrile solvent. Alkyl group rearrangement or loss ultimately leads to benzoxazole products. [Pg.12]

Mercuration of aromatic compounds can be accomplished with mercuric salts, most often Hg(OAc)2 ° to give ArHgOAc. This is ordinary electrophilic aromatic substitution and takes place by the arenium ion mechanism (p. 675). ° Aromatic compounds can also be converted to arylthallium bis(trifluoroacetates), ArTl(OOCCF3)2, by treatment with thallium(III) trifluoroacetate in trifluoroace-tic acid. ° These arylthallium compounds can be converted to phenols, aryl iodides or fluorides (12-28), aryl cyanides (12-31), aryl nitro compounds, or aryl esters (12-30). The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. [Pg.793]

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... [Pg.1508]

Section 18.2). The latest generation of such catalysts (1 in Fig. 18.17) reproduces the key features of the site (i) the proximal imidazole ligation of the heme (ii) the trisi-midazole ligation of distal Cu (iii) the Fe-Cu separation and (iv) the distal phenol covalently attached to one of the imidazoles. As a result, binding of O2 to compound 1 in its reduced (Fe Cu ) state appears to result in rapid reduction of O2 to the level of oxides (—2 oxidation state) without the need for outer-sphere electron transfer steps [Collman et ah, 2007b]. This reactivity is analogous to that of the heme/Cu site of cytochrome c oxidase (see Section 18.2). [Pg.676]

SRNl substitution include ketone enolates,183 ester enolates,184 amide enolates,185 2,4-pentanedione dianion,186 pentadienyl and indenyl carbanions,187 phenolates,188 diethyl phosphite anion,189 phosphides,190 and thiolates.191 The reactions are frequently initiated by light, which promotes the initiating electron transfer. As for other radical chain processes, the reaction is sensitive to substances that can intercept the propagation intermediates. [Pg.1055]

Another point of importance about the film structure is the degree to which it can be permeated by various ions and molecules. It is of course essential that supporting electrolyte ions be able to penetrate the film, else the electrical double layer at the electrode/polymer interface could not be charged to potentials that drive electron transfers between the polymer and the electrode. The electroneutrality requirements of porphyrin sites as their electrical charges are changed by oxidation or reduction also could not be satisfied without electrolyte permeation. With the possible exception of the phenolic structure in Fig. 1, this level of permeability seems to be met by the ECP porphyrins. [Pg.412]

Minero, C., Mariella, G., Maurino, V., and Pelizzetti, E. (2000) Photocatalytic transformation of organic compounds in the presence of inorganic anions. 1. Hydroxyl-mediated and direct electron-transfer reactions of phenol on a titanium dioxide-fluoride system. Langmuir,... [Pg.124]

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. [Pg.142]

Reported redox potentials of laccases are lower than those of non-phenolic compounds, and therefore these enzymes cannot oxidize such substances [7]. However, it has been shown that in the presence of small molecules capable to act as electron transfer mediators, laccases are also able to oxidize non-phenolic structures [68, 69]. As part of their metabolism, WRF can produce several metabolites that play this role of laccase mediators. They include compounds such as /V-hvdi oxvacetan i I ide (NHA), /V-(4-cyanophenyl)acetohydroxamic acid (NCPA), 3-hydroxyanthranilate, syringaldehyde, 2,2 -azino-bis(3-ethylben-zothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), violuric acid, 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpipperidin-iV-oxide radical and acetovanillone, and by expanding the range of compounds that can be oxidized, their presence enhances the degradation of pollutants [3]. [Pg.142]

MnP is the most commonly widespread of the class II peroxidases [72, 73], It catalyzes a PLC -dependent oxidation of Mn2+ to Mn3+. The catalytic cycle is initiated by binding of H2O2 or an organic peroxide to the native ferric enzyme and formation of an iron-peroxide complex the Mn3+ ions finally produced after subsequent electron transfers are stabilized via chelation with organic acids like oxalate, malonate, malate, tartrate or lactate [74], The chelates of Mn3+ with carboxylic acids cause one-electron oxidation of various substrates thus, chelates and carboxylic acids can react with each other to form alkyl radicals, which after several reactions result in the production of other radicals. These final radicals are the source of autocataly tic ally produced peroxides and are used by MnP in the absence of H2O2. The versatile oxidative capacity of MnP is apparently due to the chelated Mn3+ ions, which act as diffusible redox-mediator and attacking, non-specifically, phenolic compounds such as biopolymers, milled wood, humic substances and several xenobiotics [72, 75, 76]. [Pg.143]

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... [Pg.499]

Aromatic nitrosation with nitrosonium (NO + ) cation - unlike electrophilic nitration with nitronium (NO ) cation - is restricted to very reactive (electron-rich) substrates such as phenols and anilines.241 Electrophilic nitrosation with NO+ is estimated to be about 14 orders of magnitude less effective than nitration with N02+. 242 Such an unusually low reactivity of NO+ toward aromatic donors (as compared to that of NO ) is not a result of the different electron-acceptor strengths of these cationic acceptors since their (reversible) electrochemical reduction potentials are comparable. In order to pinpoint the origin of such a reactivity difference, let us examine the nitrosation reaction in the light of the donor-acceptor association and the electron-transfer paradigm as follows. [Pg.287]

The electrochemical reduction of aryllead triacetates was smdied by Chobert and Devaud82, as a re-examination of some previous work83 to detect the role of intermediates such as [ArPb(OAc)2]V The reductions were carried out by polarography in acetic acid or acidic alcohol solutions and show three diffusion controlled waves. The first step involves a single electron transfer to produce a radical anion which dimerizes, arylates the electrode or hydrolyzes to phenol ... [Pg.679]

