Protonolysis of the carbon–palladium bond in palladium(II)-catalyzed enyne cyclization in imidazolium-type ionic liquids

The Pd-catalyzed cyclizations of 1,6-enynes are efficient reactions for the synthesis of α-methylene-γ-butyrolactones and lactams. The effects of solvent, proton source, chloride concentration, and temperature on the protonolysis of the carbon–Pd bond were investigated and the optimal reaction conditions were identified. We showed that imidazolium-type ionic liquids played an important role in the reaction both as a ligand for the palladium catalyst and as a solvent. The crystal structure of the Pd complex was obtained and the reaction mechanisms were discussed.