Oxygen Donor-Mediated Equilibration of Diastereomeric Alkene−Palladium(II) Intermediates in Enantioselective Desymmetrizing Heck Cyclizations

This investigation examines the origin of enantioselection in the desymmetrization of an acyclic prochiral Heck cyclization precursor. High asymmetric induction (97−98% ee) is attributed to a temporary interaction of a Lewis basic oxygen donor with weakly Lewis acidic palladium(II). A series of control experiments combined with quantum-chemical model calculations provided sound evidence for a mechanism involving oxygen donor-mediated, rapid equilibration of diastereomeric alkene−palladium(II) complexes prior to the selectivity-determining ring-closing event, a Curtin-Hammett scenario. Our study also highlights the importance of the cationic pathway (triflate counter anions versus halido ligand) and alkene stereochemistry (E versus Z) in asymmetric Heck reactions.