Oxidation-reduction indigo

Fig. 3.12 The dependence on pH of the oxidation-reduction potential for c0x = cRcd (1) 6-dibromphenol indophenol, (2) Lauth s violet, (3) methylene blue, (4) ferricytochrome c/ferrocytochrome c, (5) indigo-carmine... 

Scheme 3.1 Schematics for the electrochemical oxidation of indigo to dehydroindigo and the electrochemical reduction of indigo to leucoindigo [218]...

Oxidation of indigo results in dehydroindigo (4). Oxidation with permanganate or chromate splits the molecule, forming isatin (5). Oxidation and reduction of the indigo system are accompanied by corresponding changes in the spectroscopic properties (see Section 2.4). 

Recently, Moody et al. reported a biomimetic synthesis of calothrixin B (378) by oxidation of Hibino s 6-formylindole[2,3-fl]carbazole 1555 (870). The key intermediate 6-formyl-indole[2,3-fl]carbazole was readily obtained in six steps from indigo (1458). Using Somei s procedure, indigo (1458) was transformed to the cis-chlorohydrin 1461 in three steps and 50% overall yield (see Scheme 5.247). The reduction of the chlorohydiin 1461 gave 5-hydroxy-indolo[2,3-fl]carbazole 1564, and subsequent Vilsmeier formylation delivered the desired 6-formyl-indole[2,3-fl]carba-zole 1565 in 45% yield. Reaction of hydroxy-indolocarbazole 1565 with an excess of chloromethyl methyl ether (MOMCI) afforded the tiis-MOM-protected compound 1555. Following Hibino s approach, the tris-MOM-protected indolocarbazole 1555... 

Topographic AFM examination of organic dyes under the appiication of reductive and oxidative potentials appears to be consistent with the above modeling [168], This can be seen in Fig. 2.25 where in situ AFM images from the upper face and sides of crystals of indigo (a) before, and (b) after application of a linear potential step between 0.0 and +0.75 V at a potential scan rate of lOmV/s are shown. Here,... 

Many heterocyclic dyes behave analogously to ordinary indigo in that they are decolorized when treated with reducing agents and are regenerated on reoxidization. This applies to the azines, thiazines, ox-azines, and other similar dyes. In contrast to these, the triphenylmethane dyes are decolorized easily by reduction, but their reduction products are usually considerably more difiicult to reoxidize by air, i.e., the leuco compounds in this series are relatively stable. However, they can be very easily and quantitatively oxidized by chloranil (see page 146). 

Isatin, Indole, etc.—By oxidation indigo yields isatin which by successive reductions as recently explained yields di-oxindole, oxindole and finally indole. Also indoxyl, the isomer of oxindole, yields indigo by oxidation. The composition formulas of indigo and these last compounds are as follows ... 

Vat dyeing depends upon the reduction of some dyes (e.g., indigo) to a colorless, or leuco, derivative, which is soluble in dilute alkali. If fabric is immersed in this alkaline solution, the leuco compound is adsorbed by hydrogen bonding. On exposure to air the leuco compound is oxidized to the dye, which remains fixed to the cloth. Vat dyes are all quinonoid substances that are readily reduced to colorless hydroquinonoid compounds that are reoxidizable by oxygen in the air. 

Most vat dyes are sold in the insoluble oxidized form. The first operation, therefore, consists of reduction to the leuco compound and dissolving the latter in alkali, a process commonly referred to as vatting. The classical natural vat dyes such as Indigo and Tyrian Purple were reduced in a fermentation vat. The operation, as described in Chapter 1, was exceedingly laborious and variable in result. There was, therefore, a constant search for an easier and more reliable method. 

To cause these changes, take the yellow solution and slowly pour it from one beaker into another (thus allowing oxygen from the air to dissolve in). The solution will turn red, and then green and then back to yellow. These changes in color are the result of reversible oxidation and reduction reactions that indigo carmine undergoes under the experimental conditions. 

The permanganate index is based on the reduction of permanganate by the polyphenols using the indicator carmino indigo, the color of which varies between blue and yellow according to its oxidation state. The correct endpoint - when the polyphenols are oxidized but not the sugars - is difficult to identify, and furthermore the reproducibility of the method is poor between operators. Nonetheless, it is possible to classify wines according to their... 

Baeyer s most famous researches, on the constitution and synthesis of indigo, began in 1865. Indigo had been investigated by Laurent and by Erdmann, who obtained isatin from it by oxidation (see p. 389). Baeyer showed that isatin on reduction forms dioxindole and then oxindole, which when distilled with zinc dust (a favourite method with Baeyer) is reduced to the parent substance indole ... 

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