N‑Heterocyclic Carbene Catalysis Exploiting Oxidative Imine Umpolung for the Generation of Imidoyl Azoliums

Although NHC-catalyzed
umpolung of imines are known, the related
reactions under oxidative conditions are limited. Described herein
is the two-step process involving the initial formation of aldimines
from the corresponding aldehydes and 2-amino benzyl alcohols followed
by NHC-catalyzed cyclization proceeding via the imidoyl azoliums under
oxidative conditions. The reaction allowed the synthesis of trifluoromethylated
3,1-benzoxazines in good yields and broad scope. The role of NHC in
the intramolecular cyclization and preliminary mechanistic experiments
are also provided.