Alkylaromatic side-chain alkylation

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). 

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],... 

The mechanism of alkenylation is similar to that of side-chain alkylation of alkylaromatics and can be illustrated as follows ... 

Alkylaromatics-Alkylation with Methanol.- Basic catalysts lead to the side chain alkylation of toluene with methanol. In the presence of acid solid catalysts xylene isomers are formed. 

Liquid-phase side-chain alkylation of alkylaromatics with alkenes proceeds over NaNs/zeolite (59) and Na/Na0H/Al203 (33). Alkylation of o-xylene with butadiene over Na/K2C03 yields o-tolylpent-2-ene, which is a precursor for 2,6-dimethylnaphthalene (116). 

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... 

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 ... 

The acyloxylation of A -alkylsubstituted amides has received considerable attention [70,75,118], from both the mechanistic and the preparative points of view. In electrolytes containing alkali metal carboxylates the direct mechanisms probably operates, whereas in the case where a nitrate salt is the supporting electrolyte an indirect mechanism involving hydrogen abstraction from the A -alkyl group by anodically generated NO3 is indicated. The same mechanistic problem is encountered in side-chain acetoxylation of alkylaromatic compounds in the presence of N03 [123,152,153]. The method for large-scale substitution into A -formyl derivatives, which works so well for methoxylation, fails when applied to acetoxylation, probably because of the acid sensitivity of the products [80]. 

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... 

Wathever the actual mechanism, oxide clusters, including aklali oxides appeared to be able to catalyze the formation of a carbide species which could react with methanol, thus resulting in an overall alkylation of the side chain of substituted aromatics. Further dehydrogenation of the resulting alkylaromatic was also achieved with such oxide species. 

There is now increasing commercial interest in the dimerization of olefins over supported alkali metals, via a carbanion mechanism. Propylene selectively produces 4-methylpent-l-ene and alkylaromatics are alkylated on the side-chain (a-carbon) with these materials. ... 

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. 

So, the ethylene production does correlate with coke presence, in particular with aromatics formation as far as the diffusion limitations are not significant. However, it seems that the majority of ethylene is not always formed directly from MeOH [115]. The aromatics and other coke species could be the products of the conversion of primary carbenium ions, which are mobile and could equilibrate each other [28]. This may explain the isotopic distribution in products and retained coke molecules and the coexistence of aromatics and carbenium ions [28], In addition to the coproduction of ethylene with aromatics in olefins interconversion cycle, formation of ethylene via alkylation-dealkylation of methyl aromatics with heavy olefins or with the equivalent carbenium ions like ethyP, propyP, and butyP could be an option. The alkyl aromatics with the side chain length of two carbons or longer are not stable in the pore and dealkylates on the acid sites due to too long residence time and steric hindrances. This may lead to formation of ethylene, other olefins, and alkylaromatics with different structure, namely PMBs [129]. In other words, the ethylene is formed via interaction of the carbenium ions like ethyP, propyP, and butyP formed from MeOH or heavy olefins with aromatics and other coke precursors followed by cracking and in a less extent by a direct alkylation of PMBs with methanol. The speculation is based properly on analysis of the prior arts and is not contradictory with the concept of the aromatic cycle for ethylene formation. 

N-Chloropolymaleimide (22) chlorinates alkylaromatic compounds on the aromatic ring in preference to the alkyl side chains as in equation (3) in yields of 70-88%. The solution analogue, N-chlorosuccinimide, gives both aromatic and alkyl chlorination in lower yields. 

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