Aromatic substitution side-chain oxidation

The oxidation of m-cresol was carried out in Parr autoclave at 353 K using a 3 1 mixture of H2O and acetonitrile as solvent and Sn-silicalites with Si/Sn ratio of 70 as catalysts. A slightly higher efficiency for H2O2 is seen with Sn-ZSM-12 sample (Table 4). The dihydroxylated products, viz., 2-methylhydroquinone and 4-methylcatechol are found to be in excess over the products of side chain oxidation, viz., 3-hydroxybenzyl alcohol and the aldehyde in the product mkture. The aromatic hydroxylation on Sn-silicalites may follow an ionic mechanism as both the -CH3 and -OH groups in m-cresol are favourably placed for electrophilic substitution reaction. Interestingly, the product distribution on all the three Sn-molecular sieves is almost similar. This shows that in all the three types, the Sn ions are dispersed uniformly and possess identical catalytic property due to similar environment around them. 

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). 

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. 

Chang-Chien, G., Terminally carboxyl oligo(ethylene oxide) monomethyl ether-substituted side chain liquid crystalline polysiloxane polymer as stationary phase in capillary gas chromatography for the separation of polynuclear aromatic hydrocarbons, J. Chromatogr. A, 808, 201-209, 1998. 

Substituted aromatic molecules can be oxidized under mild conditions using Ti or V substituted molecular sieves. The nature and the selectivities of the products formed strongly depend on the oxidant used in the catalytic reaction. H2O2 favours the hydroxylation of the aromatic ring whereas tert-butyl hydroperoxide is very selective in the side-chain oxidation. For V-substituted ole ar sieves, we have proposed a mechanism which involves the redox system... 

The subject of this chapter is the hydroxylation of all kinds of aromatic rings, either substituted or unsubstituted. The ring(s) may be in an external substrate added to a metal complex - dioxygen system, or may be part of a ligand coordinated to the metal ion within the catalyst complex. Aromatic side-chain oxidations are treated in the sections for alkane, alkene or alkyne oxidation, depending on the substituent. 

One of the most important reactions in the production of industrial aromatics is electrophilic aromatic substitution another prominent type of reaction is nucleophilic substitution, which is favored for aromatics with electron-withdrawing groups. Free radical reactions, which occur especially in thermal pyrolysis processes and in side-chain oxidation and chlorination reactions, are even more important, in quantitative terms, than electrophilic and nucleophilic substitution reactions. Typical examples are thermal cracking of naphtha and gas-oil fractions, the oxidation of naphthalene to phthalic anhydride, and the side-chain chlorination of toluene. Rearrangement reactions are less significant. 

One of the most important selective oxidation processes in the production of chemicals is the side chain oxidation of alkyl aromatics, which are then further reacted to higher value products that end up in a variety of polymer compositions and specialised chemicals.The largest oxyfunctionalised aromatic products with regard to world production are terephthalic acid, phthalic anhydride and benzoic acid which are produced worldwide with capacities of >30,000, 5,000 and 500 kt a respectively. Worldwide production capacities for benzaldehyde and pyromel-litic dianhydride do not rise above 50 kt a In smaller capacities as specialty chemicals for the pharmaceutical industry, halo-substituted oxyfunctionalized aromatics are also produced. Of the aforementioned products, phthalic anhydride and pyromellitic dianhydride are produced from gas-phase processes using V20s-Ti02 catalysts and a liquid-phase alternative process does not appear immediately desirable. On the other hand, terephtalic and benzoic acids, and benzaldehyde are... 

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]. 

A useful application in the manufacture of ion-exchange resins may well be possible which avoids the use of carcinogenic chloromethyl ether. Here, a polymer of p-methyl styrene is chlorinated on the side chain with aqueous NaOCl and a phase-transfer catalyst. Sasson et al. (1986) have shown how stubborn . substituted aromatics like nitro/chlorotoluenes can be oxidized to the corresponding acids by using aqueous NaOCl containing Ru based catalyst. 

Anodic side chain substitution is a competing reaction to nuclear substitution of aromatic compounds. In side chain substitution, the first formed acidic radical cation is deprotonated at the a-carbon atom of an alkyl group to form a radical. This is further oxidized to a benzyl cation, which reacts with a nucleophile (Scheme 9, path d). The factors that influence the ratio of nuclear to side chain substitution have been described in 5.4.1. 

Benzylic CH bonds Benzylic CH bonds can be preferentially substituted at the anode by oxidation of the aromatic ring to a radical cation, which can undergo side-chain substitution at the benzylic carbon atom and/or nuclear substitution. Benzylic substitution preponderates, when there is an alkyl substituent at the aromatic carbon bearing the highest positive charge density in the radical cation, while a hydrogen at this position leads to a nuclear substitution [16]. Anodic benzylic substitution is used in technical processes for the conversion of alkyl aromatics into substituted benzaldehydes [17, 18]. Anodic benzylic substitution has been used for the regioselective methoxylation of estratrienone at C9 (Fig. 4) [19]. 

Oxidation of Other Arenes. Aromatic compounds with longer alkyl side chains can be converted to ketones or carboxylic acids. All the previously discussed reagents except Cr02Cl2 usually afford the selective formation of ketones from alkyl-substituted arenes. Oxidation with Cr02Cl2 usually gives a mixture of products. These include compounds oxidized in the P position presumably formed via an alkene intermediate or as a result of the rearrangement of an intermediate epoxide.110,705... 

Oxidation with peroxydisulfate in AcOH in the presence of catalytic amounts of iron and copper salts gives benzylic acetates in good yields.785,861 The reaction of lead tetraacetate with alkylarenes in AcOH provides benzylacetates in moderate yields.693 Most of these oxidations usually involve methyl-substituted benzenes since aromatics with longer chain produce different side products. 

According to this hypothesis, the results are modified from what would be expected from classical radical reactions. The interest in this hypothesis is that, with the sole exception of saturated hydrocarbons, it could apply to all the compounds that can be coordinated at the Tiiv center, such as alkenes, aromatics, alcohols, and sulfides. According to this hypothesis, the weak Lewis acidity of Tilv would help to bring the reactant into its coordination sphere. The initial coordination of the reactant would explain the oxidation of methyl-substituted aromatics in the aromatic ring and not in the side chain, even with a radical-type mechanism. 

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