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

Abstract

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-phenylene diamines and salicylaldehydes undergo intramolecular cyclization in the presence of a chiral NHC catalyst resulting in the for-mation 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 pho-tophysical studies on the synthesized dihydroquinoxalines revealed that these molecules can be used for the sensing of vari-ous acids and bases.