Quinone dioximes

Quinone dioximes, alkylphenol disulfides, and phenol—formaldehyde reaction products are used to cross-link halobutyl mbbers. In some cases, nonhalogenated butyl mbber can be cross-linked by these materials if there is some other source of halogen in the formulation. Alkylphenol disulfides are used in halobutyl innerliners for tires. Methylol phenol—formaldehyde resins are used for heat resistance in tire curing bladders. Bisphenols, accelerated by phosphonium salts, are used to cross-link fluorocarbon mbbers. 

Vulcanisation may also be brought about by zinc and calcium peroxides, p-quinone dioxime, epoxide resins, phenolic resins and di-isocyanates. 

Other mild oxidising agents which abstract the terminal hydrogen atoms and thus facilitate disulphide formation may be used as vulcanising agents. They include benzoyl peroxide, p-nitrosobenzene and p-quinone dioxime. 

Benzofuroxan may be obtained by oxidation of o-quinone dioxime. The first benzofuroxan derivative, 1,2-naphthofuroxan, was obtained by this method. Suitable oxidizing agents include alkaline ferri-cyanide, bromine water, chlorine, and nitric acid. The method is of practical value only when the o-quinone or its monooxime (o-nitrosophenol) is readily available, and since this is not generally the case, other routes, e.g., the oxidation of o-nitroanilines and the thermal decomposition of o-nitrophenyl azides/ are more commonly used. 

The reduction of benzofuroxans can lead to a variety of products, depending upon the conditions. Deoxygenation to benzofurazans (40) can be effected either directly, using trialkyl phosphites, -tributyl or triphenyl - phosphine, or indirectly, via o-quinone dioximes (41), using methanol and potassium hydroxide, or hydroxyl-amine and alkali. - - The dioximes may be isolated, but... 

Reduction of benzofuroxans is usually the most convenient route to benzofurazans and o-quinone dioximes (see Section VI, C). Reduction of 4-nitrobenzofuroxan would seem to be a method of synthesis of 1,2,3-triaminobenzene, while the haloalkoxy-substitution reaction (Section VTT,B) and the rearrangements of Section VIII provide compounds accessible only with difficulty by other methods. Apart from these reactions, the benzofuroxans appear to be of limited synthetic utility. 

The other common synthetic procedure for Bfx and Fx preparation is the oxidative cyclization of 1,2-dioximes. 1,2-Dioximes are excellent starting materials for the syntheses of the 1,2,5-oxadiazole N-oxide system in presence of oxidizing conditions to promote the cyclization. Its utiUty is restricted for Bfxs syntheses because the restriction of o-quinone dioximes availability, contrarily a-glyoximes, which are useful to prepare Fx, are more easily to prepare. In Table 1, products, conditions, and comments for the most recent Fx synthesis using 1,2-dioximes are shown. 

The second study was performed using either cytosolic or microsomal fractions from rat liver as the in vitro metabolic mammal models [238]. The studied compound, benzofuroxan (128, Fig. 20), is metabolized to o-quinone dioxime and 2,3-diaminophenazine (Scheme 4). 

The synthesis of polysulfide elastomers involves the use of a small amount of trichloroalkane in addition to dichloroalkane and sodium sulfide in order to form a branched polymer. The prepolymer is treated with a mixture of sodium hydrosulfide and sodium sulfite followed by acidification to convert all end-groups to thiol groups. Further polymerization and crosslinking is achieved by oxidative coupling of the thiol end-groups by treatment with lead dioxide, p-quinone dioxime, or other oxidizing agent... 

Quinone and some of its derivatives may be used in the non-sulphur vulcanisation of natural rubber. The best-known derivative is para-quinone dioxime used as a curing agent for butyl rubbers. 

The organic substrates in Chart 8 can be divided into two main categories in which (i) the oxidation of olefins, sulfides, and selenides involves oxygen atom transfer to yield epoxides, sulfoxides, and selenoxides, respectively, whereas (ii) the oxidation of hydroquinones and quinone dioximes formally involves loss of two electrons and two protons to yield quinones and dinitrosobenzenes, respectively. In order to provide a unifying mechanistic theme for the seemingly disparate transformations in Chart 8, we note that nitrogen dioxide exists in equilibrium with its dimeric forms, namely, the predominant N—N bonded dimer 02N—N02 and the minor N—O bonded isomer ONO—N02 (equation 88). 

