Synthesis and characterization of a chiral, aza-15-crown-5-functionalized ferrocenyldiphosphine ligand for asymmetric catalysis

A chiral ferrocenyldiphosphine ligand that is functionalized with an aza crown ether, (S)1-[(R)-1',2-bis(diphenylphosphino)ferrocenyl]ethyl-1-aza-2,3-benzo-15-crown-5 (1), has been synthesized. Both the resolved and racemic ligands react rapidly with Pt(II) precursors to form stable metal-ligand adducts; the complexes PtMeI(rac-1) and PtMe2(rac-1) have been characterized crystallographically. Reaction of rac-1 with [Rh(NBD)(2)]OTf yields [Rh(NBD)(rac-1)]OTf. The three-dimensional solution structure of [Rh(NBD)(rac-1)]OTf has been determined by NOESY experiments and analysis using the two-dimensional conformer population analysis algorithm (2DCPA). The NOESY data reveal a rapid, pairwise chemical exchange between vinyl protons. The complex [Rh(NBD)(rac-1)]OTf is a catalyst precursor for hydrogenation reactions. However, we demonstrate that the lability of the aza crown ether may limit the ability of these catalysts to control selectivity via secondary interactions.