Memory effects in palladium-catalyzed allylic Alkylations of 2-cyclohexen-1-yl acetate

The objective of this work was to characterize the enantiospecificity of the allylic alkylation of enantioenriched 2-cyclohexen-1-yl acetate with the enolate ion of dimethyl malonate catalyzed by unsymmetrical palladium catalysts. The precatalysts employed were (eta(3)-allyl)PdLCl, where L is a mono-phosphine ligand [PPh3, PCy3, P(2-BiPh)Cy-2, or P(t-Bu)(3)], all of which afforded enantiospecificity to some extent (5-47 %). Quantum mechanical calculations show that, theoretically, the enantiospecificity should be high due to a preference for the trans to P transition state in both formation of the eta(3)-allyl intermediate and nucleophilic attack. However, the observed enantiospecificity is relatively low due to isomerization of the eta(3)-allyl intermediate and/or dynamic equilibria between the catalytically active (eta(3)-allyl)PdLCl species and [(eta(3)-allyl)PdL2](+) or [(eta(3)-allyl)PdCl](2). It was also observed experimentally that increasing the bulk of the phosphine inhibits formation of the [(eta(3)-allyl)PdL2](+) complexes, significantly increasing the observed enantiospecificity for some of the ligands.