Indole cations, tautomerism

C-Aminoindoles autoxidize extremely rapidly. Consequently, comparatively few chemical reactions have been examined. The 2-amino derivative exists in the 3H-indole tautomeric form (473) and is protonated and alkylated on the annular nitrogen atom (72HC(25-2)179). The 1-methyl derivative (474) exits predominantly as such and not as the alternative 2-imino-3//-indole tautomer and is protonated at the 3-position to give a cation having the same electronic structure as that of the protonated (473). Acylation of (473) yields l-acetyl-2-acetylaminoindole, via the initial acylation of the annular nitrogen atom. Confirmation of this route has been established by the observation that 2-acetylaminoindole, obtained by hydrolysis of the diacetylated compound, is acetylated under identical conditions... 

This reaction involves the elelctrophilic attack of an aryldiazonium cation on the enol carbon atom of an active methinyl compound to give an azo intermediate, which then hydrolyzes to form an arylhydrazone. Tautomerization of aryl hydrazone yields the vinyl hydrazine, which undergoes [3,3] sigmatropic rearrangement and subsequently cyclizes to give an indoline derivative which, upon evolution of ammonium ion, yields the indole derivative, as shown below. 

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