Palladium(Ⅱ)-Catalyzed Enyne Coupling Reaction Initiated by Acetoxypalladation of Alkynes and Quenched by Protonolysis of the Carbon-Palladium Bond

摘要

Divalent palladium-catalyzed inter- and intramolecular enyne coupling reactions initiated by acetoxypalladation of alkynes were developed. The reaction involves the acetoxypalladation of the alkyne, followed by the insertion of the alkene and the protonolysis of the carbon-palladium bond. The protonolysis of the carbon-palladium bond in the presence of bidentate nitrogen containing ligands is the key step in completing the Pd(Ⅱ) catalytic cycle. The nitrogen-containing ligands, like halides, served to favor the protonolysis of the carbon-palladium bond over the β-H elimination in the Pd(Ⅱ)-mediated reactions. The intermolecular coupling reactions provide an efficient method for synthesizing γ,δ-unsaturated carbonyls. The intramolecular coupling reactions offer a method to construct a variety of synthetically important carbo- and heterocycles. The asymmetric version of such a cyclization was developed with moderate enantioselectivity when employing pymox (pyridyl monooxazoline) as the ligand.