Since the substituted hydroquinones and quinone dioximes are better electron donors than hexamethylbenzene (as established by cyclic voltammetric studies), donor-induced disproportionation (to generate NO+ NOf) is even more favored. Furthermore, either two successive one-electron oxidations of hydro-quinone (or quinone dioxime) by NO + followed by the loss of two protons from the dication or two sequential oxidation/deprotonation steps complete the oxidative transformation in equation (97). Importantly, the ready aerial oxidation of NO to NO provides the basis for the nitrogen oxide catalysis of hydroquinone (or quinone dioxime) autoxidation as summarized in Scheme 26. 

Benzofurazans can be obtained by several methods (1) by dehydration of o-quinone dioxime (2) from o-substituted nitrosoarenes and (3) deoxygenation of benzofuroxans <1984CHEC(6)393, 1996CHEC-II(4)229>. For example, trihydroxyimino derivative 280 treated with sodium hypobromite, or when boiled in aqueous ammonia, affords the corresponding substituted tetrahydrobenzofurazan 281 (Equation 54) <2000CHE996>. 

The most synthetically useful methods for benzofuroxans are (1) oxidation of o-quinone dioximes (2) decomposition of o-nitroaryl azides and (3) oxidation of o-nitroanilines. Benzofuroxans can also be formed as a result of Boulton-Katritzky rearrangement (see Section 5.05.5.2.1). 

Vulcanization The treatment of natural rubber with sulfur to reduce its tackiness and improve its strength and elasticity. Invented independently by C. Goodyear and N. Hayward in the United States in 1839, and by T. Hancock in London in 1842-1843. Various chemicals other than elemental sulfur are effective, for example, sulfur monochloride, selenium, and p-quinone dioxime. 

The nitrosation of oximes can also serve to produce N-hydroxy-N-nitrosamines (Scheme 3.9). The acidified nitrite procedure can be applied to oximes derived from terpenes [132] and hydroxyguanidine [133] for the synthesis of N-hydroxy-N-nitrosamines. However, some a, 5-uri saturated oximes were converted into pyrazole-1,2-dioxides [134]. The alkyl nitrites under basic conditions have produced interesting results. Quinone dioximes yielded only monodiazeniumdiolates [135], while simple aliphatic oximes gave products resulting from addition to the imine double bond... 

The principal approaches to benzofurazans comprise the dehydration of o-quinone dioximes, the cyclization of o-disubstituted arenes, and the deoxygenation of benzofuroxans (Scheme 21). As for the monocyclic furazans these are long-established routes and have been covered in previous reviews <84CHEC-I(6)393, 84H(22)1571>. 

The principal methods for forming the heterocyclic ring of benzofuroxans involve oxidation of o-quinone dioximes, thermolysis of o-nitroaryl azides, and oxidation of o-nitroanilines (Scheme 25). Ring chain tautomerism (Section 4.05.5.2.1) for the A -oxides of asymmetrically substituted benzofuroxans is more facile than for monocyclic analogues and mixtures of isomers may result. Benzofuroxans are also formed by Boulton-Katritzky rearrangement of 7-nitro-2,l-benzisoxazoles and 4(7)-nitrobenzofuroxans (Section 4.05.5.2.5). 

Benzofuroxans can be prepared by oxidation of o-quinone dioximes with, for example, fer-ricyanide or hypohalite in a process which closely parallels the formation of monocychc furoxans from glyoximes. Its utility is restricted by the availability of the starting materials which are themselves often best made by reduction of the furoxan. However, it is a valuable approach when the parent quinone or its monooxime is accessible by other means. It was, for example, the route originally used for naphtho[l,2-c]furoxan, the first aromatic-fused derivative <1886CB176>, and it is the method of choice for acenaphthofuroxans (11). In other cases oxidation of o-nitroanilines or thermolysis of o-nitroaryl azides are more suitable. 

Chemistry of Polysulfide Polymers. Propellant chemistry based on chemically crosslinked binders had its beginning at the Jet Propulsion Laboratory in the winter of 1946 when potassium and/or ammonium perchlorate were mixed into Thiokol LP-3 polysulfide liquid polymer, to which had been added an oxidative curative, p-quinone dioxime. This polysulfide polymer, as described by Jorczak and Fettes (13), is prepared... 

There is limited evidence in experimental animals for the carcinogenicity of 1,4-benzo-quinone dioxime. 

D. Preparation of Quinoxaline jV-Oxides from Benzofurazan 1-Oxides and o-Quinone Dioximes... 

A further variation on this general method for preparing quinoxaline dioxides is the use of o-quinone dioximes (54) rather than benzofurazan 1-oxides. The dioxime undergoes cycloaddition with cr-dicarbonyl and a-hydroxycarbonyl compounds, and hydroxamic acids of type 55 are particularly easily prepared by this method.56... 

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