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 0.5h 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.