Aromatics, halogenated Subject

In aromatic halogenation processes with molecular halogen, either alone or in the presence of a carrier or catalyst, or with positively charged halogenating species, there are great differences in relative reactivity of these species. Discussion of this subject is covered extensively by some of the standard works . ... 

Figure 1.11 Tyrosine residues are subject to nucleophilic and electrophilic reactions. The unprotonated phe-nolate ion may be alkylated or acylated using a variety of bioconjugate reagents. Its aromatic ring also may undergo electrophilic addition using diazonium chemistry or Mannich condensation, or be halogenated with radioactive isotopes such as 12iI.

As observed in mammalian models, the immune system of fishes is a sensitive target organ system to evaluate toxicity. For a more thorough review of environmental immunotoxicology in fishes, with reference to specific classes of xenobiotics, readers are referred to several reviews that deal with the subject over a span of nearly three decades [45-47, 54-57], While fish in the environment may be exposed to a variety of xenobiotics, the most frequently investigated xenobiotics are the polycyclic aromatic hydrocarbons (PAHs) and halogenated aromatic hydrocarbons (HAHs) due to the presence and activation of the aryl hydrocarbon receptor (AhR) in fish, and heavy metals due to their ubiquitous environmental distribution. 

Substituted 1,2,3-triazole 1-oxides 448 have been reported to undergo electrophilic and nucleophilic aromatic substitution and are subject to debromination, proton-metal exchange, and halogen-metal exchange followed by electrophilic addition. Transmetallation and cross-coupling have not been described. 3-Substituted 1,2,3-triazole 1-oxides 448 can be proton-ated or alkylated at the O-atom and they can be deoxygenated and deal-kylated. The individual reactions are described in Section 4.2.7.1-4.2.7.14. 

Aromatic compounds in the presence of alkali halides or solutions of halogen acids are usually substituted when subjected to electrolysis. Phenols are readily substituted, and several antiseptics have been obtained in this manner. Prom thymol in alkali solution, and in the presence of... 

Aromatic hydrocarbons are subject to cytochrome P-450-catalyzed hydroxylation in a process that is similar to olefin epoxidation. As discussed in Section IV. G, halogen migration observed during the hydroxylation of 4-ClPhe and similar substrates, led to the discovery of a general mechanism of oxidation that invokes arene oxide intermediates and the NIH shift. Arene oxides and their oxepin tautomers have not been isolated as products of metabolism of benzenoid compounds, but their presence has been inferred by the isolation of phenols, dihydrodiols and dihydrophenolic GSH conjugates derived therefrom262. 

You have now seen how enols and enolates react with electrophiles based on hydrogen (deuterium), carbon, halogens, silicon, sulfur, and nitrogen. What remains to be seen is how new carbon-carbon bonds can be formed with alkyl halides and carbonyl compounds in their normal electrophilic mode. These reactions are the subject of Chapters 26-29. We must first look at the ways aromatic compounds react with electrophiles. You will see similarities with the behaviour of enols. 

The halogenated biphenyls represent a class of compounds where reductive conditions [51] (solvated electron, aromatic radical anions, ketyl radicals) answer the purpose much better than the OH radical. However, this is to be seen in the context that the toxicity of the halogenated biphenyls exceeds that of biphenyl itself by such a wide margin that the latter compound is considered as relatively harmless. Complete removal of the pollutant would still have to rely on the oxidative pathway. A similar situation exists with respect to nitro-aromatics [52] which are also subject to reductive attack, indirectly by the OH radical via a-hydroxyethyl radical generated from the additive ethanol [53]. 

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