N‐Heterocyclic‐Carbene‐Catalyzed Redox Lactonization of o ‐Hydroxycinnamaldehydes and o ‐Hydroxycinnamyl Alcohols to Coumarins

An N‐heterocyclic carbene derived from 1,3‐dibenzylbenzimidazolium bromide (NHC‐ G ) not only allowed the intramolecular esterification of o ‐hydroxycinnamaldehydes, but also favored the one‐pot domino allylic oxidation/redox lactonization of both ( E ) and ( Z )‐ o ‐hydroxycinnamyl alcohols, thus these two protocols can provide coumarins in 30–98 % yields in 0.5 h under air. Moreover, the mechanism was also speculated, and it was shown that the presence of oxidants accounted for the formation of coumarins over dihydrocoumarins. This strategy provides a simple, efficient and sustainable route for coumarin synthesis.