Phenoxyl radicals (PhO ), like the thyl radicals discussed above, are widespread reactive intermediates. The corresponding phenols (PhOH) typically have pK i values around 10, so the properties of the PhO /PhO-redox couples become highly relevant above pH 10. Standard potentials for a few of these redox couples have been determined by use of pulse radiolysis to generate the unstable phenoxyl radicals in the presence of appropriate electron donors. These conditions lead to the rapid establishment of electron-transfer equilibria as in,... [Pg.368]

An example where electron transfer from PhO- is important comes from a related publication on the reaction of phenol with O2 where [Ru(bpy)3]2+ is used as a photosensitizer (14). In acidic media the reaction involves generation of 02 by quenching of excited [Ru(bpy)3]2+ reaction of 2 with phenol leads to the production of benzoquinone. The quantum yields for benzoquinone production are highly pH dependent, showing a sharp peak at pH 8.4. This unusual pH dependence arises from the competition of several pathways, and one of the most important being the electron-transfer quenching of [ Ru(bpy)3]2+ by PhO- ... [Pg.369]

The two cyclic voltammograms shown in Fig. 13 of [Scm(LBu2)] (b) and Scln(LMe-)] (a) show an important feature. Whereas the cyclic voltammetry (CV) of the former compound displays three reversible one-electron transfer waves, the latter shows only two irreversible oxidation peaks. Thus methyl groups in the ortho- and para-positions of the phenolates are not sufficient to effectively quench side reactions of the generated phenoxyls. In contrast, two tertiary butyl groups in the ortho- and para-positions stabilize the successively formed phenoxyls, Eq. (5)... [Pg.166]

With the aim of mimicking, on a basic level, the photoinduced electron-transfer process from WOC to P680+ in the reaction center of PSII, ruthenium polypyridyl complexes were used (182-187) as photosensitizers as shown in Fig. 19. These compounds are particularly suitable since their photophysical and photochemical properties are well known. For example, the reduction potential [Rum(bpy)3]3+/-[Run(bpy)3]2+ (bpy = 2,2 -bipyridine) of 1.26 V vs NHE is sufficiently positive to affect the oxidation of phenols (tyrosine). As traps for the photochemically mobilized electron, viologens or [Co(NH3)5C1]2+ were used. [Pg.180]

Figure 21. Photoinduced electron transfer in the synthetic Ru-phenol-Mn triads. (a) from (185) and (b) from (186,187). In both cases, the Ru(II) is oxidized by photoinduced electron transfer to an extraneous electron acceptor e.g., [Co(NH3)5C1]2+ and the electron is recaptured from the tyrosyl moiety that oxidizes a Mn ion in a bimolecular reaction (left) or intramolecularly as shown on the right-hand side. Figure 21. Photoinduced electron transfer in the synthetic Ru-phenol-Mn triads. (a) from (185) and (b) from (186,187). In both cases, the Ru(II) is oxidized by photoinduced electron transfer to an extraneous electron acceptor e.g., [Co(NH3)5C1]2+ and the electron is recaptured from the tyrosyl moiety that oxidizes a Mn ion in a bimolecular reaction (left) or intramolecularly as shown on the right-hand side.
In addition, all complexes display a reversible, one-electron reduction at a very negative potential Em —1.70 to -1.90 V vs Fc+/Fc, which is metal centered and nearly invariant with respect to the substitution pattern of the coordinated pheno-lates. It demonstrates the enormous stabilization of the high-spin ferric state by three phenolato ligands. The electrochemistry also nicely shows that unprotected ortho- or para positions of these phenolates lead to irreversible electron-transfer waves on the time scale of a cyclic voltammogram and that methyl substituents are inefficient protecting groups. [Pg.184]

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... [Pg.224]

Aromatic halides react with crown ether-complexed K02 by an electron-transfer mechanism and not by nucleophilic attack, as was shown by Frimer and Rosenthal (1976) using esr spectroscopy. The corresponding phenol is the main reaction product (Yamaguchi and Van der Plas, 1977). Esters are saponified by the K02/18-crown-6 complex in benzene, presumably by an addition-elimination pathway (San Fillippo et al., 1976). The same complex has been used to cleave cr-keto-, or-hydroxy-, and or-halo-ketones, -esters, and -carboxylic acids into the corresponding carboxylic acids in synthetically useful quantities (San Fillippo et al., 1976). [Pg.358]

All the surface recombination processes, including back reaction, can be incorporated in a heavy kinetic model [22]. The predicted, and experimentally observed, effect of the back reactions is the presence of a maximum in the donor disappearance rate as a function of its concentration [22], Surface passivation with fluoride also showed a marked effect on back electron transfer processes, suppressing them by the greater distance of reactive species from the surface. The suppression of back reaction has been verified experimentally in the degradation of phenol over an illuminated Ti02/F catalyst [27]. [Pg.369]


See other pages where Phenols electron transfer is mentioned: [Pg.43]    [Pg.1199]    [Pg.57]    [Pg.382]    [Pg.364]    [Pg.114]    [Pg.227]    [Pg.321]    [Pg.328]    [Pg.592]    [Pg.645]    [Pg.408]    [Pg.71]    [Pg.444]    [Pg.14]    [Pg.159]    [Pg.1037]    [Pg.577]    [Pg.180]    [Pg.187]    [Pg.448]    [Pg.457]    [Pg.369]   
See also in sourсe #XX -- [ Pg.3 , Pg.661 ]

See also in sourсe #XX -- [ Pg.661 ]

See also in sourсe #XX -- [ Pg.3 , Pg.661 ]




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