Electrophilic aromatic substitution Wheland intermediate

Protonated polymethylbenzenes281 and the chlorohexamethylbenzenium cation,282 intermediates in aromatic electrophilic substitutions known as Wheland intermediates, have been isolated as crystalline salts, allowing investigators to obtain their X-ray crystal structure. Nitrosoarenium a complexes of various arenes were directly observed by transient absorption spectroscopy.283 Kochi presented a method combining appropriate instrumental techniques (X-ray crystallography, NMR, time-resolved UV-vis spectroscopy) for the observation, identification, and structural characterization of reactive intermediates fa and n complexes) in electrophilic aromatic substitution.284... 

Resonance stabilization is important in electrophilic aromatic substitution as well. While each of the canonical forms of the Wheland intermediate has a sextet carbon atom, the charge is distributed over the remaining five atoms of the ring by resonance and is thus greatly stabilized. 

Electrophilic aromatic substitution normally proceeds via a positively charged intermediate 71 (known as a Wheland intermediate or cr-complex) (equation 27)84. 

Another class of gitonic superelectrophiles (based on the 1,3-carbodica-tion structure) are the Wheland intermediates or sigma complexes derived from electrophilic aromatic substitution of carbocationic systems (eq 8). 

The most widely accepted mechanism for electrophilic aromatic substitution involves a change from sp2 to sps hybridization of the carbon under attack, with formation of a species (the Wheland or a complex) which is a real intermediate, i.e., a minimum in the energy-reaction coordinate diagram. In most of cases the rate-determining step is the formation of the a intermediate in other cases, depending on the structure of the substrate, the nature of the electrophile, and the reaction conditions, the decomposition of such an intermediate is kinetically significant. In such cases a positive primary kinetic isotope effect and a base catalysis are expected (as Melander43 first pointed out). 

Electrophilic aromatic substitution represents one of the most important applications of the transformation of an arene via a CT (precursor) complex. The process is considered to proceed via an (encounter) -complex between the electrophile (E+) and the aromatic substrate (ArH), which collapses in a single rate-limiting step to the Wheland intermediate or a-... 

The mechanism of alkylation of arenes can be best understood as a carbocationic electrophilic aromatic substitution — a review on this aspect is available. The alkylating agent and the catalyst first form an alkyl cation or related polarized complex, which then reacts with the aromatic ring via a Wheland intermediate (arenium ion Scheme 1). 

The remaining two steps proceed along very similar lines in most electrophilic aromatic substitution reactions. The attack by the electrophile is usually the rate limiting step. The cationic intermediate is called a Wheland intermediate, or o-complex or an arenium ion, and can sometimes be isolated. 

A significant aspect of the chemistry of furans is the occurrence of 2,5-additions initiated by electrophilic attack a Wheland intermediate is formed normally, but then adds a nucleophile, when a sufficiently reactive one is present, instead of then losing a proton. Conditions can, however, usually be chosen to allow the formation of a normal a-substitution product. The occurrence of such processes in the case of furan is generally considered to be associated with its lower aromatic resonance stabilisation energy - there is less to regain by loss of a proton and the consequent return to an aromatic furan. 

The spectroscopic manifestation of symmetry effects in cyclic hyperconjugation was reported in electron spin resonance (ESR) spectra. " Symmetric LUMOs show cooperative effects to hyperconjugative interactions, while interactions with antisymmetric LUMOs are cancelled by symmetry (Figure 8.28). Such effects can be extended to spin-paired molecules, Wheland intermediates of electrophilic aromatic substitution, metal cation/arene complexes, and n -cyclopentadieneylmetal compounds. ... 

The Hammett equation usually predicts substituent effects with reasonable accuracy, provided the correct choice is made between o and o. a values are based on a system in which there is little change in the degree of conjugation between the substituent and the remainder of the molecule on moving from one side of the transition state to the other. Electrophilic aromatic substitutions pass through a positively charged complex, the Wheland intermediate (I), in which the electronic distribution is different from that of either reactants or products. A generalised sequence is shown in Equation (10). The standard reaction on... 

The alternative meehanism involves a more classical electrophilic aromatic substitution (SuAr) process, involving a Wheland-lype intermediate. 

We consider as dihydro derivatives those rings which contain either one or two 5p3-hybridized carbon atoms. According to this definition, all reactions of the aromatic compounds with electrophiles, nucleophiles or free radicals involve dihydro intermediates. Such reactions with electrophiles afford Wheland intermediates which usually easily lose H+ to re-aromatize. However, nucleophilic substitution (in the absence of a leaving group such as halogen) gives an intermediate which must lose H and such intermediates often possess considerable stability. Radical attack at ring carbon affords another radical which usually reacts further rapidly. In this section we consider the reactions of isolable dihydro compounds it is obvious that much of the discussion on the aromatic heterocycles is concerned with dihydro derivatives as intermediates. 

Strong differences in the reactivity of the aromatic C=C double bond compared to the reactivity of the C=C double bond of olefins are observed olefinic electrophilic additions are faster than aromatic electrophilic substitutions. For instance, the addition of molecular bromine to cyclohexene (in acetic acid) is about 1014 times faster than the formation of bromobenzene from benzene and bromine in acetic acid113,114. Nevertheless, the addition of halogens to olefins parallels the Wheland intermediate formation in the halogenation of aromatic substrates. 

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