Oxidation of quinones

As the hydroxyl radical (OH ) is a non-selective oxidising agent, it is normally converted into some secondary radical by using a suitable chemical. 

Mukheijee et al [72] have shown that the decay of the one-electron oxidized naphthazarin (NzH and Nz ) proceeds by a complicated mechanism not fully understood in spite of extensive studies. At low dose condition, there was no visible change in the concentration of naphthazarin. The pK of the semi-oxidised naphthazarin was shown as 4. The most interesting observation was the evidence for the disproportionation (40) proceeding to an equilibrium level in favour of the semi-oxidised quinone, accompanied by reaction (41). The possibility of dimerisation of the one-electron oxidised species was not fully explored. 

Contrary to the naphthazarin case, dihydroxyanthraquinones (QH2) form semi-oxidised quinones which undergo simple bimolecular disproportionation to the diquinone and parent quinone [74]. Both the above reactions have been wrongly given due to printers devil type error m a recent review [12]. The pK for the semi-oxidised anthraquinone derivatives was measured and shown to be around 8. 

The third aromatic ring in the anthraquinone derivatives possibly renders all secondary reactions very slow in comparison with the timescale for radical decay. Hydroxyl radicals react wi OH-substituted quinones or other hydroquinones mainly by addition, followed by acid or base catalysed water elimination [55,72]. 

Comparison of spectroscopic characteristics and pK of one-electron oxidised quinones in 

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In the reduction or oxidation of quinone/ quinol systems, free radicals also appear as intermediate steps, but these are less reactive than flavin radicals. Vitamin E, another qui-none-type redox system (see p.l04), even functions as a radical scavenger, by delocalizing unpaired electrons so effectively that they can no longer react with other molecules. 

Wurster in 1879 had already prepared crystalline salts containing radical cation 23 (equation 12). Subsequently, radical cations of many different structural types have been found, especially by E. Weitz and S. Hunig, and recently these include a cyclophane structure 24 containing two radical cations (Figure 3). Leonor Michaelis made extensive studies of oxidations in biological systems, " and reported in 1931 the formation of the radical cation species 25, which he designated as a semiquinone. Michaelis also studied the oxidation of quinones, and demonstrated the formation of semiquinone radical anions such as 26 (equation 13). Dimroth established quantitative linear free energy correlations of the effects of oxidants on the rates of formation of these species. ... 

Arylation of quinones, Oxidation of quinones with Pd(OAc)2 in HOAc in the presence of an arene results in coupling to form arylated quinones in moderate to high yield.4... 

Benzylquininium chloride has shown good to excellent selectivity in the epoxidation of a,p-unsaturated ketones. Oxidation of quinone (2) in the presence of (1) with aqueous t-Butyl Hydroperoxide and Sodium Hydroxide in toluene gave rise to a 95% chemical yield of epoxide (3) in 78% ee (eq 1). Recrystallization improved the ee to 100% with 63% mass recovery. Aqueous Hydrogen Peroxide decreased both the yield (89%) and enantioselectivity (50% ee). 

Proton-coupled electron transfer is a prominent theme in biological redox systems. There are three basic mechanisms for these processes (Figure 18). In the first mechanism (path A), electron transfer occurs prior to proton transfer. This mechanism is commonly observed for the electrochemical reduction and oxidation of quinones and flavins in protic media [52], In this interfacial environment, proton transfer is manifested as an ECE (E represents an electron transfer at the electrode surface and C represents a homogeneous chemical reaction) two-electron reduction of these systems to their fully reduced states (Figure 19). As electron transfer occurs prior to the proton transfer event, proton transfer does not affect either the redox potential or the electron transfer rate to or from the cofactor. 

G Gellerstedt, HL HardeU, and EL Lindfors. The Reactions of Lignin with Alkaline Hydrogen Peroxide. Part fV. Products from the Oxidation of Quinone Model Compounds. Acta Chem. Scand. B 34 669-673, 1980. 

G Gellerstedt, H-L Hardell, and E-L Lindfors. The reactions of lignin with alkaline hydrogen peroxide. Part IV. Produets from he oxidation of quinone model compounds. Acta. Chem. Scand. B 34 669-673, 1980. 

Indeed, it has been found 70 that under the conditions of the experiments, the proportion of quinone to maleic acid remains fixed and is not materially altered by introduction of quinone with benzene. However, the proportion of quinone is never large in tire case where solid catalysts are used. The mechanism of the further oxidation of quinone to maleic anhydride is somewhat speculative since the isolation of any of the intermediate compounds in this step has not been reported in the vapor phase oxidation experiments. However, the formation of a poly-ketone by the following reactions seems a possibility since it may be assumed that a continuation of the hydroxylation process followed by the molecular rearrangement, would be expected. 

