Alkylaromatic side-chain reactions

Alkanes can be simultaneously chlorinated and chlorosulfonated. This commercially useful reaction has been appHed to polyethylene (201—203). Aromatics can be chlorinated on the ring, and in the presence of a free-radical initiator alkylaromatic compounds can be chlorinated selectively in the side chain. King chlorination can be selective. A patent shows chlorination of 2,5-di- to 2,4,5-trichlorophenoxyacetic acid free of the toxic... 

The side chain alkylation of alkyl benzenes is usually performed with alkenes as alkylating agents in the presence of strong-acid catalysts (256,257). The use of highly basic catalysts such as alkali metals, their hydrides, and sodium and potassium complexes for alkylation of alkylaromatic hydrocarbons has also been reported (256,257). The reaction mechanism proposed by Pines et al. (258) involves the addition of a benzylic carboanion to the alkene (Scheme 41). 

Alkylaromatic compounds possessing a sufficiently long side chain may also undergo dehydrocyclization.216 In fact, the aromatic ring enhances this reaction alkylaromatics undergo dehydrocyclization faster than do alkanes. Depending on the side chain, condensed or isolated ring systems may be formed. 

Among the different reactions which have been studied Dn solid bases, isomerization of linear butenes [ref. 6], aidolic condensation [ref. 7], and side chain alkylation in alkylaromatics [ref. 8],... 

Commercial reforming catalysts have both metal and acid sites. Both could contribute to cyclization. If there are four or more carbon atoms in the side chain of a mono-alkylaromatic or ortho-substituted dialkylaromatic hydrocarbon, cyclization can yield either five- or six-membered rings. This multiplicity of reaction pathways helps to clarify the roles of the metal and acid components in dehydrocyclization and other reactions. 

The dimerization of toluene and substituted toluenes leads to diaryl-ethanes 2S.28.32) Electron-attracting substituents favour the reaction 32) while electron-donating groups reduce the yield. In alkylaromats with straight or branched alkyl groups it is almost always the weakest bond of the side chain which is broken 25 >28>. The resulting radicals combine to diarylethanes or substituted diarylethanes ... 

A further useful application of SC-CO2 as a reaction medium is the free-radical side-chain bromination of alkylaromatics, replacing conventional solvents such as tetra-chloromethane or chlorofluorohydrocarbons having no abstractable hydrogen atoms [920]. For example, bromination of ethylbenzene in SC-CO2 at 40 °C and 22.9 MPa yields 95 cmol/mol (1-bromoethyl)benzene, with practically the same regioselectivity as obtained in conventional tetrachloromethane as the solvent. Even the classical Wohl-Ziegler bromination of benzylic or allylic substrates using A-bromosuccinimide (NBS) can be conducted in SC-CO2 [920]. Irradiation of a solution of toluene, NBS, and AIBN (as initiator) in SC-CO2 at 40 °C and 17.0 MPa for 4 hours gave (bromomethyl)-... 

The ammoxidation of methylaromatic compounds has been shown to be a convenient route to produce many nitriles required for further syntheses of side-chain functionalized products. This method is versatile and can be carried out very easily because of the stability, and undamaged desorption, of the nitriles formed under severe gas-phase conditions. A series of interesting new patents on catalysts for the ammoxidation of alkylaromatics have been issued, indicating that there is still commercial interest in this reaction [117]. 

Although a previous chapter in this volume provides a broader perspective on the reactivity of radical cations, in this section we will examine intramolecular electron-transfer reactions coupled with or followed by cleavage of a bond in odd electron species such as radical cations, radical zwitterions and radical anions. In particular, this paragraph will be divided in oxidative and reductive bond-cleavage processes. Because this field is however too large to be covered extensively here, the discussion will be limited to selected examples—for oxidative cleavages, side-chain fragmentation reactions of alkylaromatic radical cations and decarboxylation reactions of radical zwitterions derived from benzoic and arylalkanoic acids, and for reductive... 

