Metalloporphyrin-catalysed oxidation of azonaphthol dyes: the mechanism of oxidative bleaching by oxoiron(IV) porphyrins in aqueous solution

Abstract

Publisher Summary The generally accepted mechanism for phenolic oxidations catalyzed by horseradish peroxidase (HRP), the most thoroughly studied peroxidase, involves an initial two-electron oxidation of the iron(III) resting state of the enzyme by the hydroperoxide (or hydrogen peroxide) to give compound I (HRP I), that is, subsequently reduced back to the iron(III) haem, in two one-electron steps, via compound II (HRP II). The phenoxyl radical intermediates react to give products that depend on their structure and may further oxidize on their own. This chapter discusses the results from the investigations of the mechanism of oxidation of 1-phenylazo-2-naphthol-6-sulfonates by oxoiron(IV) tetra(2,6- ichloro-3-sulfonatophenyl) porphyrin in aqueous solution. The research discussed in the chapter shows that, at pH 7.6, oxidation proceeds by hydrogen atom abstraction by the oxometallo(IV) species from the phenol rather than electron transfer from the phenolate anion. Preliminary studies with commercial azo dyes has revealed that these are also substrates for oxoiron(IV) porphyrins. The azo dyes used in the present experiment were 1-phenylazo-2-naphthol-6-sulfonate and seven derivatives with substituents in the meta or para positions of the phenyl ring. These were selected as representative 1-azo-2-naphthol sulfonate dyes and because the substituents on phenyl ring would allow a systematic study of the mechanism of their oxidation by metalloporphyrin-catalysed systems in aqueous solution. Dye 2 is commercially available (as Acid Orange 12) and is purified by recrystallization, while the others were prepared by standard diazonium ion/2-naphthol coupling reactions.