Oxidations of aromatics

Studies of benzene and toluene oxidation in the turbulent flow reactor at Princeton University have provided valuable information on the mechanisms of oxidation at temperatures in excess of 1000 K [213]. The first extensive study of benzene and toluene at temperatures of about 750 K was made by Burgoyne [35]. Apart from CO and CO2, the major products of benzene oxidation that were detected were phenols and acids. An autocatalytic reaction was observed by Burgoyne [35] presumably driven by H2O2 formation and decomposition. Amongst the main products of toluene slow oxidation were benzyl alcohol, benzaldehyde and benzoic acid. Phenolic compounds were also reported. This reaction also showed an autocatalytic development. An equilibrium constant for the equilibrium between benzyl and benzylperoxy radicals has been measured by Fenter et al. [214], but this cannot be followed by an isomerization in the way that is possible in alkanes. 

Increases of the length of the alkyl side-chain open the possibility of alkylperoxy radical formation and isomerization, so there is then some 

Reaction (61) would now be expected to be an equilibrium. The RO2 radicals involved in these types of reactions are likely to have a considerably enhanced stability as a result of the delocalized character of the aromatic rings. The rate of oxidation of p-xylene was found, by Barnard and Ibbison [216], to be very similar to that of toluene. The slow oxidation of all of the xylene isomers and of toluene were found to be autocatalytic [36, 215], and a mechanism through formaldehyde oxidation was proposed, since it was found to be one of the intermediate products. Hydrogen peroxide must also be formed from the predominantly HO2 propagation, and this is likely to be an important contributor to the autocatalysis. 

Hexamethylbenzene has been photo-oxidized by singlet oxygen in a two-step process and each step consumes one molecule of oxygen. Step one is a [4 -I- 2]cycloaddition and this is followed by an ene-reaction to give (45)  

In MeCN the products of photo -oxidation of Ph2C=CH2, cis- and trans-PhCH=CHPh, and Ph2C=CPh2 using 9,10-dicyanoanthracene and 9-cyano-anthracene as sensitizers include benzophenone, benzaldehyde, epoxides, and products of cu-trfln5-isomerization. A correlation is established between the rate constants for electron-transfer processes and those determined from the acceptor concentration-dependence of product formation. These observations appear to implicate a sensitizer radical anion that subsequently reduces O2 to Oj- 

Oxidative photocyclization of l-(2-naphthyl)-2-(3-phenanthryl)ethylene has been carried out in a chiral liquid crystal and an optically active helicene obtained. A synthetically useful conversion of 2-()S-arylvinyl)pyrazines to azaphenanthrenes has also been described using the same oxidative procedure. The position at which ring closure occurs is dependent on the structme of the aryl group. 

Sakuragi, and K. Tokumaru, Kokagaku Toronkai Koen Yoshishu, 1979, 184. 

Hasegawa, K. Amano, C. Mori, A. Ohsawa, K. Ikeda, and T. Watanabe, Chem. Pharm. Bull., 1979, 27, 2596. 

When working in acidic solution, arenes are generally oxidized to quinones reasonable yields are obtained with symmetric hydrocarbons as starting molecules, but 

Lanthanide and Actinide Chemistry S. Cotton 2006 John Wiley Sons, Ltd. 

Working in other media, the rings can be functionalized instead of being converted into quinones. 

Ce Compounds oxidize side chains of aromatic compounds effectively and selectively, methylene carbons at the benzylic positions being oxidized to carbonyl groups. Polymethy-lated aromatics generally are oxidized to a single aldehyde group. 

If alkylbenzenes are oxidized in non-aqueous media, other products are obtained working in ethanoic acid, acetates are formed in alcohol, ethers result. 

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Phenol biosynthesis in plants proceeds from carbohydrate precursors whereas the pathway in animals involves oxidation of aromatic rings... 

Original routes involving the direct oxidation of aromatic precursors (14,15) iato quiaols (16,17) followed by a thermal transformation of the latter have been patented for the synthesis of methyLhydroquiaone [95-71-6] (10) andphenyLhydroquiaone [1079-21-6] (11) (80,81). 

