Phenol reduction potentials

MbFe(IV)=0 and with lipid peroxyl radicals (Castellucio et al, 1995). It may accordingly be concluded that the most relevant single parameter for predicting the antioxidative activity of a new plant phenol would be the standard reduction potential, E . 

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. 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

One-Electron Reduction Potentials (E°) of the Phenoxyl-Phenolate and Phenoxyl-Phenol Couples... 

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. 

The H64Y variant of Mb is an example of the former situation in that the tyrosyl side chain coordinates to the heme iron of the oxidized variant. As expected for a variant with an anionic phenolate ligand, the reduction potential of this variant is 40 mV lower than that of the wild-type protein (Table I). Although this change is consistent with stabilization of the oxidized form of the protein, the fact that the tyrosyl ligand is not coordinated in the reduced protein complicates quantitative interpretation of this shift in potential. 

Another facet of the reactivity of aminoxyl radicals has been brought to attention by recent studies of the oxidation of substrates endowed with low redox potential. Some aminoxyl radicals, depending on the reduction potential value of the couple >N—O / >N—O, can behave as moderate one-electron abstractor towards substrates endowed with appropriate oxidation potential. This rather unprecedented reactivity feature, outlined in Scheme 10, has been substantiated by the oxidation of aniline or phenol derivatives, whose redox potential is conveniently located in the 0.4-1.0 V/NHE range ". 

Swain and Goldstein (6, 7) noted a rather large difference in the molar color yield from different phenols with the Folin-Denis reagent. They attributed this to differences in relative oxidation-reduction potentials of the different phenols, but under their conditions pyrogallol gave about half the color of catechol and more than resorcinol. However, they also reported that the molar absorptivity produced by a flavonoid was approximately equal to the sum of the values for the separate phenolic moieties which it contained. 

The situation with pendant phenol groups is much less ambiguous. The crystal structure of the nickel(II) complex [Ni H 1[14]ane-N4C6H5OH]+ (Fig. 5) shows it to be a square-pyramidal, high-spin species and the reduction potential of the nickel(III) complex is 0.15 V lower than that of the parent saturated 14-membered macrocyclic complex (100). Strong coordination by phenolate oxygen is even more... 

Although an ET from phenolates is highly exothermic (for reduction potentials Lind et al. 1990 Jonsson et al. 1993) and ET is thermodynamically favored over addition (Lundqvist and Eriksson 2000), the usually preferred mode of reaction is addition rather than ET. Yet, addition and ET are in competition (Tripathi 1998), and, when the ortho- and the para-positions which are the relevant positions of addition for the electrophilic OH are blocked by a bulky substituent [e.g., reaction (34)] ET may become dominant (Table 3.3). Thus, also for these reactions a short-lived tx-complex [cf. reaction (6)] may be postulated as common precursor wherefrom the competition between addition and ET occurs. 

Land EJ, Ebert M (1967) Pulse radiolysis studies of aqueous phenol. Water elimination from dihy-droxycyclohexadienyl radicals to form phenoxyl. Trans Faraday Soc 63 1181-1190 Lind J, Shen X, Eriksen TE, Merenyi G (1990) The one-electron reduction potential of 4-substituted phenoxyl radicals in water. J Am Chem Soc 112 479-482 Loft S, Poulsen HE (1999) Markers of oxidative damage to DNA antioxidants and molecular damage. Methods Enzymol 300 166-184... 

Eugenol, like other phenolic compounds, is a structurally non-specific drug. The pharmacological action is not directly subordinated to chemical structure, except to the extent that structure affects physicochemical properties, as adsorption, solubility, pKa, and oxidation-reduction potential, factors which influence permeability, depolarization of the membrane and protein coagulation [34],... 

In both cases, the reduction potential is directly related to the Br0nsted acidity, which depends on the basicity of the solvent [e.g, phenol (PhOH) has a pjfa of 16 in MeCN and 21 in Me2SO see Chapter 8]. 

Br0nsted Acids. Carboxylic acids, phenols, and alcohols are electrochemi-cally reduced by means of their Br0nsted acidity at a reduction potential that is a direct measure of their acidity (p/fj in a given solvent (see Chapter 8) ... 

A practical access to 2-biphenyl alcohols 95 has been discovered by Petrillo (Scheme 36) [152]. Azo sulfides 96, which are employed as masked diazonium salts, do not undergo azo coupling reactions. In combination with 4-substituted phenolates 97, photochemically initiated arylations can even be conducted as chain processes according to the SrnI mechanism. Given their high reductive potential, the cyclohexadienyl intermediates 98 are able to rearomatize by a single electron... 

Phenol Hydrogenation. In principle, appropriate lignin deconstruction processes will provide a stream of mixed phenols. Reduction of these phenols will lead to a new source of cyclic aliphatic alcohols of potential use in the manufacture of adipic acid derivatives. Several catalytic processes for these types of reductions have appeared for phenol and should be applicable to lignin-derived mixed phenols. Phenol itself is reduced to cyclohexanol in the presence of various heterogeneous catalysts based on Pd.530-535... 

