Steric and Electronic Effects on the Configurational Stability of Residual Chiral Phosphorus-Centered Three-Bladed Propellers : Tris-aryl Phosphanes

A series of tris-arylphosphanes, structurally designed for existing as residual enantiomers or diastereoisomers, bearing substituents differing in size and electronic properties on the aryl rings, have been synthesized and characterized. Their electronic properties have been evaluated on the basis of their electrochemical oxidative potential determined by voltammetry. The configurational stability of residual phosphanes, evaluated either by dynamic HPLC on a chiral stationary phase or/and by dynamic 1H- and 31P-NMR, was found rather modest (barriers of about 18-20 kcal mol-1), much lower than that shown by the corresponding phosphane-oxides (barriers of about 25-29 kcal mol-1). For the first time, the residual antipodes of a tris-arylphosphane have been isolated in an enantiopure state and the absolute configuration assigned to them by single-crystal anomalous X-ray diffraction analysis. In this case, the racemization barrier could be calculated also by CD signal decay kinetics. A detailed computational investigation was carried out to clarify the helix reversal mechanism. Calculations indicated that the low configurational stability of tris-arylphosphanes can be attributed to an unexpectedly easy phosphorous pyramidal inversion which, depending upon the substituents present on the blades, can occur even on the most stable of the four conformers constituting a single residual stereoisomer.