Chirality in the Absence of Rigid Stereogenic Elements: The Absolute Configuration of Residual Enantiomers of C₃‐Symmetric Propellers

Abstract

Two new tris(aryl)phosphane oxides existing as configurationally stable residual enantiomers have been synthesised and their racemates resolved by semipreparative HPLC on a chiral stationary phase (CSP HPLC). One of them, recognised as a conglomerate, could be resolved by fractional crystallisation at a preparative scale level. In this case, the absolute configuration of the propeller-shaped molecule was determined by anomalous X-ray scattering. The problem of the correlative assignment of the absolute configuration to all known C(3)-symmetric three-bladed propeller-shaped molecules existing as stable residual enantiomers is discussed. The configurational stability of the new chiral phosphane oxides and of the corresponding phosphanes was evaluated by CD signal decay kinetics and dynamic (1)H NMR spectroscopy. The racemisation barriers in phosphanes were found about 10 kcal mol(-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)phosphanes was found to be influenced by the electronic availability of the phosphorus centre, as evaluated by electrochemical CV experiments.

Synthesis of a New C3-Symmetric Tripodal P4-Tetradentate Ligand and Its Application to the Formation of Chiral Metal Complexes

“…The structure of this compound was established by means of NMR spectroscopy (see Figures S1 and S2 in the Supporting Information) and finally confirmed by X-ray analysis of a single crystal grown in CH 2 Cl 2 /hexane (Figure ). The perspective view showed the three-blade propeller shape of the molecule …”

Chirality in the Absence of Rigid Stereogenic Elements: Steric and Electronic Effects on the Configurational Stability of C3 Symmetric Residual Tris‐Aryl Phosphanes

“…a, dotted curves). The configuration descriptors P res and M res have been specifically coined by us for describing the residual stereogenicity of three‐bladed propellers …”

“…The helix inversion barriers have been evaluated by using classical off‐column kinetic experiments in CHCl 3 solution at 55°C, combined with enantio‐ or diastereoselective HPLC. All the phosphane‐oxides were found to be about 10 kcal mol ‐1 more stable than the corresponding phosphanes, proving that the phosphane‐oxides are configurationally much more stable even if the dynamic gearing in phosphane‐oxides was slightly lower than in phosphanes (Table ) …”

Steric and Electronic Effects on the Configurational Stability of Residual Chiral Phosphorus‐Centered Three‐Bladed Propellers: Tris‐aryl Phosphane Oxides

“…[8] We recently reported the synthesis, structural characterization, and resolution of phosphane oxides 1 and 7. [6] We were able to assign the absolute configuration to the residual antipodes of 7 by anomalous X-ray scattering and, by correlative methods, to all other residual enantiomers, both carbon-and phosphorus-centered, known so far, and they display configurational stability high enough to allow their survival during resolution by HPLC on a chiral stationary phase (CSP). We also coined configurational descriptors for the residual enantiomers of three-bladed propellers.…”

“…We have focused our recent attention on phosphorus-centered C 3 propellers, [5,6,7] since access to a unconventional class of chiral tris-aryl phosphanes as ligands for transition metals in homogeneous stereoselective catalysis appeared to be an exciting challenge, particularly considering that hindered tris-aryl phosphanes are the promoters of choice in several metal-catalyzed reactions. [8] We recently reported the synthesis, structural characterization, and resolution of phosphane oxides 1 and 7.…”