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, ortho-iodination 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 1a 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 .