Trans effect of different coordinated atoms of planar chiral ferrocene ligands with the same backbone in palladium-catalyzed allylic substitutions

A series of planar chiral P,S-, P,(C=N)-, and S,(C=N)-bidentate ferrocenyl ligands 4, 5, 8, and 9 with the same central and planar chiralities were prepared from the commercially available starting material N,N-dimethyl-(S)-alpha-ferrocenylethylamine. A Pd-catalyzed asymmetric allylic alkylation reaction was used as a model reaction to study the trans effect of the coordination atoms by these ligands. On the basis of the reaction results and the study of X-ray diffraction as well as H-1 and C-13 NMR spectra of the complexes (12-15), the sequence of the trans effect was established as C=N > P > S, which was supported by molecular modeling at the PM3 level.