Phosphorus–olefin chelation in coordinated atropisomeric chiral auxiliaries

Abstract

The atropisomeric chelating auxiliaries MeO-Biphep, Binap, MOP and selected monodentate phosphoramidite type ligands are all capable of using differing aromatic fragments as donors to stabilize coordinatively unsaturated 14- or 16-electron species. MeO-Biphep and Binap make use of a double bond immediately adjacent to one of the P-donors to turn these chelate ligands into six-electron donors. In addition to the P-atom, the MOP class uses the π-electrons of the naphthyl group, not attached to the phosphorus atom, to form a 4e chelate ligand. The MeO–MOP ligands often form weak σ-bonds, derived from the electrons in this naphthyl ring, rather than π-olefin complexes. Phosphoramidites, and some related ligands such as “simple-phos”, use a pendant phenyl group to form an η²-arene, P-chelate. The various bonding modes have been investigated via X-ray, NMR and DFT studies.