Hydroquinone/benzoquinone mediator

DNA breaks in human MCF-7 cells [65], Damaging effect of menadione was probably mediated by hydroxyl radicals as it was demonstrated by ESR spin-trapping method. The analogs of menadione 2-methylmethoxynaphthoquinone and 2-chloromethylnaphtho-quinone also stimulated DNA damage through the formation of superoxide and other free radicals [66]. Similar effects have been shown for hydroquinone, catechol, benzoquinone, and benzenetriol [67,68]. 

The palladium(II)-mediated oxidative cyclization is also applied to the synthesis of carbazole-l,4-quinone alkaloids. The required arylamino-l,4-benzo-quinones are readily prepared by arylamine addition to the 1,4-benzoquinone and in situ reoxidation of the resulting hydroquinone [131]. 

The Ti02-mediated photocatalytic oxidation reaction can be described by the radical mechanism involving OH as the major reaction species. The reaction mechanism follows the ortho pathway, so that the main intermediate found is 4-chlorocatechol, whereas the formation of 4-chlororesorcinol and hydroquinone is only a minor pathway. Further degradation of 4-chloroca-techol leads to production of hydroquinone, which can be further oxidized and mineralized to carbon dioxide. In contrast, the direct photolysis of 4-chlorophenol follows the para pathway, which leads to the formation of hydroquinone and p-benzoquinone as the major products. 

The //-benzoquinone was used as a catalytic oxidant and electron-transfer mediator with good conversion and selectivity at room temperature. It was also found that the choice of the solvent was critical, the oxidation of hydroquinone to benzoquinone catalyzed by Co(salophen)(PPh3) being more than ten times faster in methylene chloride than in benzene. A significant improvement was obtained by the use of RuClfOAcXPPhs) and molecular sieves to remove the water formed. A further improvement can be expected from the encapsulation of Co(salophen) catalyst. 

Photocatalytic degradation of 4-chlorophenol in TiOz aqueous suspensions produces 4-chlorocatechol, an ortho hydroxylated product, as the main intermediate. This result disagrees with data reported by other researchers, who proposed the formation of a para-hydroxylated product, hydroquinone, as the major intermediate. Results also indicated that further oxidation of 4-chlorocatechol yields hydroxy hydroquinone, which can readily he oxidized and mineralized to carbon dioxide. Complete dechlorination and mineralization of 4-chlorophenol can be achieved. In contrast, direct photolysis of 4-chlorophenol produces hydroquinone and p-benzoquinone as the main reaction products. The photocatalytic oxidation reaction, initially mediated by TiOz, is generated by an electrophilic reaction of the hydroxyl radical attacking the benzene ring. 

As with shikimic acid, considerable improvements in the microbe-cat yzed synthesis of quinic acid from glucose need to be and likely can be achieved. The subsequent oxidation of quinic acid to hydroquinone is another area of research in which fundamental advances in catalysis have to be achieved. The current oxidation of quinic acid to benzoquinone using Mn02 is a stoichiometric oxidation. This runs counter to the current trend toward development of metal-catiyzed oxidations that are catalytic in the amounts of metal required. Although a variety of cooxidants can be employed, elaboration of a metal-catalyzed oxidation in which O2 is the cooxidant would be particularly desirable. Sufficiently mild conditions for oxidation of quinic acid to hydroquinone may be identified whereby overoxidation to benzoquinone is avoided and hydroquinone is obtained directly from quinic acid. Metal-catalyzed oxidations have to be compatible with use of water as the reaction solvent. F referably, elaborated catalysts should be sufficiently robust to mediate the oxidation of quinic acid in clarified, crude fermenter broth. 

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