In Search of Non-conventional Stereogenic Elements for Designing Chiral Ligands for Homogeneous Stereoselective Catalysis: the Residual C3 Phosphines and Phosphine Oxides

All kinds of rigid tridimensional stereogenic elements, centers, axes, planes and screw helicity, have been employed in designing the architectures of chiral phosphine and diphosphine ligands to be employed as mediators in asymmetric catalysis. Tris-arylphosphines and phosphine oxides bearing three identical aromatic or heteroaromatic rings are generally considered operationally achiral compounds, even though it is known that the preferred arrangement of the aryl rings is described by a three-bladed propeller in which the phosphorus atom is the hub. Chemists have always accepted their configurational instability as an unavoidable event due to the very easy helix reversal, which corresponds to an enantiomerization process, and rarely contemplated the possibility of taking profit of their inherent chirality. The research demonstrates that, on the basis of an accurate structural design, it is possible to accede to C3 symmetric tris-arylphosphines and phosphine oxides 1 and 2, which are configurationally stable even at quite high temperature.1 Rotation of the rings is fast, but helix reversal is a disfavored process. The physical principle that gives rise to configurationally stable enantiomers (residual enantiomers) is the correlated rotation of the rings, a stereodynamic phenomenon studied by Mislow in the Eighties2 and considered so far an interesting facet of organic theoretical stereochemistry devoid of any possibility for applicative extensions.