Electrophilic aromatic substitution described

Reduction of arenes by catalytic hydrogenation was described m Section 114 A dif ferent method using Group I metals as reducing agents which gives 1 4 cyclohexadiene derivatives will be presented m Section 1111 Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes the entire subject matter of Chapter 12... 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

The orbital and resonance models for bonding in arylamines are simply alternative ways of describing the same phenomenon Delocalization of the nitrogen lone pair decreases the electron density at nitrogen while increasing it m the rr system of the aro matic ring We ve already seen one chemical consequence of this m the high level of reactivity of aniline m electrophilic aromatic substitution reactions (Section 12 12) Other ways m which electron delocalization affects the properties of arylamines are described m later sections of this chapter... 

The two mam methods for the preparation of aryl halides halogenation of arenes by electrophilic aromatic substitution and preparation by way of aryl diazomum salts were described earlier and are reviewed m Table 23 2 A number of aryl halides occur natu rally some of which are shown m Figure 23 1... 

Figure 12.1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution. For electrophilic aromatic substitution reactions to... 

In general, if condensation polymers are prepared with methylated aryl repeat units, free radical halogenatlon can be used to introduce halomethyl active sites and the limitations of electrophilic aromatic substitution can be avoided. The halogenatlon technique recently described by Ford11, involving the use of a mixture of hypohalite and phase transfer catalyst to chlorinate poly(vinyl toluene) can be applied to suitably substituted condensation polymers. 

A diverse group of organic reactions catalyzed by montmorillonite has been described and some reviews on this subject have been published.19 Examples of those transformations include addition reactions, such as Michael addition of thiols to y./bunsatu rated carbonyl compounds 20 electrophilic aromatic substitutions,19c nucleophilic substitution of alcohols,21 acetal synthesis196 22 and deprotection,23 cyclizations,19b c isomerizations, and rearrangements.196 24... 

In CHEC-II(1996), a very similar acid-catalyzed conversion of 3,5-diamino-l,2,4-thiadiazolidines to 2-guanidino-benzothiazoles was reported, and this was described as an electrophilic aromatic substitution reaction <1996CHEC-II(4)307>. 

The first product is derived from a normal electrophilic aromatic substitution reaction of the kind described in the text. The second product is derived from ipso electrophilic aromatic substitution. The mechanism is exactly the same, but in the last step z-Pr+ is lost instead of H+. 

Other metals capable of electrophilic substitution of C-H bonds are salts of palladium and, environmentally unattractive, mercury. Methane conversion to methanol esters have been reported for both of them [29], Electrophilic attack at arenes followed by C-H activation is more facile, for all three metals. The method for making mercury-aryl involves reaction of mercury diacetate and arenes at high temperatures and long reaction times to give aryl-mercury(II) acetate as the product it was described as an electrophilic aromatic substitution rather than a C-H activation [30],... 

As described in the previous sections, a variety of nucleophiles attack the Cy atom of ruthenium-allenylidene intermediates. Aromatic compounds should also be suitable candidates and this was found to be the case [30]. Thus, reactions of propargylic alcohols with heteroaromatic compounds such as furans, thiophenes, pyrroles, and indoles in the presence of a diruthenium catalyst such as la proceeded smoothly to afford the corresponding propargylated heteroaromatic compounds in high yields with complete regioselectivity (Scheme 7.25). The reaction is considered to be an electrophilic aromatic substitution if viewed from the side of aromatic compounds. 

The monomers described so far have all been prepared by starting with 4-bromobenzocyclobutene, 2. A different approach to the preparation of monomers begins with the parent hydrocarbon benzocyclobutene 1 by carrying out electrophilic aromatic substitution reactions [36]. Benzocyclobutene readily undergoes a Friedel-Crafts benzoylation reaction with a variety of substituted acid chlorides (Fig. 7). 

The quantum-chemical calculation of charge-transfer states as possible intermediates in electrophilic aromatic substitution reactions, making allowance for solvation effects, has been reviewed.6 It has been shown that a simple scaled Hartree-Fock ab initio model describes the ring proton affinity of some polysubstituted benzenes, naphthalenes, biphenylenes, and large alternant aromatics, in agreement with experimental values. The simple additivity rule observed previously in smaller... 

Probably, the electrophilic addition to olefins and the electrophilic aromatic substitution are more similar than is generally thought. Nevertheless, the two reactions are described in textbooks in separate chapters with the implicit idea that they are completely different reactions. Mechanistic investigations reveal that some steps show parallel behaviour in both reactions. In particular, the initial steps are similar for both reactions. 

Substituted imidazole 1-oxides 228 are predicted to be activated toward electrophilic aromatic substitution, nucleophilic aromatic substitution, and metallation as described in Section 1. Nevertheless little information about the reactivity of imidazole 1-oxides in these processes exists. The reason for this lack may be the high polarity of the imidazole 1-oxides, which makes it difficult to find suitable reaction solvents. Another obstacle is that no method for complete drying of imidazole 1-oxides exists and dry starting material is instrumental for successful metallation. Well documented and useful is the reaction of imidazole 1-oxide 228 with alkylation and acylation reagents, their function as 1,3-dipoles in cycloadditions, and their palladium-catalyzed direct arylation. 

For benzo[b]furan and indole no such precise data are available, but it is possible to adduce some information from the various reactions described below. The positional reactivity orders for these molecules and also for benzo[b]thiophene, which have been calculated by various methods, are given in Table 8.1. In principle the ab initio calculations should be the more reliable, but neither the tt nor the (a + it) order is correct for benzo[6]thiophene, suggesting that these are incorrect for the other molecules also. The calculations using the STO-3G basis set certainly wrongly predict the site of most rapid protonation. Notably, only the Hiickel calculations give the correct order for benzo[b]thiophene and indeed they are usually the most reliable indicators for electrophilic aromatic substitution. 

We have already described how nitration leads eventually to aromatic amines by reduction of the nitro group. In the next chapter you will meet the further development of these amines into diazoni-um salts as reagents for nucleophilic aromatic substitution by the S l mechanism with loss of nitrogen. In this chapter we need to address their potential for electrophilic aromatic substitution without the loss of nitrogen as this leads to the important azo dyes. Treatment of the amine with nitrous acid (H0N=0) at around 0°C gives the diazonium salt. 

The second route utilizes the introduction of the chlorosulfonyl substituent directly onto the aromatic nucleus. The reaction of substituted benzenes with chlorosulfonic acid gives good yields of arylsulfonyl chlorides however, the aryl substituent dictates the position of attachment of the chlorosulfonyl function in this electrophilic aromatic substitution.7 The method described herein allows replacement of a diazotized amine function by the chlorosulfonyl group. The ready availability of substituted anilines makes this the method of choice for the preparation of arylsulfonyl chlorides. 

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