Steric and Electronic Effects on The Configurational Stability of Chiral Residual Phosphorus-Centered Three-Bladed Propellers: The tris-Arylphosphane-oxides

A series of tris-aryl phosphane-oxides existing as residual enantiomers or diastereoisomers, bearing substituents on the aryl rings differing in size and electronic properties, have been synthesized and characterized. Their electronic properties have been evaluated on the basis of their electrochemical oxidation and reduction potentials together with those of the corresponding "blade bromides" (i.e. the naphthalene derivatives displaying the same substitution pattern of the tris-naphthylphosphane-oxide blades, with a bromo substituent where the phosphorus atom is located) determined by CV. The residual stereoisomeric phosphane-oxides have been isolated in a stereochemically pure state and have been found highly configurationally stable at room temperature (stereomerization barriers of about 27 kcal mol-1). The chiroptical properties of the residual stereoisomers and the absolute configuration assignments are discussed. The configurational stability of residual tris-arylphosphane-oxides was found to be scarcely influenced by the electronic properties of the substituents present on the aromatic rings constituting the blades, while steric effects play the most relevant role. Detailed theoretical calculations are in agreement with the experimental results contributing also to a rational interpretation of the stereodynamics of these systems.