Chirality in the absence of rigid stereogenic elements: the absolute configuration of residual enantiomers of C3-symmetric propellers.

Two new tris(aryl)phosphine oxides existing as configurationally stable residual enantiomers have been synthesized and their racemates resolved by semipreparative HPLC on a chiral stationary phase (CSP HPLC). One of them, recognized as a conglomerate, could be resolved by fractional crystn. at a preparative scale level. In this case, the abs. configuration of the propeller-shaped mol. was detd. by anomalous x-ray scattering. The problem of the correlative assignment of the abs. configuration to all known C3-sym. three-bladed propeller-shaped mols. existing as stable residual enantiomers is discussed. The configurational stability of the new chiral phosphine oxides and of the corresponding phosphines was evaluated by CD signal decay kinetics and dynamic 1H NMR spectroscopy. The racemization barriers in phosphines were found about 10 kcalmol-1 lower than those found for the corresponding oxides, though geometry and inter-ring gearing would be very similar in the two series. Configurational stability of residual tris(aryl)phosphines was influenced by the electronic availability of the phosphorus center, as evaluated by electrochem. CV expts