Chirality in the absence of rigid stereogenic elements: Steric and Electronic Effects on the Configurational Stability of C3 symmetric residual tris-aryl phosphanes

Residual stereoisomers result whenever closed subsets of appropriately substituted interconverting isomers (the residual stereoisomers) are generated from a full set of stereoisomers under the operation of a favoured stereomerization mechanism.1 In the case of the three-bladed propellers, differentiation of the edges of the blades and strict correlation in the motion of the rings are the prerequisites for the existence of residual stereoisomers. In these systems, the two-ring flip mechanism is the lowest energy process. It does not interconvert all possible conformational stereoisomers generated by helicity and the three blade-hub rotors. In the case of systems bearing three identical blades, two non interconverting subgroups (the residual stereoisomers) are generated, each one constituted by quickly interconverting diastereoisomers.2 A series of tris-aryl phosphanes and phosphane-oxides, structurally designed for existing as residual enantiomers or diastereoisomers,3 bearing substituents differing in size and electronic properties on the aryl rings, have been synthesized and characterized (Figure 1). The residual antipodes of a trisarylphosphane were isolated in enantiopure state and the absolute configuration assigned to them by single-crystal anomalous X-ray diffraction analysis. The electronic properties have been evaluated through theoretical calculations and on the basis of the 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 modest (stereomerization barriers of about 17 kcal m-1), much lower than those shown by the corresponding phosphane-oxides (about 28 kcal m-1). 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.