Structural and kinetic effects of chloride ions in the palladium-catalyzed allylic substitutions

Abstract

Addition of ligands to [Pd(η 3-RCHCHCH 2)(μ-Cl)] 2 or chloride ions to cationic [(η 3-RCHCHCH 2)PdL 2] +BF 4 − induces the formation of neutral complexes η 1-RCHCHCH 2PdClL 2 (R=H with L=(4-ClC 6H 4) 3P, (4-CH 3C 6H 4) 3P, (4-CF 3C 6H 4) 3P or L 2=1,2-bis(diphenylphosphino)butane (dppb), 1,1′-bis(diphenylphosphino)ferrocene (dppf); R=Ph with L=(4-ClC 6H 4) 3P), instead of the expected cationic complexes [(η 3-RCHCHCH 2)PdL 2] +Cl −. In the presence of chloride ions, the reaction of morpholine with the cationic complexes [(η 3-allyl)Pd(PAr 3) 2] +BF 4 − (Ar=4-ClC 6H 4, 4-CH 3C 6H 4) goes slower and involves both cationic [(η 3-allyl)Pd(PAr 3) 2] + and neutral η 1-allyl-PdCl(PAr 3) 2 complexes as reactive species in equilibrium with Cl −. The cationic complex is more reactive than the neutral one. However, their relative contribution in the reaction strongly depends on the chloride concentration, which controls their relative concentration. The neutral η 1-allyl-PdCl(PAr 3) 2 may become the major reactive species at high chloride concentration. Consequently, [Pd(η 3-allyl)(μ-Cl)] 2 associated with ligands or cationic [(η 3-allyl)PdL 2] +BF 4 −, used indifferently as precursors in palladium-catalyzed allylic substitutions, are not equivalent. In both situations, the mechanism of the Pd-catalyzed allylic substitution depends on the concentration of the chloride ions, delivered by the precursor or purposely added, that determines which species, [(η 3-allyl)PdL 2] + or/and η 1-allyl-PdClL 2 are involved in the nucleophilic attack with consequences on the rate of the reaction and probably on its regioselectivity. Consequently, the chloride ions of the catalytic precursors [Pd(η 3-allyl)(μ-Cl)] 2 must not be considered as ‘innocent’ ligands.