Complexes in electrophilic aromatic

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... 

The theory that electrophilic aromatic substitution proceeds via a CT complex has been proposed for a long time in the literature117118. In a recent paper, Kochi examines the relevance of the presence of a CT complex in electrophilic aromatic nitration119. 

Rationalization of the observed selectivity can be based on several considerations [95]. The negative charge of the intermediate adduct (analogous to the u-complex in electrophilic aromatic substitution) has to be stabilized. This... 

The formation of the sigma complex in electrophilic aromatic substitution has a higher activation energy than the formation of a carbocation in electrophilic addition to an alkene (Figure 13.1). Therefore, the rates of electrophilic aromatic substitution reactions are slower than the rates of electrophilic addition reactions to aUtenes for the same electrophile. For example, bromine reacts instantly with alkenes, but does not react at all with benzene except in the presence of a strong Lewis acid catalyst. 

Reactivity and orientation in electrophilic aromatic substitution can also be related to the concept of hardness (see Section 1.2.3). Ionization potential is a major factor in determining hardness and is also intimately related to the process of (x-complex formation when an electrophile interacts with the n HOMO to form a new a bond. In MO terms, hardness is related to the gap between the LUMO and HOMO, t] = (sujmo %omo)/2- Thus, the harder a reactant ring system is, the more difficult it is for an electrophile to complete rr-bond formation. 

Another drawback to the use of amino-substituted benzenes in electrophilic aromatic substitution reactions is that Friedel-Crafts reactions are not successful (Section 16.3). The amino group forms an acid-base complex with the AICI3 catalyst, which prevents further reaction from occurring. Both drawbacks can be overcome, however, b3 carrying out electrophilic aromatic substitution reactions on the corresponding amide rather than on the free amine. 

Isomeric product distributions. Isomeric product distributions obtained from toluene and anisole have been the subject of considerable mechanistic discussion in electrophilic aromatic nitration (Schofield, 1980 Olah et al., 1989). As applied to nitrations with iV-nitropyridinium ion, the yellow colour of the EDA complex immediately attendant upon the mixing of toluene and PyN02 in acetonitrile persists for about a day (in the dark), whereas the charge-transfer colour of toluene and Me2PyNOj is discharged within 10 min at 25°C. Both bleached solutions afford an identical product mixture (81), consisting of o- (62%), m- (4%) and p-nitrotoluenes (34%)... 

The fact that it and a complexes do form is not proof that either or both are intermediates in electrophilic aromatic substitution. However, Table 7.16 gives strong evidence that a complexes are the usual intermediate. Electron-donating groups greatly stabilize a complexes of benzene derivatives but only slightly... 

Now that we have determined that the intermediate in electrophilic aromatic substitution is usually a a complex (see, however, p. 394), let us return to a consideration of Reaction 7.76. Two factors probably combine to cause the observed isotope effect and base catalysis. First, the strong electron-donating groups stabilize the intermediate 76 (Equation 7.77) and make departure of the proton more difficult than proton loss in many other electrophilic substitutions. [Remember, however, that k1 < k2 (see p. 386).] Second, steric interactions between the large diazonium group and the nearby substituents increase the rate... 

Apart from these examples, the activated substrate-Fe complex is more frequently used as an electrophilic reagent, for example in electrophilic aromatic substitutions (Chapter 6). 

An intermediate in electrophilic aromatic substitution or nucleophilic aromatic substitution with a sigma bond between the electrophile or nucleophile and the former aromatic ring. The sigma complex bears a delocalized positive charge in electrophilic aromatic substitution and a delocalized negative charge in nucleophilic aromatic substitution, (p. 756)... 

For many years the question has been discussed as to whether other intermediates are involved in electrophilic aromatic substitutions in addition to a-complexes. Most claims that jt-complexes or radical pairs are intermediates are ambiguous. It is not possible to differentiate between an intermediate on the direct way from reagents to products and an adduct of the reagents in a side equilibrium, if in the formation or dissociation of such a compound no additional particle is added or transferred to another particle. Only in such a case can the steady-state equations be tested by checking the dependence of the overall rate constant on the concentration of such particles. 

Hydroarylation can also be mediated by Au(I) and Au(III) (Scheme 33) (384). In the case of aryl substituted alkynes, the Au(III) Ji complex undergoes electrophilic aromatic substitution with the electron-rich arene to give aLkenyl-Au(III) complex, which is immediately protonated by the H generated upon C C bond formation. For the Au(I)-catalyzed hydroarylation, the cationic gold complex k coordinates the alkyne, with subsequent nucleophilic attack by the arene from the opposite face leading to an alkenyl-gold complex, which is protonated to the desired products. The nature of the reaction causes the regioselectivity of this reaction to be sensitive to electronic rather than steric factors. 

In Solution. During the 1940s and 1950s, proposals were made by separate groups of workers that charge-transfer complexes had a role in electrophilic aromatic substitution (8-12). The idea was that in a reaction of an aromatic compound (ArH) with an electrophile (E), covalent bonding was preceded by transfer of charge from ArH to E ... 

An aqueous Friedel-Crafts reaction has also been used in polymer synthesis. The acid-catalyzed polymerization of benzylic alcohol and fluoride functionality in monomeric and polymeric fluorenes was investigated in both organic and aqueous reaction media. Polymeric products are consistent with the generation of benzylic cations that participate in electrophilic aromatic substitution reactions. Similar reactions occurred in a water-insoluble Kraft pine lignin by treatment with aqueous acid. A Bisphenol A-type epoxy resin is readily emulsified in aqueous medium with an ethylene oxide adduct to a Friedel-Crafts reaction product of styrene and 4-(4-cumyl)phenol as emulsifier.Electrophilic substitution reaction of indoles with various aldehydes and ketones proceeded smoothly in water using the hexamethylenetetramine-bromine complex to afford the corresponding Z A(indolyl)methanes in excellent yields.InFs-catalyzed electrophilic substitution reactions of indoles with aldehydes and ketones are carried out in water.Enzymatic Friedel-Crafts-type electrophilic substitution reactions have been reported. ... 

---