Oxidative reactions of phenols

At present the only colored product that has been identified in the reaction of syringyl alcohol with aqueous alkali is 2,6-dimethoxyquinone. No evidence exists that dimeric chromophoric structures such as a dipheno-quinone or a stilbenequinone, which have been found to be products in the oxidative reactions of phenols in other studies (/, J, I5y 23), are also products of this reaction. However, with the identification of 2,6-dimethoxyquinone and the two colorless products, bis-4-hydroxy-3,5-dimethoxyphenylmethane and syringaldehyde, one of the logical pathways for the reaction is suggested and discussed. 

The oxidation reaction of phenolic compounds can be used for the purpose of detection. While the phenolic compound is oxidized, a reagent is reduced, and the reduction can be monitored by a change in color. Two common reagents are ammoniacal silver nitrate and the Folin-Denis reagent. 

Total Synthesis via Oxidation Reactions of Phenol Derivatives... 

Mild and high yielding oxidation reactions of phenol derivatives to the corresponding quinone monoacetals, quinols, and quinones using PIDA or PIFA [Eqs. (1) - (3)] have been developed independently by Kita et al. [27], Lewis et al. [28], and Pelter et al. [29], and have been utilized extensively for the syntheses of... 

PospKil J (1981) Photo-oxidation reactions of phenolic antioxidants, In Developments in polymer photochemistry-2, Allen N S (Ed), Applied Science Publishers London, pp 53-133. 

It is generally known that the processes of reversible oxidation of phenols, i.e. the conversions of phenolic systems into quinone structures and vice versa, are of great importance in biochemical reactions. The reaction partners mentioned above can serve as donors and acceptors of electrons and protons, i.e. as antioxidant systems. The conversions of phenols into cyclohexadienones are accompanied by the loss of aromaticity and in essence are not rearrangements, although the term phenol-dienone rearrangement is found in the literature. A review which summarizes in detail the oxidation reactions of phenols under conditions of halogenation, nitration and alkylation as well as radical reactions appeared . The various transformations of phenols upon oxidation with nickel peroxide were also reviewed . Therefore, only recent reports concerning the phenols-to-quinones conversions are described in this section. 

Gay-Martin M, Diez-Arevalo E, Rodriguez-Mendez ML, Saez JAD (2013) Electrochemical quartz crystal microbalance analysis of the oxidation reaction of phenols found in wines at lutetium bisphthalocyanine electrodes. Sensor Actuator B 185 24—31... 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

Solutions of Moiseev s giant Pd colloids [49,161-166] were shown to catalyze a number of reactions in the quasi homogeneous phase, namely oxidative ace-toxylation reactions [162], the oxidative carbonylation of phenol to diphenyl carbonate [166], the hydrogen-transfer reduction of multiple bonds by formic acid [387], the... 

Alkylsulfonic acids are active oxidative agents like other organic peracids. Several oxidative reactions of seodecylsulfonic peracid were studied by Safiullin et al. [41]. Peracid was found to oxidize benzene to phenol as the first intermediate product. The formed sulfonic acid accelerates the reaction. Oxidation occurs according to the stoichiometric equation... 

Assuming that ks = k9 = 3x 108Lmol 1s 1 (the key value), the diversity of the rate constants of the reactions of phenols and phenoxyl radicals (7, —7, 10, 11, and 12) can be reduced to only two parameters, k7 and T. This allows one to get the universal formulae for the oxidation rate v, into which these parameters enter as functions of k2, k7, T, and ambient conditions (Table 14.7). When considering this table, it should be taken into account that mechanism VII is possible only for 2,4,6-tris-alkylphenols, while mechanism IX holds only for o- and p-alkoxyphenols. 

Generally, an inhibitor reacts with both dioxygen and hydroperoxide. For the inhibitor to be efficient, the overall rate of these reactions must be low. The competition between reactions (11) and (12) depends on the temperature and concentrations of O2 and ROOH. At typical temperatures of oxidation (35(M150 K) and [ROOH] 0.1 [O2], the reaction of phenol with hydroperoxide is predominant. 

