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.