Electrophilic substitution of aromatic

The complex that is formed can dissociate to form a cation (n-tr-complex) and an iodide anion, with the iodide ion reacting with the excess iodine molecules that are present. In addition the decomposition of the n-cr-complex can lead to the formation of highly reactive iodine cations, which can initiate further reactions — e.g. oxidations or electrophilic substitutions of aromatic systems [11, 13]. 

If we are correct in our assumption that the electrophilic substitution of aromatic species involves such a complexes as intermediates—and it has proved possible actually to isolate them in the course of some such substitutions (p. 136)—then what we commonly refer to as aromatic substitution really involves initial addition followed by subsequent elimination. How this basic theory is borne out in the common electrophilic substitution reactions of benzene will now be considered. 

On the other hand, the involvement of vinyl cationic species in the reaction cannot be ruled out in some cases, as shown in Scheme 4. In this context, it was found that the reaction of 3-butyn-2-one with mesitylene can occur without Pd(OAc)2, clearly indicating the involvement of vinyl cations generated from alkynes and H+ in this reaction.47 1 The yield difference in the presence and in the absence of Pd(OAc)2 may be explained by the competition between [Pd(n)02CF3]+ and vinyl cationic species in the electrophilic substitution of aromatic G-H bonds. Recent kinetic isotope experiments suggest a mechanism involving alkyne coordination to Pd(n) followed by electrophilic aromatic substitution.476... 

Friedel-Crafts reactions involving electrophilic substitution of aromatic compounds have been reported on solid base catalysts such as thallium oxide and MgO. The rates of benzylation of toluene by benzyl chloride over MgO nanocrystals were found to be of the order CP-MgO > CM-MgO > AP-MgO.56 An important observation in the study was that x-ray diffraction of the spent catalyst... 

In addition to these kinetic investigations, which were in part carried out with a view to the basicity of the aromatic substances, numerous investigations have also been carried out on the subject of electrophilic substitution of aromatic systems, and all of these ultimately show, directly or indirectly, a dependence on the basicity of the aromatic substance (cf. Mason, 1958, 1959 Gould, 1962 Brown and Stock, 1962). 

Study of the reactivity of aromatic C-H bonds in the presence of transition metal compounds began in the 1960s despite the quite early discovery of Friedel-Crafts alkylation and acylation reactions with Lewis acid catalysts. In 1967, we reported Pd(II)-mediated coupling of arenes with olefins in acetic acid under reflux [1], The reaction involves the electrophilic substitution of aromatic C-H bonds by a Pd(II) species, as shown in Scheme 2, and this is one of the earliest examples of aromatic C-H bond activation by transition metal compounds. Al-... 

The parallels between Cgo solubility in alkyl derivatives of benzene and reactivity of these derivatives to the reactions of electrophilic substitution have been established. The parallels allow the Cgo dissolution to be considered as a reaction of electrophilic substitution of aromatic hydrocarbons. 

It is well known that the reaction of electrophilic substitution of aromatic hydrocarbons is a two-stage process to form 7t-complexes at an intermediate... 

C120 is a most potent chlorinating agent in radical side-chain chlorination of deactivated methylbenzenes and also in the electrophilic substitution of aromatics bearing electron-withdrawing groups819. 

Ortho effect In electrophilic substitution of aromatic rings that already have two substituents, one of which is a meta directing group and that positioned meta to an ortho/para directing group, then the new substituent tends to go to the ortho position. In mass spectrometry, the ortho effect is the loss of a neutral species, usually a stable molecule such as water, derived from the ortho-... 

Electrophilic substitution of aromatic (and heteroaromatic) molecules proceeds via a two-step sequence, initial addition (of EF) giving a positively charged intermediate (a o-complex, or Wheland intermediate), then elimination (normally of H ), of which the former is usually the slower (rate-determining) step. Under most circumstances such substitutions are irreversible and the product ratio is determined by kinetic control. 

Effenberger, R and Epple, G. 1972. Electrophilic substitution of aromatic compounds. 1. Trifluoromethanesulfonic-carboxylic anhydrides, highly active acylating agents. Angew. Chem., Int. Ed. Engl. 11 299-300. 

Butler and Crossley (102) studied the reactivity of PAH, naturally present on soot particles, generated in a flame from an eth-ylene-air burner, in a reaction chamber with ambient air containing 5-10 ppm of sulfur dioxide or a mixture of nitrogen oxides. While exposure of up to three months to SO did not yield any significant loss of PAH, degradation by NO occurred readily and resulted in half lives varying between 7 and 30 days (for benzo(a)-pyrene and phenanthrene respectively),in correspondence with the order of reactivity for electrophilic substitution of aromatic systems. 

Electrophilic substitution of aromatic nuclei in tyrosine and tryptophan side chains has frequently been reported in connection with acidolytic removal of blocking groups. C-Benzylation and tert.butylation of the tyrosine side chain and N-alkylation of the indole nucleus in tryptophan are often attributed to the alkyl cations generated in the reaction. This common side reaction is caused, however, mainly by the alkylating agents formed in the process, such as benzyl bromide or tert.butyl trifluoroacetate. The same is true for the S-alkylation of the methionine side chain. Conversion of the thioether to a sulfonium salt can... 

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