The Reduction of Indigo

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... 

The reduction of indigo by electrolytic hydrogen in alkaline suspension, the fluid being warmed, has already been carried out by Fr. Goppelsroder3 and v. Wartha.4 Mullerus 5 easily reduced indigosulphonic acid. 

Thorough studies regarding the process of the reduction of indigo by the electric current have recently been made by A. Binz,6 and Binz and Hagenbach7 these show that most probably zinc and not hydrogen plays the chief part in the reduction. Thus when indigo is electrolytically reduced in... 

Sodium hydrosulfite can be used as the white crystalline product of commerce, or it can be prepared in the. course of the reaction by adding zinc dust to a solution of sodium bisulfite. The latter procedure has been employed for the reduction of indigo and for the preparation of o-aminophenol from the corresponding nitro derivative. The reaction involved is as follows ... 

Before the availability of sodium dithionite (1871), the reduction of indigo was brought about by bacteria with reducing properties (fermentation vat). The vat of indigo has a brown-yellow colour. The cloth is dipped in the vat and then exposed to air to allow reoxidation to indigo which is precipitated and finely distributed onto the fibre. A consequence of this process is the low rubbing fastness of the dyes. It causes the faded appearance of indigo-dyed jeans and makes possible the manufacture of faded jeans . 

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. ... 

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. 

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... 

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. 

An interesting application of electrolytic reduction to vat-dyeing is the process of the Farbwerke vorm. Meister Lucius and Briining, in which alkali sulphite is electro-lytically reduced in the presence of indigo. In an acid solution, indigo white is precipitated.2... 

Aniline (also know as aminobenzene and benzamine) was first produced in 1826 by Unverdorben by the dry distillation of indigo, the oldest known vat dye. Fritsche also obtained aniline from indigo by heating it with potash (K2O), and he named it aniline. Hofmann obtained aniline by the reduction of nitrobenzene in 1843 and was able to prove the structure. Aniline is a colorless, oily, flammable liquid that is slightly soluble in cold water and infinitely soluble in alcohol and ether. Its physical properties are summarized in Table 20.1. 

From Di-phenyl Di-acetylene.—The proof that the two isatin or indole residues are linked together by the carbons is in the synthesis of indigo from di-phenyl di-acetylene, CeHs—C = C—C = C—CeHs. This compound by conversion into the di- (ortho-nitro) product, and treatment with sulphuric acid and reduction with ammonium sulphide, yields indigo. This indicates that the chain of carbon linkings remains as in the above compound. 

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. 

Indol was first obtained by reduction of indigo blue. It is also produced in the pancreatic digestion of albuminoids [2], and by melting the latter with potash. 

From the derivatives of indigo blue (sulphonic acids, c.), substitution-derivatives of indigo white are obtained on reduction. 

In turn, the reduction of palygorskite-attached indigo to palygorskite-attached... 

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