Electrophilic aromatic substitution reactions Bronsted acids

The mechanism of the conventional electrophilic aromatic substitution reaction is expected to consist of four steps (i) formation of an electrophile (E ) by activation with a Lewis- or Bronsted acid (A), (ii) attack of the electrophile on the aromatic compound (ArH) resulting in a TT-complex (iii) conversion of the TT-complex into a o-complex, (iv) followed by the loss of a proton from the o-complex to give the product (Eq 2.16). The formation and attack of the electrophile are separate steps i.e. the electrophile is formed and dispersed in the reaction medium before attack on the aromatic compound takes place. 

No investigations have as yet been undertaken to elucidate the mechanism of these reactions, although ordinary electrophilic aromatic substitutions are the most likely. The attacking species is apparently RfS" " or an activated complex formed through coordination of the per-fluorohalogenosulfenyl chloride with a Lewis or a Bronsted acid (4). 

A third possible explanation for the reduced reactivity of anisole and phenol compared to that of toluene could be that the former two substrates are deactivated towards electrophilic aromatic substitution by reaction of the alcohol or ether moiety with either the Lewis (BF3) - or Bronsted ( HBFV) acid. In this instance the first equivalent of acid would be consumed by reaction with the oxygen (Scheme 2.4), and the second equivalent effecting protonation of CO. Inductive deactivation of the aromatic ring by the cationic oxygen will result in reduced reactivity of the aromatic system. Since the positive charge resulting from complexation of phenol with BF3 on the phenolic entity could be neutralised by proton loss to the reaction medium, phenol is expected to be more reactive than anisole under the reaction conditions (Scheme 2.4). Additional support for this explanation is provided by the reported isolation of an anisole/HF/BFs complex containing a 1 1 2 ratio of reactants. 

The alkylation of benzene with ethylene is an electrophilic substitution on the aromatic ring. Alkylation reactions are commonly considered as proceeding via carbenium-ion-type mechanisms. On a Bronsted acid site ethylene is protonated to form the active species. The latter can follow two major routes ... 

Substituted aromatics are essential chemical feedstocks. Among the xylenes, for example, p-xylene is in great demand as a precursor to terephthalic acid, a polyester building block. The pura-isomer is therefore more valuable than the o- and m-xylenes, so there is a powerful incentive for conversion of o- and m-xylene to p-xylene. Isomerisation over solid acids occurs readily as a result of alkyl shift reactions of the carbenium-ion-like transition state. The initial protonation occurs by interaction of the Bronsted acid site with the aromatic 71 system, by an electrophilic addition. Over non-microporous solid acids, at high conversion, xylenes are produced at their thermodynamically determined ratios, which favour the meta rather than the ortho or para isomers. In addition, unwanted transalkylation reactions occur, giving rise, for example, to toluene and trimethylbenzenes. Zeolite catalysts can be much more selective. 

Friedel-Crafts alkylation is one of the most frequently used and widely studied reactions in organic chemistry. Since the initial discovery by Charles Friedel and James Mason Crafts in 1877, a large number of applications have emerged for the construction of substituted aromatic compounds. Friedel-Crafts alkylation processes involve the replacement of C—H bond of an aromatic ring by an electrophilic partner in the presence of a Lewis acid or Bronsted acid catalyst. Particularly, catalytic asymmetric Friedel-Crafts alkylation is a very attractive, direct, and atom-economic approach for the synthesis of optically active aromatic compounds. However, it took more than 100 years from the discovery of this reaction until the first catalytic asymmetric Friedel-Crafts (AFC) alkylation of naphthol and ethyl pyruvate was realized by Erker in 1990. Nowadays, owing to continued efforts in developing... 

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