The first mechanism is often found in the electrochemical reduction and oxidation of quinones and flavins in aprotic media.Using cyclic voltammetry (CV) and simultaneous electrochemistry and electron paramagnetic resonance (SEEPR),... 

The following oxidation of camphor to camphor-quinone illustrates the oxidising action of selenium dioxide, and readily gives a crystalline product. 

Benzoquinone ( quinone ) is obtained as the end product of the oxidation of aniline by acid dichromate solution. Industrially, the crude product is reduced with sulphur dioxide to hydroquinone, and the latter is oxidised either with dichromate mixture or in very dilute sulphuric acid solution with sodium chlorate in the presence of a little vanadium pentoxide as catalyst. For the preparation in the laboratory, it is best to oxidise the inexpensive hydroquinone with chromic acid or with sodium chlorate in the presence of vanadium pent-oxide. Naphthalene may be converted into 1 4-naphthoquinone by oxidation with chromic acid. 

When a solution of, say, 1 g. of hydroquinone in 4 ml. of rectified spirit is poured into a solution of 1 g. of quinone in 30 ml. of water, qulnhydrone C,HA.C,H (0H)3, a complex of equimolecular amounts of the two components, is formed as dark green crystals having a gfistening metallic lustre, m.p. 172°. In solution, it is largely dissociated into quinone and hydroquinone. Quinhydrone is more conveniently prepared by the partial oxidation of hydroquinone with a solution of iron alum. 

In addition to CuCfi, some other compounds such as Cu(OAc)2, Cu(N03)2-FeCl.i, dichromate, HNO3, potassium peroxodisulfate, and Mn02 are used as oxidants of Pd(0). Also heteropoly acid salts comtaining P, Mo, V, Si, and Ge are used with PdS04 as the redox system[2]. Organic oxidants such as benzo-quinone (BQ), hydrogen peroxide and some organic peroxides are used for oxidation. Alkyl nitrites are unique oxidants which are used in some industrial... 

Unexpectedly, a completely different reaction took place in the oxidation of 2-(l-propenyl)phenol (111) with PdCh. Carpanone (112) was obtained in one step in 62% crude yield. This remarkable reaction is explained by the formation of o-quinone, followed by the radical coupling of the side-chain. Then the intramolecular cycloaddition takes place to form carpanone[131]. 

Quaternary structure (Section 27 22) Description of the way in which two or more protein chains not connected by chemical bonds are organized in a larger protein Quinone (Section 24 14) The product of oxidation of an ortho or para dihydroxybenzene denvative Examples of quinones include... 

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

Oxidation of acetaminophen yields a reactive quinone intermediate. 

Synthesis by oxidation remains the first choice for commercial and laboratory preparation of quinones the starting material (1) provided the generic name quinone. This simple, descriptive nomenclature has been abandoned by Chemicaly hstracts, but remains widely used (2). The systematic name for (2) is 2,5-cyclohexadiene-l,4-dione. Several examples of quinone synonyms are given in Table 1. Common names are used in this article. 1,2-Benzoquinone (3,5-cydohexadiene-l,2-dione) (3) is also prepared by oxidation, often with freshly prepared silver oxide (3). Compounds related to (3) must be prepared using mild conditions because of their great sensitivity to both electrophiles and nucleophiles (4,5). 

Excellent evidence of the gende nature of quinones as oxidants in the presence of the thiophene ring, eg (39), has been found (26). 

Recognition of the thio group s key role in biochemistry has led to studies of l,4-ben2oquinone with glutathione, a tripeptide 7-Glu-Cys-Gly (GSH). The cross-oxidation of the initial addition product by excess quinone leads, under physiological conditions, to all three isomeric products (46), ie, the 2,3-and 2,6-isomers as well as the 2,5-disubstituted l,4-ben2oquinone shown. 

The importance of quinones with unsaturated side chains in respiratory, photosynthetic, blood-clotting, and oxidative phosphorylation processes has stimulated much research in synthetic methods. The important alkyl- or polyisoprenyltin reagents, eg, (71) or (72), illustrate significant conversions of 2,3-dimethoxy-5-methyl-l,4-ben2oquinone [605-94-7] (73) to 75% (74) [727-81-1] and 94% (75) [4370-61-0] (71—73). 

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. 

The oxidation of 4-bromophenols to quinones can also be accompHshed using periodic acid (113). A detailed study of this reagent with stericaHy hindered phenols provided insight about the quinonoid product (114). The highest yield of 2,6-di-/-butyl-l,4-ben2oquinone [719-22-2] is for the case of R = OCH. The stilbene stmcture [2411-18-9] is obtained in highest yield for R = H. 

In the case of l,4-ben2oquinone, the product is steam-distilled, chilled, and obtained in high yield and purity. Direct oxidation of the appropriate unoxygenated hydrocarbon has been described for a large number of ring systems, but is generally utilized only for the polynuclear quinones without side chains. A representative sample of quinone uses is given in Table 5. 

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