Indoxyl tautomerism

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. 

The C-2 and C-3 hydroxy derivatives of pyrrole are special in the sense that the tautomeric equilibria favor the pyrrolinone structures (see Section 3.04.6.2). Furthermore, the general synthetic methods are not usually applicable so that we will call attention in this section not only to the methods of directly introducing these substituents, which are rare, but also to those ring construction processes which specifically give the pyrrolinones and indolinones. The indole derivatives have widely used trivial names, oxindole (5) for indolin-2-one and indoxyl (6) for indolin-3-one, Carbocyclic hydroxy substituents in indole and carbazole, on the other hand, for the most part act as normal aromatic phenolic groups. These compounds are usually prepared by application of the standard ring syntheses. 

Hydroxyindole certainly contribntes in the tautomeric equilibrium with the carbonyl form, though it is the minor component. Indoxyl, 10.46, ° is more acidic than oxindole, the anion produced is ambident reactions with electrophiles at both oxygen and carbon are known. ... 

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