Pseudorotationen bei spirozyklischen Phosphoranen

Abstract

The free energies of activation of the pseudorotational processes in some organyl-bis-(4.4'-dimethyl-2.2'-biphenylylen)-phosphoranes are evaluated by temperature-dependent ¹H-NMR-spectroscopy. The energy barriers range from  ΔF^≠_−48º = 11.9 kcal/mol (organyl = β-naphthyl) to ΔF^≠_+54º = 17.1 kcal/mol (organyl = α-naphthyl) and show significant dependence on the steric requirements of the organyl-group. By combining these results with other published work it is shown, that the most stable conformations of these phosphoranes are chiral trigonal bipyramids which equilibrate over a tetragonal pyramidal transition state.