N-Heterocyclic-Carbene-Catalyzed Imine Umpolung for the Cross-Coupling of Quinoxalin-2-ones with Isatins.

The N-heterocyclic carbene (NHC)-catalyzed umpolung of aldimines using quinoxalin-2-ones for intermolecular reactions is demonstrated. Specifically, NHC-catalyzed cross-coupling of quinoxalin-2-ones with isatins proceeds via the generation of aza-Breslow intermediates by the addition of carbene to the C=N moiety of quinoxalinones followed by interception with isatins to afford diverse oxindoles in moderate to good yields and good functional group compatibility. Moreover, detailed mechanistic studies involving the isolation and characterization of the imidoyl azoliums (oxidized form of the aza-Breslow intermediates) are provided. Considering the significance of scaffolds bearing both quinoxalin-2-one and oxindole moieties in medicine and natural products, the synthesized molecules employing the NHC-catalyzed imine umpolung strategy are likely to find promising applications.