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.