Indoxyl, oxidation

It is an important intermediate in the indigo industry. When phenylglycine is fused with sodamide a good yield of indoxyl is obtained. The indoxyl is easily oxidized to indigo. 

Based on this observation, K. Heumann treated N-phenylglycine [103-01 -5] with alkali and obtained indoxyl (38) (keto form), which on aerial oxidation converted to indigotin ... 

Hydroxyindole (181) represents a well known example of a compound in which the hydroxyl group is to the ring heteroatom. The equilibrium mixture again contains mainly the carbonyl form (182), indoxyl. Deprotonation gives a reactive ambident anion which can be methylated either on oxygen or C-2 (Scheme 73). Indoxyl is easily oxidized to indigo (184), which may be formed by dimerization of the radical (183) produced by electron loss from the anion. 

The quindoline 224 may be prepared by the condensation of indoxyl-2-carboxylic acid with 6-aminopiperonaldehyde in the presence of hydrochloric acid, when decarboxylation and cyclization take place. Nitric acid oxidation of 224 gave an unstable nitrodicarboxylic acid which decarboxylated readily to a nitromonocarboxylic acid formulated as 8-nitro-8-carboline-3-carboxylic acid (225). ... 

From its alkali salt, which is partially hydrolysed, the large indigo white molecule is adsorbed by the fibre and then, in this finely divided condition, dehydrogenated by the oxygen of the air, so that the dye now remains as a fast blue pigment. The oxidation is analogous to that of indoxyl. 

Isatogens (1) and indolones (2) are brightly colored solids that do not occur naturally. Isatogens (I) are more comprehensively named as 3-oxo-3//-indole 1-oxides, or as the 1-oxides of indolones (2), 3-oxo-3H-indoles, or 3//-indol-3-ones. Both series of compounds are numbered in accordance with IUPAC rules. Isatogens were first reported in 18811,2 and the first indolone in 1912.3 Isatogens (1), indolones (2) and indoxyls (3) form an interrelated redox system. Indolones in which there is a methylene or methine substituent in the 2-position tautomerize to the preferred 2-methylene-3//-indol-3-one (indogenide) structures (4).4 Only passing reference will be made to 3 and 4 in this review. 

Indoles (75), indolones (80), indolone hydrates (81), and indoxyls (82) could theoretically be formed in these reactions by loss of water from 77, or by preferential oxidation of the indole or indoline at the 3-... 

The reduction of indolones is described together with the reduction of isatogens. The major products of reduction are indoxyls and diindoxyls. In some cases the indoxyls are oxidized by air to benzoxazines (Section III,B). Indolones are comparable with isatogens as oxidizing agents88... 

Indoxyl and thioindoxyl are easily oxidized, e.g. by K3Fe(QsT respectively, via dimerization of radical intermediates (263). 6... 

Oxidation of indoles.5 Indoles react with K02 in conjunction with 18-crown-6 or benzyl(triethyl)ammonium chloride in THF to give products (1, 2, 3) that arise from an indoxyl hydroperoxide intermediate (a). 

After the ring-closure reaction, the yellow indoxylate present as the di-Na/K salt in the alkaline melt is hydrolyzed with water. Oxidation of the mono salt of indoxylate takes place in air at 80-90° C. A suspension of blue indigo in an aque-... 

The enzyme responsible for cleaving indican is indoxyl- 3-D-glucosidase, which is formed on the plant by microorganisms. The resulting indoxyl (16), which under alkaline conditions is soluble in water, is discharged from the fermentation vat and oxidized to indigo (1) by aeration. The insoluble dye is isolated by precipitation and air drying. 

Russell, G. A. Kaupp, G. Oxidation of carba-nions. IV. Oxidation of indoxyl to indigo in basic solution./. Am. Chem. Soc. 1969, 91, 3851-3859. 

The oxidation of ethyl indoxylate to ethyl indox-anthinate6 and of naphthol to dinaphthol may be accomplished by ferric chloride ... 

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