Both nuclear and side-chain substitutions of alkylaromatics by NCO have been observed [204]. Thus, electrolysis of anisole in MeOH-KOCN results in substitution of hydrogen by a methylcarbamyl group, via the isocyanate. The reaction has been pictured as a direct oxidation process. Yields are fairly low (<20%). 

Partial catalytic oxidation of alkylaromatic hydrocarbons is interesting both from the industrial and the scientific point of view. The industrial interest is due to the availability of these substances from the petrochemical industry and to a number of applications for the possible oxidation products. Conventional gas phase oxidation concerns the side chain and leads mainly to benzoic acid or even to destruction of the aromatic ring. Oxidation under mild conditions could cease the reaction at earlier stages and reduce the number of the products formed. However, the appropriate catalyst for such partial oxidation has not been found yet. 

Nonacidic Catalysts. The hydrocracking of alkylaromatics over nonacidic catalysts is relatively simple and straightforward (5,39,42,46). Little or no isomerization occurs. Successive removal of methyl groups from the side chains is the principal reaction. The larger side chains are attacked first. For example, the reaction of p-ferf-butyltoluene over Rh on neutral alumina is reported to proceed sequentially as follows p-ferf-butyltoluene -> p-proplytoluene - p-ethyltoluene -> p-xylene — toluene benzene. In the hydrogenolysis of indane, the first reaction is the cleavage of the 1-2 (or 3-4) carbon-carbon bond of the five-... 

Cobalt(III) acetate oxidizes thianthrene and 9,10-diphenyl-anthracene in acetic or trifluoroacetic acid to the cation radicals, as shown by esr (Heiba et al., 1969b). In flow systems, Co(III) acetate in trifluoroacetic acid has also given well-resolved esr spectra of polyalkylbenzene cation radicals (hexa-, penta-, and all tetra-methylbenzenes, 1,3,5-tri-t-butyl-, and 1,4-di-t-butylbenzene) which are difficult to make in other ways (Dessau et al., 1970). When similar oxidations of alkylaromatics are carried out in static systems, further reactions occur, and these are sometimes of preparative value, since among the products are nuclear and side chain acetates. The reactions have been clarified most recently by the work of Heiba and Dessau. 

Alkylaromatics-Alkylation with Olefins.- The base catalysed alkylation of alkyaromatics with olefins results in selective addition of an olefin to the side chain. These reactions have... 

We have already seen in this chapter that we can substitute bromine and chlorine for hydrogen atoms on the benzene ring of toluene and other alkylaromatic compounds using electrophilic aromatic substitution reactions. We can also substitute bromine and chlorine for hydrogen atoms on the benzylic carbons of alkyl side chains by radical reactions in the presence of heat, light, or a radical initiator like a peroxide, as we first saw in Chapter 10, (Section 10.9). This is made possible by the special stability of the benzylic radical intermediate (Section 15.12A). For example, benzylic chlorination of toluene takes place in the gas phase at 400-600 °C or in the presence of UV light, as shown here. Multiple substitutions occur with an excess of chlorine. 

Other applications for aromatic hydroxylation are tiny in comparison with phenol, but the same technology can in principle be used. Side-chain oxidation is a possible side reaction for alkylaromatics. 

There is no further, destructive, oxidation as can be observed in cobalt-acetic acid systems, and the remaining unreacted ethylbenzene may be recycled. The reaction can be extended to a range of substituted ethylbenzenes. It may also be possible selectively to oxidise the side chain of other alkylaromatics, although toluenes are largely resistant to oxidation except under forcing conditions. 

The above examples are mostly related to either acidic forms of zeolite-like materials or supported metals and complexes. The basic forms of zeolites are known to exhibit interesting properties in side chain alkylation of alkylaromatic hydrocarbons as well as in diverse condensation reactions (Knoevenagel, Michael, etc.). One example was given in the paper by Corma et al. [163] related to the condensation of benzaldehyde with diethyl malonate on... 

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