Oxidation of the Benzene Ring

Except under extreme conditions, oxidation of the benzene ring requires the presence of strongly electron-donating groups such as hydroxyl or amino. These groups are simultaneously oxidized. The best known products of this oxidation process are the quinones benzo-l,4-quinone (p-benzoquinone, cyclohexadiene-l,4-dione, 10) and benzo-l,2-quinone (o-benzoquinone, cyclohexadiene-l,2-dione, 11). 

Benzyl ethers are Cleaved urider both hydrogenation and Birch reduction conditions. Both methods are used in the deprotection of benzyl-protecteo alcohols. The by-product is toluene  

Vilamin K.. . hich plays a olc in the coagulation ol blood, is a naphtho-1,4-quinone derivative. Co-eozymes Q, the ubiquinones, which are present in the cells of organisms, are involveti in the transportation of electrons, for example in the oxidation of NADH to NAD, Comniercially important quinones include the anihraquinone dyes (see Chapter 12) and the dyes which produce the colours in conventional photography. 

Quinones can he prepared hy the oxidation of phenols, dihydroxy-henzenes, dimethoxyhenzenes and anilines. For example, 1,4-dihydroxy-henzene (hydroquinone) can he oxidized in good yield using sodium chlorate in dilute sulfuric acid in the presence of vanadium pentoxide and also hy manganese dioxide and sulfuric acid and hy chromic acid. Other reagents which convert hydroquinones to quinones include Fremy s salt [potassium nitrosodisulfonate, (KS03)2N0] and cerium(IV) ammonium nitrate [CAN, Ce(NH4)2(N03)J. 

Benzoquinones are highly conjugated molecules and are consequently coloured. 8enzo-1,4-quinone itself is yellow and benzo-1,2-quinones are often red. Chloranil forms glistening yellow platelets on crystallization from fuming nilric acid Quinones occur naturally and some natural pigments contain a quinone unit 

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The transformation of isoquinoline has been studied both under photochemical conditions with hydrogen peroxide, and in the dark with hydroxyl radicals (Beitz et al. 1998). The former resulted in fission of the pyridine ring with the formation of phthalic dialdehyde and phthalimide, whereas the major product from the latter reaction involved oxidation of the benzene ring with formation of the isoquinoline-5,8-quinone and a hydroxylated quinone. 

Figure 8-40. The prolonged reaction of [Co(phen)3]3+ or [Co(5-nitrophen)3]3+ with a mixture of nitric and sulfuric acids yields a quinone, derived from the oxidation of the benzene ring of the ligand.

Benzothiophene. There have been several reports on the aerobic co-metabolism of benzothiophene (42, 46, 47). Bohonos et al. (46) identifed some benzothiophene metabolites by GC-MS and the structures of these are shown in Figure 8. Although the only compounds found were oxidized on the thiophene ring, they could not exclude the possibility of oxidation of the benzene ring. Finnerty et al. (48) found that benzothiophene was transformed to unidentified water-soluble products by a dibenzothiophene-oxidizing bacterium. 

Brucine behaves similarly but, under much more vigorous conditions (hydrogen peroxide and formic acid at 60°), oxidation of the benzene ring occurs, and a 48% yield of C19 Hannsen acid is obtained, a much higher yield than is obtained with Cr03 (143). 

Partial oxidation of the benzene ring resulted when 2-cyclopropylphenol was treated with lead(IV) acetate. ... 

From a comparison of the growth response of Neurospora to kynurenic and xanthurenic acids with that of a number of nicotinic acid derivatives, it was concluded that the pyridine ring of nicotinic acid is not formed by kynurenine first being converted to kynurenic acid or xanthurenic acid, and this in turn being followed by the oxidation of the benzene ring moiety to yield nicotinic acid as a residium. ... 

By oxidation of the benzene ring. The benzene ring of an alkylbenzene can be converted to a carboxyl group by ozonolysis, followed by treatment with hydrogen peroxide (Section 15.13D) ... 

Figure 5.33 Ring-opening oxidation of the benzene ring in poiystyrene...

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