Electrophilic arene substitution

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

Antiaromaticity. 194 Antibonding orbitals. 15 Anti-elimination. 128ff Arenes. electrophilic substitution. 205/T niirosation, 206 Aromatic character. 193 Aromaticity. 193 Aromatic reactions. 199 Aromatic substitutions, nucleophilic. 2 Sff Aromafization. 202... 

Arenediol derivs. ox. ring opening, 87-88 Arenes electrophilic substitution at, 207-208,... 

Whereas the previous arene electrophilic substitutions take place at ortho position of pyridine coordinating group, the sulfonation with arylsulfonyl chloride of phenylpyridines but in the presence of a ruthenium(II) catalyst [RuCl2(p-cymene)]2 surprisingly led Frost et al. to observe for the first time an unexpected regioselective sulfonation at weto-position of the directing 2-pyridyl group [(Eq. 49)] [111]. 

CHAPTER TWELVE Reactions of Arenes Electrophilic Aromatic Substitution... 

---