Indoxyl oxidative dimerization

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. 

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

In the present case, this was accomplished with isatin-5-sulfonate instead of isatin, using the urea-Unked cyclodextrin dimer to alter the ratio of formation of indigo, through oxidative dimerisation of indoxyl, to indirubin-5 -sulfonate, from indoxyl and the isatin-sulfonate (Figure 3.5). The isatinsulfonate exists in solution in equilibrium with its hydrate, which deprotonates with a pA a of 9.55 to give the corresponding anion... 

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