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.
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People's Republic of China.
Delong
D
Liu
Fang
F
Xie
Wanbin
W
Zhang