Benzoquinone-Induced Stereoselective Chloride Migration in (η3-Allyl)palladium Complexes. A Theoretical Mechanistic Study Complemented by Experimental Verification

Benzoquinone-mediated Cl - migration reactions in (η 3 -allyl)palladium complexes were studied using density functional theory at the B3PW91 level. p -Benzoquinone coordinates to palladium in an η 2 fashion, exerting considerable steric and electronic effects which facilitate the ligand migration. Studies involving the commercially available benzoquinone derivatives chloranyl, fluoranyl, and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) show that the activation barrier to the ligand migration can be decreased by employing electron-withdrawing substituents on benzoquinone. Benzoquinone derivatives with large π-acceptor ability and moderately bulky substituents induce ligand migration most effectively. Alkyl substitution of the allyl moiety also lowers the activation energy for Cl - migration. The lowest activation barrier was encountered for an [η 3 -(1,2,3)-cyclohexenyl]palladium complex coordinated to DDQ. Experimental studies verified that, in such types of complexes, Cl - migration is feasible, providing stereodefined 3-chlorocyclohexene products. Since benzoquinone-mediated ligand migration in (η 3 -allyl)palladium complexes is a key step in synthetically important palladium-catalyzed transformations, the implications of the theoretical results are discussed for the cis -migration of other ligands and for the inductive ability of different benzoquinone derivatives.