Synthesis and application of chiral cyclopropane-based ligands in palladium-catalyzed allylic alkylation.

A series of chiral, cyclopropane-based phosphorus/sulfur ligands have been synthesized and evaluated in the palladium-catalyzed allylic alkylation of 1,3-diphenylpropenyl acetate with dimethyl malonate. Variation of the ligand substituents at phosphorus, sulfur, and the carbon backbone revealed 24d to have the optimal configuration for this reaction, giving the product in high yield and with good enantioselectivity (93%). A model for the observed enantioselectivity is discussed within the context of existing models, using X-ray crystallographic data, solution-phase NMR studies, and the absolute stereochemistry of the products. Selected ligands were also evaluated in the palladium-catalyzed intermolecular Heck reaction and the rhodium-catalyzed hydrogenation of a dehydroamino acid.