Steric and Electronic Effects on the Configurational Stability of Phosphorus Centred Three - Bladed Propellers

A series of tris-aryl phosphanes and phosphane-oxides 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 (Figure 1). Their electronic properties have been evaluated through theoretical calculations and experimentally on the basis of their electrochemical oxidative potential determined by voltammetry. The configurational stability of residual phosphanes, evaluated by dynamic 1H- and 31P-NMR analysis and by dynamic enantioselective HPLC was found rather modest (stereomerization barriers of about 17 kcal m-1), much lower than those shown by the corresponding phosphane-oxides which have been always obtained in an enantiopure state. Their helix reversal barriers could be inferred by off-column kinetics monitored by HPLC (about 28 kcal m-1) on enantiopure antipodes. In accordance with calculations, an unexpectedly low barrier for phosphorous pyramidal inversion was invoked as responsible for the scarce configurational stability of the residual tris-arylphosphanes. A strategy for inhibit phosphorous inversion was based on the production of a frame of hydrogen bonds amongst the blades located inside the conical space of the propeller.