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 e ffective in asymmetric allylic alkylation and amination reactions. Ligand 1 4 furnished the highest enantiomeric excess, 98.5% and 96.5% ee in alkylati on and amination reactions, respectively. The role of planar chirality in a symmetric reactions has been examined, and decisive effects on enantioselec tivity as well as the control of absolute configuration in palladium-cataly zed allylic alkylation and amination reactions were observed. To clarify wh y and how the planar chirality governed the stereochemical outcome, X-ray c rystallographic structures of eta (3)-diphenylallyl Pd complexes, H-1 NMR. P-31 NMR spectra of palladium dichloride complexes, and eta (3)-diphenylall yl 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 tha t planar chirality influences the stereochemical outcome by changing or eve n inverting the ratio of two rotamers because of the steric interaction bet ween a planar chiral group and the coordination site.