Aromatic transformation products, hydroxylate

Alkylphenols undergo a variety of chemical transformations, involving the hydroxyl group or the aromatic nucleus that convert them to value-added products. 

The transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

Aldehydes lacking an OH group at C2 are also transformed by transketolase, leading to a 3S configuration of the hydroxyl group in the deoxyketose product [7a, 9] albeit with a significantly lower rate than with the hydroxylated acceptors [6b, 10. In contrast to the transketolases from spinach and yeast [9, no conversion of aromatic aldehydes, e.g., benzaldehyde or hydroxybenzaldehydes, could be detected with purified E. coli transketolase [6b]. 

Aromatic hydroxylation. Aromatic hydroxylation such as that depicted in Figure 4.3 for the simplest aromatic system, benzene, is an extremely important bio transformation. The major products of aromatic hydroxylation are phenols, but catechols and quinols may also be formed, arising by further metabolism. One of the toxic effects of benzene is to cause aplastic... 

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