According to the Chance mechanism, the interaction of H202 with the enzyme gives compound I (E,). The oxidation of the donor molecules leads to compound II (E[I) which oxidizes the second donor molecule. The radical intermediates were detected experimentally for such substrates as amines and phenols with relatively high reduction potential (Dunford and Stillman, 1976). The one-electron steps with the formation of free radicals at oxidation of amines and phenols have been proved in the ceruloplasmin, laccase and ascorbic oxidase reactions (Malsmstrom et al., 1975). 

Reaction between phenol and hydroxyl yields the dihydroxybenzenes, which can then undergo further oxidation (hydroquinone to benzoquinone, further hydroxylated to hydroxybenzoquinone, catechol and resorcinol to trihydroxybenzenes [79,100]). The condensation products, phenoxyphenols and dihydroxybiphenyls, most likely originate from the reaction between phenol and the phenoxyl radical [101]. Their presence indicates that some phenoxyl forms in the system, due to the reaction of phenol with OH or NO2. The possibility for NO2 to oxidise phenol to phenoxyl has been the object of a literature debate [102,103] in the context of nitration processes. The problem can be tackled upon consideration of the reduction potentials of the various species. The reduction potential of phenoxyl to undissociated phenol is E = 1.34 V - 0.059 pH [104], while for the reduction of nitrogen dioxide to nitrite it is E = 0.90 V [105]. Accordingly oxidation of phenol to phenoxyl would be possible above pH 7.5, and of course in the presence of phenolate (pH > 10 [106]). 

The fine tuning of reduction potential was useful for optimising the cleavage of arenesulfonamides by electrolysis292 in acetonitrile using a mercury pool cathode and platinum anode, with phenol as the proton source and 0.1 M tetraethyl-ammonium bromide as anolyte.29 294... 

The antioxidants studied can be classified into two broad types phenolic antioxidants and non-phenolic antioxidants. Phenolic antioxidants have been found to be more promising as they are obtained from dietary sources.Vitamin E (a-tocopherol), the first known chainbreaking antioxidant, is also an o-methoxy phenol. Pulse radiolysis studies of vitamin E and its water-soluble analogue, trolox C, have been reported several years ago. a-tocopherol reacts with almost all the oxidizing free radicals, and the phenoxyl radicals produced during oxidation reactions absorb at -460 nm (Fig. 1). The regeneration reaction of a-tocopherol phenoxyl radicals back to a-tocopherol by water-soluble antioxidant ascorbic acid was also first reported by pulse radiolysis method. The one-electron reduction potential of vitamin E is -0.48 V vs. NHE. Both a-tocopherol and trolox C are used as standards for evaluating the antioxidant ability of new compounds. 

Porphyrins are often employed in sensors on account of their ability to act as cation hosts and, with a suitable metal ion coordinated, as redox catalysts. Electropolymerised poly(metalloporphyrin)s have been used as potentiometric anion-selective electrodes [131] and as amperometric electrocatalytic sensors for many species including phenols [132], nitrous oxide [133] and oxygen [134]. Panasyuk et al. [135] have electropolymerised [nickel-(protoporphyrin IX)dimethylester] (Fig. 18.10) on glassy carbon in the presence of nitrobenzene in an attempt to prepare a nitrobenzene-selective amperometric sensor. Following extraction of the nitrobenzene the electrode was exposed to different species and cyclic voltammetric measurements made. A response was observed at the reduction potential of nitrobenzene (the polyporphyrin film acts only to accumulate the analyte and not in a catalytic fashion). Selectivity for nitrobenzene compared with w-nitroaniline and o-nitroto-luene was enhanced compared with an untreated electrode, while a glassy carbon-... 

For the measurement a moderate reduction potential between — 100 and + 100 mV vs. Ag/AgCl is appUed (Fig. 2.12). In this region the potential for electrochemical interferences is very low. However, the biggest problems arise from the high reactivity of compormds I and II with reducing substrates (electron donors), which compete with the electrode for the reduction of peroxidase. Ascorbic acid, naturally occurring phenolics and aromatic amines are among those compounds. The competitive reaction of reductants should be... 

The electrochemical behavior of fuchsone (1) has been studied [62] in some detail. An unusual pathway is followed in the presence of acids (in this case phenols). The first-formed radical anion is protonated reversibly at oxygen and the triphenylmethyl radical so formed is not further reduced at the first reduction potential (Scheme 21). However,... 

While the above mechanism certainly operates in many systems, it is not universal. If each radical is surrounded by unoxidized phenol or phenolate anion molecules, then aromatic substitution becomes possible, and this becomes more likely in intramolecular cases where two units of different reduction potential are held in proximity. Scheme 2 shows the likely pathway with radical insertion into a phenox-ide and the second one-electron transfer following coupling, and such a mechanism has been shown to operate in the oxidation of 2,3,4 -trihydroxybenzophenone to 2,6-dihydroxyxanthone (ref. 20, p. 114). 

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