Biferrocene-Based Diphosphine Ligands: Synthesis and Application of Walphos Analogues in Asymmetric Hydrogenations

A total of four biferrocene-based Walphos-type ligands
have been
synthesized, structurally characterized, and tested in the rhodium-,
ruthenium- and iridium-catalyzed hydrogenation of alkenes and ketones.
Negishi coupling conditions allowed the biferrocene backbone of these
diphosphine ligands to be built up diastereoselectively from the two
nonidentical and nonracemic ferrocene fragments (R)-1-(N,N-dimethylamino)­ethylferrocene
and (SFc)-2-bromoiodoferrocene. The molecular
structures of (SFc)-2-bromoiodoferrocene,
the coupling product, two ligands, and the two complexes ([PdCl2(L)] and [RuCl­(p-cymene)­(L)]­PF6) were determined by X-ray diffraction. The structural features of
complexes and the catalysis results obtained with the newly synthesized
biferrocene-based ligands were compared with those of the corresponding
Walphos ligands.