BENZOQUINONE-INDUCED STEREOSELECTIVE CHLORIDE MIGRATION IN (ETA(3)-ALLYL)PALLADIUM COMPLEXES - A THEORETICAL MECHANISTIC STUDY COMPLEMENTEDBY EXPERIMENTAL-VERIFICATION

Benzoquinone-mediated Cl- migration reactions in (eta(3)-allyl)palladi um complexes were studied using density functional theory at the B3PW9 1 level. p-Benzoquinone coordinates to palladium in an eta(2) fashion, exerting considerable steric and electronic effects which facilitate the ligand migration. Studies involving the commercially available ben zoquinone derivatives chloranyl, fluoranyl, and 2,3-dichloro-5,6-dicya nobenzoquinone (DDQ) show that the activation barrier to the ligand mi gration can be decreased by employing electron-withdrawing substituent s on benzoquinone. Benzoquinone derivatives with large pi-acceptor abi lity and moderately bulky substituents induce ligand migration most ef fectively. Alkyl substitution of the allyl moiety also lowers the acti vation energy for Cl- migration. The lowest activation barrier was enc ountered for an [eta(3)-(1,2,3)-cyclohexenyl]palladium complex coordin ated to DDQ. Experimental studies verified that, in such types of comp lexes, Cl- migration is feasible, providing stereodefined 3-chlorocycl ohexene products. Since benzoquinone-mediated ligand migration in (eta (3)-allyl)palladium complexes is a key step in synthetically important palladium-catalyzed transformations, the implications of the theoreti cal results are discussed for the cis-migration of other ligands and f or the inductive ability of different benzoquinone derivatives.