Reactions at Aromatic Rings

In contrast with aliphatic nucleophilic substitution, nucleophilic displacement reactions on aromatic rings are relatively slow and require activation at the point of attack by electron-withdrawing substituents or heteroatoms, in the case of heteroaromatic systems. With non-activated aromatic systems, the reaction generally involves an elimination-addition mechanism. The addition of phase-transfer catalysts generally enhances the rate of these reactions. 

Aromatic substitution (see Substitution reactions at aromatic carbon atoms) Aromatization of six-membered rings Titanium(IV) chloride-Diethylaluminum chloride, 309... 

Some of the methods by which alkyl halides are prepared do not work for aryl halides because it is difficult to form C-halogen bonds at aromatic ring carbons by nucleophilic displacement reactions. The most common ways... 

In spite of the elegant solution to the problem as represented in the structure of 206, it is rather far from being optimal. In fact, the presence of the naphthalene system in this base made the latter rather vulnerable to electrophilic attack directed at aromatic rings. Owing to this complication, 206 is used in syntheses less often than other sterically hindered bases like 4-methyl-2,6-di-tcrt-butylpyridine 207, ethyldiisopropylamine, 208 (Hunig s base) or ethyldi-cyclohexylamine 209. All of these reagents are now manufactured commercially and widely used in cases where it is essential to carry out a reaction with strong electrophiles under strictly non-acidic conditions with the removal of proton acids as they are formed. In fact, the preparation of 205 was succesfully carried out in the presence of 208. °... 

The 77 bonds in aromatic compounds are also reactive toward electrophiles, although not nearly so much as alkenes. The aromatic ring attacks an electrophile to give an intermediate carbocation. The carbocation then undergoes fragmenta-tive loss of H+ (sometimes another cation) from the same C to which the electrophile added to re-form the aromatic system and give an overall substitution reaction. Thus, the predominant mechanism of substitution at aromatic rings under acidic conditions is electrophilic addition-elimination, sometimes referred to as SpAr. The reaction of toluene and nitric acid is indicative. 

The most common example of electrophilic substitution at a trigonal planar center is electrophilic aromatic substitution, which will serve as our archetype. Aromaticity was covered in Section 1.9, and the reactivity trends of aromatics will be covered in detail in Chapter 5. Chapter 8 has additional reaction examples. Aromatic rings are usually poor nucleophiles therefore excellent electrophiles are needed for the reaction to proceed. 

Photodimerization reactions involving aromatic rings of two naphthalene units are not as frequently encountered as those involving anthracene. A study of the effect of high pressure on the photodimerization of methyl 3-methoxy-naphthalene-2-carboxylate shows that the rate can be doubled (at 2000 bars), but this represents a much lower activation volume than for similar thermal reactions. An intramolecular dimer is formed when the a f/-[3.3]-(l,4)naphthalenophane (182) is irradiated the cycloadduct reverts to (182) on heating or on further irradiation. The syn isomer of (182) does not undergo this reaction. 

In acidic conditions, 2-methylpropene is protonated to give a tertiary carbocation, which can function as an electrophile in an electrophilic aromatic substitution reaction. The aromatic ring attacks the carbocation to give a resonance stabilized intermediate (sigma complex), followed by deprotonation which restores aromaticity. In aqueous acidic conditions, the proton source is H3O (in the beginning of the mechanism), and the base is H2O (at the end of the mechanism). 

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