OXIDATION OF CARBANIONS .4. OXIDATION OF INDOXYL TO INDIGO IN BASIC SOLUTION

By a combination of kinetic, stoichiometric, and spectrometric measurements it has been demonstrated that the reaction of indoxyl with oxygen in basic solution involves leucoindigo as an intermediate. A precursor to leucoindigo that is diverted to nonindigo products by either excess oxygen or base is involved. This precursor is most likely the indoxyl radical. Leucoindigo is most likely formed by the coupling of indoxyl radicals. However, a coupling reaction between indoxyl radicals and the indoxyl anion has not been excluded. Organic one-electron acceptors, such as nitrobenzene, duroquinone, or 2-chloro-2-nitropropane, can be used to bring about the oxidative dimerization of indoxyls that are resistant to oxygen (e.g., thioindoxyl). The use of these reagents is particularly beneficial in instances where the indigo itself is readily oxidized by air or peroxide, e.g., N,Nʹ-dimethyl-indigo. The reduction of indigo to indigo radical anion and thus to leucoindigo can be readily demonstrated by esr spectroscopy. Similar reductions of indirubin and thioindigo are readily observed. The reductions of thioindigo or indigo by hydrazobenzene in methanolic base are particularly noteworthy. © 1969, American Chemical Society. All rights reserved.