Application of Chiral Mixed Phosphorus/Sulfur Ligands to Palladium-Catalyzed Allylic Substitutions

Abstract

A modular approach to the synthesis of a class of mixed phosphorus/sulfur ligands was designed to identify important ligand structural features for enantioselective palladium-catalyzed allylic subsitutions of acyclic and cyclic ayllic esters. After a systematic variation of the ligand substituents at sulfur, phosphorus, and the ligand backbone, ligand 11k was found to be optimal in the palladium-catalyzed allylic substitution of 1,3-diphenylpropenyl acetate with dimethyl malonate or benzylamine in high yield and excellent enantioselectivity (95−98% ee). A similar optimization of the mixed phosphorus/sulfur ligand for the palladium-catalyzed allylic substitution of cycloalkenyl acetates showed that 49g afforded the highest enantioselectivities (91−97% ee). Application of this methodology to heterocyclic substrates was developed as an efficient approach to the enantioselective synthesis of 3-substituted piperidines and dihydrothiopyrans. Models for asymmetric induction are discussed based on the absolute stereochemistry of the products, X-ray crystallographic data, and NMR spectroscopic data for relevant π-allyl complexes.