Aromatic rings electrophilic hydroxylation

Many of the properties of phenols reflect the polarization implied by the resonance description The hydroxyl oxygen is less basic and the hydroxyl proton more acidic in phenols than m alcohols Electrophiles attack the aromatic ring of phenols much faster than they attack benzene indicating that the ring especially at the positions ortho and para to the hydroxyl group is relatively electron rich... 

In most of their reactions phenols behave as nucleophiles and the reagents that act on them are electrophiles Either the hydroxyl oxygen or the aromatic ring may be the site of nucleophilic reactivity m a phenol Reactions that take place on the ring lead to elec trophilic aromatic substitution Table 24 4 summarizes the behavior of phenols m reac tions of this type... 

Although a hydroxyl group strongly activates an aromatic ring toward electrophilic attack an oxyanion substituent is an even more powerful activator Electron delocaliza tion m phenoxide anion leads to increased electron density at the positions ortho and para to oxygen... 

The aromatic nucleus of alkylphenols can undergo a variety of aromatic electrophiUc substitutions. Electron density from the hydroxyl group is fed iato the ring. Besides activating the aromatic nucleus, the hydroxyl group controls the orientation of the incoming electrophile. 

Activating group (Section 16.4) An electron-donating group such as hydroxyl (-OH) or amino (— NH2) that increases the reactivity of an aromatic ring toward electrophilic aromatic substitution. 

Whereas the reactive species under acidic conditions are the [H2OOH] and [HO] cations, the reactive species under alkaline conditions is the [H02]e anion. Whereas the cationic oxidation species are stronger oxidants, with oxidation potential rising with increasing acidity favoring aromatic ring hydroxylation by electrophilic substitution (3,13), the anionic species mediate a nucleophilic attack on quinones according to the scheme illustrated in Fig. 6 (3,10,11). The formation of new acidic functionality results in a rapid decline in pH. 

This hydroxylation-induced intramolecular migration, known as the NIH shift, was explained by the involvement of arene oxides formed by the attack of electrophilic oxoiron(V) porphyrin on the aromatic ring.753 Intermediate 98 was also suggested to be formed in hydroxylation by the Fenton and related reagents in aprotic media after initial oxidation with an oxoiron(V) complex followed by electron transfer.744 754... 

These copper-mediated reactions very often involve dinuclear intermediates, but detailed mechanistic studies on stoichiometric systems are relatively few. The key features are the formation of p-peroxo or p-superoxo complexes by electron transfer from cop-per(i) to dioxygen. The co-ordinated oxygen may then act as an electrophile to the aromatic ring. A possible mechanism for the ortho-hydroxylation of phenol by dioxygen in the presence of copper catalysts is shown in Fig. 9-29. 

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