Formation of Coumarins by Palladium(II)-Catalyzed Reaction of Phenols with Ethyl Acrylates

The reaction of phenols with ethyl acrylates in the presence of a Pd(OAc)2 catalyst in trifluoroacetic acid did not yield dihydrocoumarins but gave coumarins, in contrast to the reaction of phenols with propiolates. The addition of K2S2O8 as an oxidant increased the yield of coumarins. The reaction of several phenols with ethyl cinnamate, ethyl crotonate, or ethyl acrylate gave the corresponding coumarin derivatives in moderate to good yields. The coumarin formation competed with the oxidative coupling of electron-rich phenols, which reduced the product yield.