Halogenated Arene Hydrocarbons

Ring fission. Ring fission of substituted catechols may take place by three pathways intradiol (ortho), extradiol (meta), or distal (1 6) fission. Although extradiol fission is generally preferred for substituted catechols, the product from extradiol fission of 

3- chlorocatechol is normally inhibitory since the acyl chloride reacts with the enzyme 

FIGURE 9.1 Alternative dioxygenation pathways for 3-substituted catechols (a) distal (b) intradiol (c) extradiol. 

FIGURE 9.2 Ring-cleavage pathways for the biodegradation of 3-substituted catechols. 

Additional limitations. Other limitations have been enconntered (a) the formation of the inhibitory protoanemonin from 4-chlorocatechols (Blasco et al. 1995) and (b) the ineffectiveness of cycloisomerases reqnired for effective degradation of 2-chlorotoluene (Polhnann et al. 2005). An example of this limitation is given later, and in the degradation of 3-flnorobenzoate in Part 3 of this chapter. 

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Aerobic degradation of chlorinated arene hydrocarbons, including the important group PCBs, and chlorobenzoates that are produced from them as metabolites, is generally initiated by dihy-droxylation of the rings to dihydrodiols followed by dehydrogenation to catechols. Halide may be lost simultaneously and for 2-halogenated benzoates, both halide and carboxyl. Salient aspects are summarized, and attention drawn to selected aspects of enzyme inhibition. 

The metabolism of 1,4-dichlorobenzene could involve the formation of an arene oxide intermediate, as has been proposed to occur in the oxidative metabolism of many halogenated aromatic hydrocarbons (Jerina and Daly 1974). 1,4-Dichlorobenzene has not been shown to be mutagenic in microbial or mammalian systems, a result that may be viewed as further suggestive evidence that an arene oxide intermediate is not involved in its metabolism. 

A second group of specific xenobiotics in Teltow Canal sediments are halogenated aromatics. Several chlorinated and brominated mono- and diaromatic hydrocarbons were detected in high amounts within the extractable organic matter as reported previously (Schwarzbauer et al. 2001). The halogenated arenes identified in the hydrolysis extracts included mono- and dichlorinated naphthalenes 12+13. mono- and dihrominated naphthalenes 14+15. tetra- to hexachlorinated biphenyls (PCB) and 2,4,6-tribomoaniline. The peak pattern of the chlorinated naphthalenes was similar to the congener distribution in technical mixtures e.g. Halowax 1000 (Falandysz 1998). 

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. 

A learning set of 1663 structurally diverse compounds was built. It contained 28 aliphatic hydrocarbons 52 aromatic hydrocarbons 74 alcohols, ethers, or phenols 27 aldehydes and ketones 42 acids and esters 66 amines and nitriles 23 amides and anilides 9 sulfur-containing hydrocarbons 18 nitro arenes 10 amino acids 54 halogenated hydrocarbons 35 nucleosides 25 nucleoside bases and 1200 multifunctional compounds. 

Using the above procedure, arene complexes of many of the block transition metals may be prepared, and substituted benzenes such as toluene, mesitylene (Mes) and hexamethylbenzene may be used instead of benzene. Generally, mesitylene and hexametl lbenzene metal complexes are more readily prepared than the unsubstituted benzene analogues. When halogenated aromatics are used, they undergo dehalogenation reactions affording hydrocarbon Ti-arene-metd complexes [25, 26]. 

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