Importance of Planar Chirality in Chiral Catalysts with Three Chiral Elements: The Role of Planar Chirality in 2‘-Substituted 1,1‘-P,N-Ferrocene Ligands on the Enantioselectivity in Pd-Catalyzed Allylic Substitution

A series of novel planar chiral 2‘-substituted 1,1‘-P,N-ferrocene ligands 9−11, 14, and 16 were
prepared with diastereopurity >99:1 and found to be effective in asymmetric allylic alkylation and amination
reactions. Ligand 14 furnished the highest enantiomeric excess, 98.5% and 96.5% ee in alkylation and amination
reactions, respectively. The role of planar chirality in asymmetric reactions has been examined, and decisive
effects on enantioselectivity as well as the control of absolute configuration in palladium-catalyzed allylic
alkylation and amination reactions were observed. To clarify why and how the planar chirality governed the
stereochemical outcome, X-ray crystallographic structures of η3-diphenylallyl Pd complexes, 1H NMR, 31P
NMR spectra of palladium dichloride complexes, and η3-diphenylallyl Pd complexes of three 1,1‘-P,N-ferrocene
ligands were analyzed with the aid of COSY and 2D NOESY experiments. All results led to the conclusion
that planar chirality influences the stereochemical outcome by changing or even inverting the ratio of two
rotamers because of the steric interaction between a planar chiral group and the coordination site.