C2-symmetric diphosphine ligands with only the planar chirality of ferrocene for the palladium-catalyzed asymmetric allylic alkylation

Novel C-2-symmetric diphosphine ligands possessing only the planar chirality of ferrocene, 1,1'-bis(diphenylphosphino)-2,2'-disubstituted-ferrocenes were prepared from 1,1'-bis(diphenylphosphino)-2,2'-bis(oxazolinyl)ferrocene by the transformation of the oxazoline moieties in the molecule. Upon complexation with palladium(II), ligands of this kind were ascertained to form only one of the two possible diastereomeric 1:1 P,P-chelating palladium complexes by H-1 NMR spectroscopy using NOE. These ligands afforded excellent enantioselectivity for the palladium-catalyzed allylic alkylation of 1,3-diphenylprop-2-en-1-yl acetate with dimethyl malonate. The substituents at the 2,2'-positions of the Cp rings of the ligands had considerable effect on the catalytic activity but minor effect on the enantioselectivity. These ligands also afforded excellent catalytic activity and good enantioselectivity for the allylic alkylation of the less sterically hindered substrate, cyclohex-2-en-1-yl acetate, with dimethyl malonate. It has been shown that the C-2-symmetric ferrocene ligands with only the planar chirality can produce an excellent chiral environment for metal-catalyzed asymmetric reactions.