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

Addition of ligands to [Pd(η3-RCHCHCH2)(μ-Cl)]2 or chloride ions to cationic [(η3-RCHCHCH2)PdL2]+BF4− induces the formation of neutral complexes η1-RCHCHCH2PdClL2 (R=H with L=(4-ClC6H4)3P, (4-CH3C6H4)3P, (4-CF3C6H4)3P or L2=1,2-bis(diphenylphosphino)butane (dppb), 1,1′-bis(diphenylphosphino)ferrocene (dppf); R=Ph with L=(4-ClC6H4)3P), instead of the expected cationic complexes [(η3-RCHCHCH2)PdL2]+Cl−. In the presence of chloride ions, the reaction of morpholine with the cationic complexes [(η3-allyl)Pd(PAr3)2]+BF4− (Ar=4-ClC6H4, 4-CH3C6H4) goes slower and involves both cationic [(η3-allyl)Pd(PAr3)2]+ and neutral η1-allyl-PdCl(PAr3)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 η1-allyl-PdCl(PAr3)2 may become the major reactive species at high chloride concentration. Consequently, [Pd(η3-allyl)(μ-Cl)]2 associated with ligands or cationic [(η3-allyl)PdL2]+BF4−, 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, [(η3-allyl)PdL2]+ or/and η1-allyl-PdClL2 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(η3-allyl)(μ-Cl)]2 must not be considered as ‘innocent’ ligands.