Formation of quinones

Oxidation of phenols results in compounds called quinones. They are conjugated, and they contain two carbonyl groups (dicarbonyl). See the example below  

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Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

In addition to the above possible mechanisms the possibility of reaction at w-positions should not be excluded. For example, it has been shown by Koebner that o- and p-cresols, ostensibly difunctional, can, under certain conditions, react with formaldehyde to give insoluble and infusible resins. Furthermore, Megson has shown that 2,4,6-trimethylphenol, in which the two ortho- and the one para-positions are blocked, can condense with formaldehyde under strongly acidic conditions. It is of interest to note that Redfam produced an infusible resin from 3,4,5,-trimethylphenol under alkaline conditions. Here the two m- and the p-positions were blocked and this experimental observation provides supplementary evidence that additional functionalities are developed during reaction, for example in the formation of quinone methides. 

In order that the rate of cure of phenolic moulding compositions is sufficiently rapid to be economically attractive, curing is carried out at a temperature which leads to the formation of quinone methides and their derivatives which impart a dark colour to the resin. Thus the range of pigments available is limited to blacks, browns and relatively dark blues, greens, reds and oranges. 

An important problem encountered with polymer electrodes is that of overoxidation. It occurs after reversible charging of the electrode at high oxidation potentials and leads to polymer degeneration. The results of thorough studies show that such degenerative mechanisms are promoted by the nucleophilicity of the solvent. Especially the activity of water leads to the formation of quinone-type compounds, to the cleavage of C—C bonds, the liberation of CO2, and the formation of carboxylic acids Hence, there is a clear tendency to avoid both nucleophile solvents... 

A similar approach to the one described above was utilized for the formation of quinone methide derivatives of osmium.14 Reaction of OsCl2(PPh3)3 with a phenolic diphosphine ligand in the presence of Et3N resulted in phosphine exchange followed by C—H activation and deprotonation by the base to form the two isomeric QM... 

Metabolic Formation of Quinones by an Initial One-Electron Oxidation of BP... 

The same pathway of activation has been postulated in the formation of quinones, although the putative 6-hydroxyBP precursor has never been isolated (19,20). In this mechanism, formation of quinones would proceed by autoxidation of 6-hydroxyBP (20). However, substantial evidence indicates that the first step in formation of quinones does not involve the typical attack of the electrophilic active oxygen to yield 6-hydroxyBP, but instead consists of the loss of one electron from BP to produce the radical cation. 

The first line of evidence derives from the predominant formation of quinones when metabolism of BP is conducted under peroxi-datic conditions, namely by prostaglandin H synthase (21) or by cytochrome P-450 with cumene hydroperoxide as cofactor T22). Under these metabolic conditions one-electron oxidation is the preponderant mechanism of activation. 

Finally, we have studied the metabolism of a series of PAH with decreasing IP. In these metabolic studies with Aroclor-induced rat liver microsomes, the formation of quinones was measured in the presence of NADPH or cumene hydroperoxide as cofactor. 

As presented in Table II, no quinones are obtained with NADPH for dibenz[a,h]anthracene and benz[a]anthracene, whereas with cumene hydroperoxide a trace amount of benz[a]anthracene quinone is observed. For the PAH with low IP, quinones are formed in the presence of both cofactors. The relationship between IP and formation of quinones constitutes further evidence that these metabolites are obtained by an initial one-electron oxidation of the PAH with formation of its radical cation. 

Ionization Formation of Quinone by Aroclor-induced Rat Liver Microsomes with ... 

We propose that the first step in the formation of quinones, as shown in Scheme 3 for BP, involves an electron transfer from the hydrocarbon to the activated cytochrome P-450-iron-oxygen complex. The generate nucleophilic oxygen atom of this complex would react at C-6 of BP in which the positive charge is appreciably localized. The 6-oxy-BP radical formed would then dissociate to leave the iron of cytochrome P-450 in the normal ferric state. Autoxidation of the 6-oxy-BP radical in which the spin density is localized mainly on the oxygen, C-l, C-3 and C-12 (19,20) would produce the three BP diones. 

The oxidation of dihydroxy aromatic compounds under the conditions used by Goldschmidt ususually leads to the formation of quinones rather than diradicals. For example, >,/> -dihydroxydiphenyl gives >-diphenoquinone. Several attempts have been made to oxidize o.o -dihydroxydiphenyl, but without success. The product would be of special interest because of the possible equilibrium among diradical, quinone, and peroxide isomers ... 

Halogen substitution is expected to increase the electrophilicity of the carbenes, and in particular lh with four fluorine substituents is expected to be highly electrophilic and of unusual reactivity. All the carbenes le-g could be matrix-isolated by irradiation of their corresponding quinone diazides 2 in argon at 8-10 K.24 68,62 Again, the thermal reaction in (Vdoped matrices results in the formation of quinone oxides 7, which show the expected photochemical rearrangement to the spiro dioxiranes 8 and finally lactones 9. 

For the cracking of catechol and 3-methylcatechol in the presence of iron oxide, reaction pathways for the secondary products appeared to follow the same trend based on the assumption that the possible identities of the products we proposed above were correct. As presented in Scheme 12.1, catechol and 3-methylcatechol were oxidized to their corresponding quinones, 1,2-benzoquinone, and methylbenzoquinone, respectively. This was followed by an expulsion of CO to form cyclopentadienones and further followed by one more CO expulsion to form possibly vinyl acetylene from catechol and pentenyne from 3-methylcatechol. These products were eventually converted to the tertiary products. The formation of quinones was also observed in other studies where the oxidation of catechols was carried out. The formation of secondary products I in our study is in agreement with the previous studies (e.g., Wornet et Therefore, it is reasonable to propose the reaction pathways de-... 

To prepare paper, felled timber is transformed into pulp. The ready-made paper, however, contains some residual lignin and undergoes slow yellowing caused by the sunlight. The coloration results from the formation of quinones. This role of quinones has already been mentioned. 

The formation of B-phenolic complexes can also affect the quantity of these compounds in the tissues. As indicated by Pilbeam and Kirkby [122], the bonding of B and caffeic acid blocks the formation of quinones, and therefore facilitates the synthesis and accumulation of phenolics. 

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