Novel C2-symmetric planar chiral diphosphine ligands and their application in Pd-catalyzed asymmetric allylic substitutions

Novel C-2-symmetric diphosphine ligands possessing only the planar chirality of ruthenocene, 1,1'-bis(diphenylphosphino)-2,2'-disubstituted-ruthenocenes (4), were prepared. With this kind of ligands, excellent enantioselectivity and especially highly catalytic activity in palladium-catalyzed asymmetric allylic substitutions of rac-1,3-diphenyl-2-propenyl acetate (9) were observed, compared to their ferrocene analogues 1. Good enantioselectivity and highly catalytic activity were also obtained with 4 in palladium-catalyzed asymmetric allylic substitutions of cyclohexen-1-yl acetate (12). Further study on the effect of R in ester group on enantioselectivity of 4 showed an opposite trend compared with their ferrocene analogues 1 in asymmetric allylic substitutions. For ruthenocene ligands 4, the one with the smaller R in the ester group gave higher enantioselectivity for the palladium-catalyzed asymmetric allylic substitutions of 9, while a converse trend had been observed with 1. However, for the palladium-catalyzed asymmetric allylic substitutions of 12, ligand 4 with a larger R in the ester group resulted in somewhat higher enantioselectivity but still an opposite trend with ligand 1. The X-ray diffraction study of crystal structures of 4 and 1 with Pd(II) was carried out and showed that the enantioselectivity was correlated to the twist angle existing in the palladium complex.