STEREOSELECTIVITY AND REGIOSELECTIVITY IN PALLADIUM-CATALYZED ALLYLIC ETHERIFICATION

The palladium(0)-catalyzed etherification of various allylic carbonates by phenols allows the easy preparation of various allylic aryl ethers. In the case of an unsymmetrical allylic carbonate, the regioselectivity of the process was temperature dependent. Under thermodynamic control, the less substituted allyl aryl ether was always obtained, while under kinetic control the stereoselectivity was influenced by steric and electronic factors. Phenols having meta or para substituents afford the allylic aryl ether with no selectivity using palladium(0)-dppb as the catalyst and with low chemical yields in the case of phenols bearing electron-withdrawing substituents. Ortho-substituted phenols gave predominantly the less substituted ethers. The phosphine ligand effect was very important on the regioselectivity, ortho-substituted phosphines giving the more substituted allyl aryl ether. The stereochemical course of the allylic etherification was found to be an overall retention of configuration.