Exploring an anomaly: the synthesis of 7,7'-diazaindirubin through a 7-azaindoxyl intermediate.

Two independent methods generating 7-azaindoxyl as an intermediate verify that 7,7'-diazaindirubin is formed exclusively over 7,7'-diazaindigo. This contrasts with long-standing knowledge related to the reactivity of indoxyl, which proceeds via a radical-initiated homodimerization process, leading to indigo. A series of experiments confirms 7-azaindoxyl as an intermediate with results suggesting a condensation pathway followed by oxidation.