C₂-Symmetric Chiral Bis(thiazoline) and Bis(oxazoline) Ligands and their Application in the Catalytic Asymmetric Allylic Alkylation

A series of novel C₂-symmetric bis(thiazoline) ligands with dibenzo[a,c]cycloheptadiene backbone as the substituent on the thiazoline ring were synthesized using the simple reagent phosphorus pentasulfide, and their application in catalytic allylic alkylation was investigated by comparison with the corresponding bis(oxazolines). Palladium-bis(oxazoline) complexes furnished good enantioselectivity (up to 87% ee), while bis(thiazolines) only gave up to moderate enantioselectivity (56% ee) with CH₂Cl₂ as solvent. Preliminary explanation of the catalysis mechanism is given for the difference of two types of Pd(0)-ligand complexes.