Indigo reduction

Aniline (hen enamine) [62-53-3] is the simplest of the primary aromatic amines. It was first produced ia 1826 by dry distillation of indigo. In 1840 the same oily hquid was obtained by heating indigo with potash, and it was given the name aniline. The stmcture of aniline was estabUshed in 1843 with the demonstration that it could be obtained by reduction of nitrobenzene. 

Addition of sodium dithionite to formaldehyde yields the sodium salt of hydroxymethanesulfinic acid [79-25-4] H0CH2S02Na, which retains the useful reducing character of the sodium dithionite although somewhat attenuated in reactivity. The most important organic chemistry of sodium dithionite involves its use in reducing dyes, eg, anthraquinone vat dyes, sulfur dyes, and indigo, to their soluble leuco forms (see Dyes, anthraquinone). Dithionite can reduce various chromophores that are not reduced by sulfite. Dithionite can be used for the reduction of aldehydes and ketones to alcohols (348). Quantitative studies have been made of the reduction potential of dithionite as a function of pH and the concentration of other salts (349,350). 

However the earliest example in the literature of a reaction that proceeds via the putative intermediates invoked in the Pfitzinger reaction can be traced to the formation of quindoline-5-carboxylic acid 10, a product of over-reduction of 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... 

Tests using indigo have shown that reduction with alkaline hydroxyacetone is most reproducible when the concentrations are as follows [237] ... 

Figure 12.23 Rate of reduction of indigo with and without ultrasound [237], 0.1 g/l Indigo, 40 °C 2.5 ml/l hydroxyacetone 5.0 g/l sodium hydroxide, pH 12.7 0.03 g/l anionic dispersing agent...

The synthesis of indigo was much more difficult than that of alizarin (6.2) [43]. In 1865 von Baeyer first attempted to obtain indigo by reductive dimerisation of isatin (6.116) he finally achieved a seven-step synthesis from phenylglycine via isatin in 1878. Many syntheses have been developed subsequently for indigo, but very few of these have achieved industrial importance. 

Scheme 112 Cathodic reduction of indigo to leuco-indigo with iron complexes.

The reduction of indigo (293), which is insoluble in water and alkaline solutions, to the disodium salt of leuco-indigo (294), which is soluble in alkaline solutions, is crucial to the application of... 

Numerous experimental systems verified the theory of surface electrode reactions. Reductions of methylene blue [92], azobenzene [79, 82] alizarine red S [93], probucol [94], molybdenum(V)-fulvic acid complex [95], molybdeniun(VI)-1,10 phenanthroline-fulvic acid complex [96], indigo [97], and reduction of vana-dium(V) [98] at a mercury electrode are some of the examples for surface electrode... 

A considerable improvement of the situation could be obtained by the use of prereduced indigo instead of the reduction of the dyestuff in a stock vat [58]. By use of prereduced indigo the sulfate concentration in the wasted water can be lowered to approximately 50% of the initial value. 

Somei et al. reported the total synthesis of the cytotoxic and antiviral 5-cyano-6-methoxy-ll-methylindolo[2,3-fl]carbazole (357) starting from indigo (1458) (786,787). A reduction of indigo (1458) with tin in acetic acid/acetic anhydride afforded 3-acetoxy-2,2 -bisindolyl (1459) in 88% yield. Heating of 1459 with dichloroacetyl chloride in ethyl acetate under reflux provided 3-acetoxy-3 -dichloroacetyl-2,2 -bisindolyl (1460), which was treated with aqueous ammonia in methanol/DMF at room temperature to give the indolo[2,3-fl] carbazole derivative 1461. N-Methylation... 

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,... 

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

Bond AM, Marken F, Hill E, Compton RG, Hiigel H (1997) The electrochemical reduction of indigo dissolved in organic solvents and as a solid mechanically attached to a basal plane pyrolytic graphite electrode immersed in aqueous electrolyte solution. J Chem Soc Perkin Trans 2 1735-1742. 

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