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

Abstract

Benzoquinone-mediated Cl- migration reactions in (η3-allyl)palladium complexes were studied using density functional theory at the B3PW91 level. p-Benzoquinone coordinates to palladium in an η2 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 [η3-(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 (η3-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.

Nature of the interaction between β-substituents and the allyl moiety in (η3-allyl)palladium complexes

“…The same regioselectivity was found for the DDQ (2,3-dichloro-5,6-dicyanobenzoquinone) mediated chloride migration in 11 (Scheme 13). 38 The regiochemistry of the nucleophilic attack was also studied for b-silyl substituted (h 3 -allyl)palladium complexes. 23,36 Complex 19 was reacted with sodium diethyl malonate (Scheme 12), providing the 1,4-substituted product 20 with very high regio-and stereoselectivity.…”

Unusual reactivity of a sterically hindered diphosphazane ligand, EtN{P(OR)2}2, (R = C6H3(Pri)2-2,6) towards (η3-allyl)palladium precursors

“…Szabo has reported the first theoretical study complemented by experimental verification on ligand assisted chloride migration in (g 3 -allyl) palladium complexes. 8 There is only one other example in the literature of chloride migration from a palladium center to the allylic carbon terminus. 9 To our knowledge, complex (2) represents the first example of nucleophilic attack by a chloride ligand at the central carbon atom of a (g 3 -allyl) palladium complex.…”

Ubiquitous Benzoquinones, Multitalented Compounds for Palladium‐Catalyzed Oxidative Reactions

“…The formation of allyl chloride from (η 3 ‐allyl)PdCl is difficult, due to the strong aptitude of Cl – for binding to palladium and its low nucleophilicity. The migration of Cl – was studied by Szabó, with use of (η 3 ‐allyl)PdClQ complexes (Scheme ) 35. DFT investigations showed that quinones with electronegative substituents facilitate the cis ‐migration of Cl – .…”