Memory Effects in Palladium‐Catalyzed Allylic Alkylations of 2‐Cyclohexen‐1‐yl Acetate

Abstract

Abstract 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 (η 3 ‐allyl)PdLCl, where L is a monophosphine ligand [PPh 3 , PCy 3 , 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 η 3 ‐allyl intermediate and nucleophilic attack. However, the observed enantiospecificity is relatively low due to isomerization of the η 3 ‐allyl intermediate and/or dynamic equilibria between the catalytically active (η 3 ‐allyl)PdLCl species and [(η 3 ‐allyl)PdL 2 ] + or [(η 3 ‐allyl)PdCl] 2 . It was also observed experimentally that increasing the bulk of the phosphine inhibits formation of the [(η 3 ‐allyl)PdL 2 ] + complexes, significantly increasing the observed enantiospecificity for some of the ligands.