Quinone, from hydroquinone oxidation

In the presence of pyridine, nicotinium dichromate is also a sufficiently strong oxidant for the preparation of quinones from hydroquinones,... 

Derivatives of phenol or aniline can be oxidized to quinones, the yield and ease of oxidation depending on the substituents. If an amino or hydroxyl group is in the para position, the reaction proceeds readily, as illustrated by the synthesis of quinone from hydroquinone by oxidation with a sodium chlorate-vanadium pentoxide mixture (5>6%) or with chromic-sulfuric acid mixture (92%). A para halogen atom usually has a favorable effect. Any group in the para position is eliminated or oxidized. o-Quinones are usually prepared from the corresponding catechols. A survey of procedures for the synthesis of benzoquinones by oxidation has been made. ... 

Reaction of Aniline with Organic Functional Groups. In the absence of catalysis by enzymes or metals, aniline undergoes nucleophilic addition reactions to quinone and other carbonyl groups in humic substances to form both heterocyclic and nonheterocyclic condensation products (9). In aqueous solution, aniline undergoes 1,4-addition (Michael addition) to both 1,2- and 1,4-quinones (10-14). The reaction of aniline with 1,4-benzoquinone, from the oxidation of hydroquinone, and with 4-methyl-1,2-quinone, from the oxidation of 4-methylcatechol, are illustrated here. 

In small-scale syntheses, a wide variety of oxidants have been employed in the preparation of quinones from phenols. Of these reagents, chromic acid, ferric ion, and silver oxide show outstanding usefulness in the oxidation of hydroquinones. Thallium (ITT) triduoroacetate converts 4-halo- or 4-/ f2 -butylphenols to l,4-ben2oquinones in high yield (110). For example, 2-bromo-3-methyl-5-/-butyl-l,4-ben2oquinone [25441-20-3] (107) has been made by this route. 

This deprotection-oxidation can also be applied to the preparation of quinones from trialkylsilyl-pro-tected hydroquinones. This method has wider applicability than that reported employing PCC, since substrates with electron-releasing and those with electron-withdrawing groups are oxidized. Use of the reagent prepared in situ appears to be preferable to the preformed reagoit. 

Phenols are rather easily oxidized despite the absence of a hydrogen atom on the hydroxylbearing carbon. Among the coloured products from the oxidation of phenol by chromic acid is the dicarbonyl compound p-benzoquinone (also known as 1,4-benzoquinone or simply quinone). Dihydroxybenzenes, hydroquinone (7.30) and catechol (7.32) are oxidized to p-benzoquinone (7.31) and o-benzoquinone (7.33), respectively, by milder oxidants such as Jones reagent. Fremy s radical (7.34) is an excellent and very specific oxidizing agent for the oxidation of phenols to o- or p-benzoquinones. (m-Quinones do not exist.)... 

Recently, the use of sulfolane solvent allowed better kinetic control of the oxidation chain, with an increase of the selectivity to 80% or greater, at ca 8% benzene conversion. The by-products were catechol (7%), hydroquinone (4%), 1,4-benzo-quinone (1%) and tar (5%) [53, 54]. According to these authors, a rather stable complex, formed by hydrogen bonding with sulfolane, promoted desorption and hindered the re-adsorption of phenol, protecting it from consecutive oxidation (Equation 18.7). Actually, the rate of oxidation of phenol in the presence of sulfolane was only 1.6 times that of benzene, while it was 10 times higher in the presence of acetone. 

Electrochemical oxidation of hydroquinone diesters generally gives only the corresponding quinones [20]. However, with hydroquinone monoesters, a useful anodic trans-acylation occurs when the reaction is carried out in alcohol solvents, probably via loss of acylium ion from the oxidized phenol monoester [107]. An example of the utility of the reaction is the conversion of hydroquinone monopivalate to highly hindered f-butyl piva-late in excellent yield ... 

Fe(CN)6]3-.332 Another case to obtain quinone and hydroquinone by electrochemical oxidation of benzene with a high current efficiency, an anode modified by a bipolar ion exchange membrane, composed of a protonated poly-4-vinylpyridine layer and a perfluorocarbon sulfonic acid layer, is used. [Fe(CN)6]3 is incorporated in the inner layer (protonated poly-4-vinylpyridine) and is not contained in the solution. Release of [Fe(CN)6]3 from the inner layer is prevented by the outer layer. As a result, redox catalysts, i.e., Cr6+/Cr3+, or [Fe(CN)6]4-/ [Fe(CN)6]3 incorporated in the bipolar ion exchange membrane on the electrode surface accomplish catalysis more efficiently than if they were dissolved in the electrolyte solution.333... 

Catechols, hydroquinones, and their methyl ethers readily afford quinones on Ce oxidation.Partial demethylative oxidation is feasible, as shown in the preparation of several intramolecular quinhydrones (eq 5) and a precursor of daunomy-cinone. Sometimes the dual oxidant system of CAN-NaBrOs is useful. In a synthesis of methoxatin (eq 6) the o-quinone moiety was generated from an aryl methyl ether. 

The solvents should have a good solubility profile for both the quinone and hydroquinone, and should be stable in both the oxidizer and hydrogenator. They must also be easy to separate from H2O2, that is, have low solubility in water and H2O2 solution, and have low volatility. 

Oxidation of hydroquinone (1,4-benzenediol) produces a compound known as p-benzo-quinone. The oxidation can be brought about by mild oxidizing agents, and, overall, the oxidation amounts to the removal of a pair of electrons (2 e ) and two protons from hydroquinone. (Another way of visuahzing the oxidation is as the loss of a hydrogen molecule, H H, making it a dehydrogenation.)... 

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