Rawal s group developed an intramolecular aryl Heck cyclization method to synthesize benzofurans, indoles, and benzopyrans [83], The rate of cyclization was significantly accelerated in the presence of bases, presumably because the phenolate anion formed under the reaction conditions was much more reactive as a soft nucleophile than phenol. In the presence of a catalytic amount of Herrmann s dimeric palladacyclic catalyst (101) [84], and 3 equivalents of CS2CO3 in DMA, vinyl iodide 100 was transformed into ortho and para benzofuran 102 and 103. In the mechanism proposed by Rawal, oxidative addition of phenolate 104 to Pd(0) is followed by nucleophilic attack of the ambident phenolate anion on o-palladium intermediate 105 to afford aryl-vinyl palladium species 106 after rearomatization of the presumed cyclohexadienone intermediate. Reductive elimination of palladium followed by isomerization of the exocyclic double bond furnishes 102. 

The Cu-complex-catalyzed oxidative polymerization of phenol derivatives has been selected here as a model reaction in which a polymer-metal complex acts as a catalyst. The catalytic cycle is illustrated in Scheme 3, the example used being the oxidative... 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Benzene-l,4-diols are oxidized to quinones by benzyltrimethylammonium tribromide under mild conditions in almost quantitative yields [6]. With an excess of the tribromide further reaction produces the 2-bromo-l, 4-quinones. This oxidation is in contrast to the analogous reaction of phenols, which produces bromophenols (see Section 2.3). Hindered 4-methyl-phenols are oxidized to the corresponding benzyl alcohols, benzaldehydes, bromomethyl derivatives and 4-bromo-4-methylcyclo-hexa-2,5-dien-l-ones [7]. Benzylic alcohols are oxidized under neutral or basic conditions to yield the corresponding aldehydes (>70%) oxidation with an excess of the reagent produces the benzoic acids (>90%) [8],... 

The earlier literature on oxidative coupling of phenols is reviewed in Ref. [168] and that on anodic coupling in Ref. [169, 170] some examples of the coupling reactions are summarized in Table 11, see also Chapter 6. 

Oxidation of phenols and aromatic amines using HRP is generally of little synthetic value, as oligomers and polymers are the main products (5, 260). Under certain conditions oxidative coupling of phenols or naphthols to give biaryls can be achieved, but with low selectivity (262). In contrast, HRP can catalyze a number of useful oxidative N-and 0-deaIkyIation reactions that are relatively difficult to carry out synthetically. This area has been described in detail by Meunier (263). A method for the preparation of optically active hydroperoxides using HRP C has been developed (264). Optically pure (S)-hydroperoxides... 

Oxidative coupling involves condensation reactions catalyzed by phenol oxidases. In oxidative coupling of phenol, for example, arloxy or phenolate radicals are formed by the removal of an electron and a proton from an hydroxyl group. The herbicide 2,4-D is degraded (Fig. 15.5) to 2,4 dichlorophenol, which can be oxidatively coupled by phenol oxidases (Bollag and Liu 1990). 

Phenol-induced oxidative stress mediated by thiol oxidation, antioxidant depletion, and enhanced free radical production plays a key role in the deleterious activities of certain phenols. In this mode of DNA damage, the phenol does not interact with DNA directly and the observed genotoxicity is caused by an indirect mechanism of action induced by ROS. A direct mode of phenol-induced genotoxicity involves covalent DNA adduction derived from electrophilic species of phenols produced by metabolic activation. Oxidative metabolism of phenols can generate quinone intermediates that react covalently with N-1,N of dG to form benzetheno-type adducts. Our laboratory has also recently shown that phenoxyl radicals can participate in direct radical addition reactions with C-8 of dG to form oxygen (O)-adducts. Because the metabolism of phenols can also generate C-adducts at C-8 of dG, a case can be made that phenoxyl radicals display ambident (O vs. C) electrophilicity in DNA adduction. 

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