Lead dioxide oximes

Various mononitrones, e.g. (41), and symmetrical and unsymmetrical dinitnmes, e.g. (42) and (43), respectively, have been synthesized. The dinioones could be reduced to their corresponding bis-(A(-hydroximino)-derivatives. Such sugar hydroxylamines are precursOTS free radicals, these being formed on contact with air or by oxidation with lead dioxide, and able to be observed by e.sx spectroscopy. In the case of the bis(A/-hydroximino)-derivatives, only mono-radical species were observed, any di-radicals being present in concentrations insufficient for detection. The nitrones (44) and (45) were obtained on condensation of the oximes of protected 2-acetamino-2,4-dideoxy-D JO /t>-hexose and hexuronate derivatives with 4-methoxybenzaldehyde. ... 

Condensation of the 5-methyl group in (80) (R = Me, Et, Ph, SMe) with aromatic aldehydes leads to 5-styrylthiadiazoles (79). The action of carboxylic acid esters gives ethoxalyl derivatives (81) and that of isoamyl nitrite produces the oxime (82) (Scheme 20) <82AHC(32)285>. These reactions are restricted exclusively to the 5-methyl group in (80) (R = Me), reflecting the greater reactivity of substituents in the 5-position compared to the 3-position in 1,2,4-thiadiazoles. This point is further illustrated when (80) (R = Me) is selectively converted into the carboxylic acid (83) on treatment with n-butyllithium and carbon dioxide (Scheme 20) <84CHEC-I(6)463). 

Nitrosation of mesityl oxide oxime leads to the dioxide 96, correctly formulated by Freeman.119 Earlier workers had formulated the product as a nitrimine128 and as a nitrosonitrone.118 Hypobromite oxidation of 97 leads to the same compound129 (Scheme 35), whereas the 4-bromo analog of 96 has been prepared by base elimination of HBr from a dibromopyrazoline N,N -dioxide precursor.119... 

The oxidation of cyclohexanone by nitric acid leads to the generation of nitrogen dioxide, nitric oxide, and nitrous oxide. The first two gases can be recycled for the synthesis of nitric acid. Nitrous oxide, however, is an ozone depleter and cannot be recycled. Indiscriminate nitrous oxide emission from this process is therefore the cause of considerable concern. As shown by 8.9, part of the cyclohexanone can also be converted to the corresponding oxime and then to caprolactam—the monomer for nylon 6. Phthalic acids are one of the monomers for the manufacture of polyesters. As shown by Eq. 8.10, it is made by the oxidation of p-xylene. A general description of polyamides (nylons) and polyesters are given in Section 8.4. 

Addition of anionic nucleophiles to alkenes and to heteronuclear double bond systems (C=0, C=S) also lies within the scope of this Section. Chloride and cyanide ions are effieient initiators of the polymerization and copolymerization of acrylonitrile in dipolar non-HBD solvents, as reported by Parker [6], Even some 1,3-dipolar cycloaddition reactions leading to heterocyclic compounds are often better carried out in dipolar non-HBD solvents in order to increase rates and yields [311], The rate of alkaline hydrolysis of ethyl and 4-nitrophenyl acetate in dimethyl sulfoxide/water mixtures increases with increasing dimethyl sulfoxide concentration due to the increased activity of the hydroxide ion. This is presumably caused by its reduced solvation in the dipolar non-HBD solvent [312, 313]. Dimethyl sulfoxide greatly accelerates the formation of oximes from carbonyl compounds and hydroxylamine, as shown for substituted 9-oxofluorenes [314]. Nucleophilic attack on carbon disulfide by cyanide ion is possible only in A,A-dimethylformamide [315]. The fluoride ion, dissolved as tetraalkylammo-nium fluoride in dipolar difluoromethane, even reacts with carbon dioxide to yield the fluorocarbonate ion, F-C02 [840]. 

Benzoyl Oxime Esters Few works have been devoted to the photochemistry of these photoinitiators [2f,24—26]. The cleavage of 1-phenyl 2-propanedione-2 (ethoxycarbonyl) oxime PDO occurs at the N-O bond (10.13) a further cleavage process leads to a benzoyl radical, a nitrile and carbon dioxide. The photochemistry of other keto oxime derivatives is rather complex. A suitable substitution on the phenyl ring allows to shift the absorption. 

Hydrogenation. Gold-on-titanium dioxide is a special catalyst with which nitro-alkenes are converted into saturated oximes. Thus only the sidechain is affected when o,P-dinitrostyrene is subjected to the hydrogenation conditions in its presence. Conventional hydrogenation (Pd/C, Pt/C) of the same compound leads to indole and diamine products. 

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