N‑Heterocyclic Carbene-Catalyzed Umpolung of Imines for the Enantioselective Synthesis of Dihydroquinoxalines

N-heterocyclic
carbene (NHC) organocatalysis is widely employed
for the umpolung of aldehydes and recently to the umpolung of Michael
acceptors and aldimines. Described herein is the NHC-organocatalyzed
umpolung of aldimines for the enantioselective synthesis of nitrogen
heterocycles. The bisimines generated from the condensation of 1,2-phenylenediamines
and salicylaldehydes undergo intramolecular cyclization in the presence
of a chiral NHC catalyst, resulting in the formation of dihydroquinoxalines
in moderate to good yields and er values. Detailed DFT studies shed
light on the role of −OH groups in stabilizing the initially
generated aza-Breslow intermediates via intramolecular hydrogen bonds.
Preliminary photophysical studies on the synthesized dihydroquinoxalines
revealed that these molecules can be used for the sensing of various
acids and bases.