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Obsolete uses of urea peroxohydrate, as a convenient source of aqueous hydrogen peroxide, include the chemical deburring of metals, as a topical disinfectant and mouth wash, and as a hairdresser s bleach. In the 1990s the compound has been studied as a laboratory oxidant in organic chemistry (99,100). It effects epoxidation, the Baeyer-Villiger reaction, oxidation of aromatic amines to nitro compounds, and the conversion of sodium and nitrogen compounds to S—O and N—O compounds. 

Nitroso compounds are formed selectively via the oxidation of a primary aromatic amine with Caro s acid [7722-86-3] (H2SO ) or Oxone (Du Pont trademark) monopersulfate compound (2KHSO KHSO K SO aniline black [13007-86-8] is obtained if the oxidation is carried out with salts of persulfiiric acid (31). Oxidation of aromatic amines to nitro compounds can be carried out with peroxytrifluoroacetic acid (32). Hydrogen peroxide with acetonitrile converts aniline in a methanol solution to azoxybenzene [495-48-7] (33), perborate in glacial acetic acid yields azobenzene [103-33-3] (34). 

Azo Coupling. The coupling reaction between an aromatic diazo compound and a coupling component is the single most important synthetic route to azo dyes. Of the total dyes manufactured, about 60% are produced by this reaction. Other methods iaclude oxidative coupling, reaction of aryUiydraziae with quiaones, and oxidation of aromatic amines. These methods, however, have limited iadustrial appHcations. 

Oxidation of Aromatic Amines. The technically important dye Direct Yellow 28 (23) [10114-47-3] (Cl 19555) for cotton usage is manufactured by oxidation of dehydrothio- i ra-toluidinesulfonic acid sodium salt with sodium hypochlorite ia aqueous alkaline solutioa. 

From N-oxides of aromatic bases oxaziridines were obtained only at very low temperatures, but oxaziridines were often postulated as intermediates in the photoconversion of such N-oxides (Section 5.08.3.1.2). Isolation of the more stable photoisomers of nitrones also causes some problems due to their thermal and photochemical instability leading to acid amides, e.g. (69TL2281), or, by fragmentation, to carbonyl compounds and products of stabilization of nitrenes, e.g. from (260) (69ZN(B)477). 

INDICATIVE OXIDATION OF AROMATIC AMINES IN PLATINUM METALS CATALIMETRY... 

Oxidation of monophenols to polyphenols or oxidation of aromatic methyl groups by persulfates (Caro s acxJ)... 

TRAHANOVSKY Ether oxidation Oxidation of aromatic ethers to carbonyl compounds with cemim ammonium nitrate... 

Attenlion should be drawn to ihe use of tin oxide systems as heterogeneous catalysts. The oldest and mosi extensively patented systems are the mixed lin-vanadium oxide catalysis for the oxidation of aromatic compounds such as benzene, toluene, xylenes and naphthalene in the. synthesis of organic acids and acid anhydride.s. More recenily mixed lin-aniimony oxides have been applied lo the selective oxidaiion and ammoxidaiion of propylene to acrolein, acrylic acid and acrylonilrile. 

Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

Iodosobenzene diacetate is used as a reagent for the preparation of glycol diacetates from olefins,9 for the oxidation of aromatic amines to corresponding azo compounds,10 for the ring acetylation of N-arylacetamides,11 for oxidation of some phenols to phenyl ethers,12 and as a coupling agent in the preparation of iodonium salts.13 Its hydrolysis to iodosobenzene constitutes the best synthesis of that compound.14... 

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

Intramolecular Friedel-Crafts acylation of diaryl ketones Oxidation of phenols or aromatic amines Oxidation of aromatic hydrocarbons... 

CgHsCl as in the absence of aromatic, suggesting auto-decomposition of the oxidant as the slow step (p. 386). The oxidation of toluene was somewhat faster, implying an additional electron-transfer pathway (c/. the oxidation of aromatic ethers and amines, p. 405). 

Hardisson C, JM Sala-Trapat, RY Stanier (1969) Pathways for the oxidation of aromatic compounds by Azotobacter. J Gen Microbiol 59 1-11. 

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