Aromatic compounds side-chain oxidation

A principally different approach for the indirect electrochemical oxidation of aromatic compounds goes via the formation of hydroxyl radicals from cathodically generated hydrogen peroxide and from reductively formed iron(II) ions. The thus in situ formed Fenton reagent can lead to side-chain as well as nuclear oxidations of aromatic compounds. Side-chain oxidations to form benzaldehydes according to Eqs. (18)—(24) can also be initiated by the redox pairs and Cu instead of... 

Scheme 12.22. Side Chain Oxidation of Aromatic Compounds... 

Scheme 38 Side-chain oxidation of aromatic compounds mediated by Fe(ll).

Side-chain oxidations of alkyl aromatic compounds to aromatic carboxylic acids by electrogenerated and regenerated chromic acid have been studied extensively in the case of saccharin formation from o-toluene sulfonamide This... 

Two-electron redox phenomena are rare for Cu, and therefore Cu is not a suitable catalyst for epoxidations, for example. However, Cu is useful for free radical reactions such as deep oxidation of organic molecules in waste streams and ring or side chain oxidation of aromatic compounds. 

Attack other than on the amino group apparently does represent a competitive reaction course when the substituents on the aromatic ring are less electronegative than the nitro group. This is demonstrated by the strong coloration and tar formation observed when N,IV-dimethyl-aniline and p-chloro-N,A7-dimethylaniline are ozonized and by the fact that more ozone is consumed by these compounds than can be accounted for by side chain oxidation (Table III, Experiments 1 and 2). 

This section will encompass the reactions of carbocyclic and heterocyclic aromatic compounds in which oxidation affects the aromatic rings and the attached side chains. 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

In many cases, peroxygen technology can be used to avoid the use of the transition metal altogether. Transition metal oxidants are traditionally used for alcohol or aldehyde oxidation and side chain oxidation of aromatic compounds such as toluenes. 

Oxidation of alkyl benzene using heteropolycompounds are effective in presence of oxidants like hydrogen peroxide and t-butylhydroperoxide. Vanadium substituted heteropolymolybdates are more effective than unsubstituted heteropoly compounds. Both side chain and products with oxidation in the aromatic ring are observed in presence of heteropoly compound-hydrogen peroxide system whereas only side chain oxidized products were observed in presence of heteropolycompound-t-butyl hydroperoxide system. This difference in activity can be due to the formation of different active intermediate species. 

Hydroxylation of benzene to phenol using hydrogen peroxide in the presence of heteropoly compounds was observed in this study. Other aromatic hydrocarbons that were used as substrates were toluene, ethylbenzene, o,p-xylene and isopropyl benzene under homogeneous conditions. Both side chain oxidation and ring hydroxylation were observed in presence of hydrogen peroxide. For example, toluene gave benzyl alcohol, benzaldehyde, o,p-cresols in presence of hydrogen peroxide, whereas benzaldehyde and benzyl alcohol were observed in presence of t-BuOOH. 

In certain cases side chains of aromatic compounds can be oxidized in an oxidizing alkali melt — the substance is heated at 200-300° with 3-4 parts of solid sodium hydroxide or potassium hydroxide to which a little water is added. This process has been recommended specifically for homologous phenols since the experiment can be made with the free phenol. An example is the conversion of 2,4-xylenol into 2-hydroxy-5-methylbenzoic acid, which shows also that the alkyl group next to the hydroxyl group is preferentially oxidized.400... 

The regularities attaching to the various oxidation reactions, and their mechanisms, have not yet been fully clarified. In general, the formation of a second carboxyl group on side-chain oxidation of dialkyl aromatic compounds is favored by alkalinity and disfavored by acidity of the medium. Beyond this, the following paragraphs give some indications of the influence of substituents. 

Aromatic side chain, oxidation of, 65. Azo-compounds, formation of, 56, 59, 60. 

Electrooxidation of aromatic compounds has been intensively investigated, and many useful fine chemicals have been prepared by both side-chain and aromatic nucleus oxidation. Side-chain oxidation of alkylbenzenes may furnish benzyl alcohols, benzyl acetates, benzyl methyl ethers, Af-benzyl acetamides, benzaldehydes, benzoic acids, and so on. For instance, electrooxidation of p-methoxytoluene affords p-methoxybenzyl methyl ether, p-methoxybenzaldehyde, and/or its dimethylacetal depending on the choice of electrolysis media [3]. Many examples of electrooxidation of aromatic nucleus have been also reported. p-Quinones and their methyl acetals and semiquinones are prepared by electrooxidation of phenol derivatives and hydroquinones [3]. Nucleus-nucleus coupling of methoxybenzene derivatives... 

Beier, M., SchimmoeUer, B., Hansen, T, et al. (2010). Selective Side-Chain Oxidation of Alkyl Aromatic Compounds Catalyzed by Cerium Modified Silver Catalysts, J. Mol Catal. A Chem., 331, pp. 40-49. 

Robert D.A., Geoffroy G.L. Compounds with heteronuclear bonds between transition metals. In Comprehensive Organometallic Chemistry, Wilkinson G., Stone F.G.A., Abel E.W. Eds., Perga-mon, Oxford, UK, 1982 Vol. 6, pp. 821-877 and references therein Rogovin, M., Neumann, R. Silicate xerogels containing cobalt as heterogeneous catalysts for the side-chain oxidation of alkyl aromatic compounds with tert-butyl hydroperoxide. J. Mol. Catal. A Chem. 1999 138 315-318... 

The nature of the mechanism of side-chain oxidation of alkyl aromatic compounds by one-electron reagents has been discussed.The reaction sequence ... 

In comparison with alkyl halogenides, halides bound to aromatic nuclei are less reactive, and, unless activated by the presence of other substituents (for example, reactive chlorine in 2,4-dinitrochlorobenzene), nucleophilic substitution reactions cannot be used for their identification. For the identification, electrophilic substitution reactions on aromatic nuclei are used predominantly, such as nitration and chlorosulfonation. Only in exceptional and special cases are other procedmes used [preparation of Grignard reagent and the conversion to anilides (12) preparation of addition compounds with picric acid-chloronaphthalenes, see p. 127 oxidation of side chains-oxidation of chlorotoluene to the corresponding chlorobenzoic acid, see p. 129]. 

When an aromatic compound having an aliphatic side chain is subjected to oxidation, fission of the side chain occurs between the first and second carbon atoms from the benzene ring, the first carbon atom thus becoming part of a carboxyl ( -COOH) group. For example ... 

Oxidation of a side chain by alkaline permanganate. Aromatic hydrocarbons containing side chains may be oxidised to the corresponding acids the results are generally satisfactory for compounds with one side chain e.g., toluene or ethylbenzene -> benzoic acid nitrotoluene -> nitrobenzoic acid) or with two side chains e.g., o-xylene -> phthalic acid). 

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