Diastereoselective ortho-metalation of stereogenic ferrocenylphosphine oxides.: Asymmetric synthesis of the first enantiopure phosphorus-chiral 2,2′-bis(diarylphosphino)-1,1′-biferrocenyls

Two novel phosphorus-chiral ligands, 2,2'-bis(arylphenylphosphino)-1,1'-biferrocenyls 1a (aryl = 1-naphthyl) and 1b (aryl = 2-biphenylyl), have been prepared in enantiopure form by stereoselective multistep synthesis. While asymmetry on phosphorus was established via nucleophilic substitution reactions at borane-protected phosphorus centers, orthoiodination of the derived optically pure ferrocenylphosphine oxides served as the key step for the introduction of planar chirality. Utilizing biphenylylphenylphosphinoxyferrocene, 5b, the latter reaction was found to proceed in a highly diastereoselective manner, resulting in a product distribution of 97:3. The absolute configuration of the predominantly formed diastereomer was confirmed by crystal structure analysis of the Ullmann-coupled bis-(arylphenylphosphinoxy)biferrocenyl 6b (aryl = biphenylyl). Reduction of dioxides 6a (aryl 1-naphthyl) and 6b gave rise to the enantiopure C-2-symmetrical title compounds comprising four adjacent stereocenters. The coordination behavior of la was investigated by a crystal structure determination of its Pt(II) dichloride complex 8a and compared with the structure of complex 8c, bearing the related 1,1'-bis(1-naphthylphenylphosphino)ferrocene ligand 1c.