Inhibition of Pseudorotation in Some Monocyclic Pentaoxyphosphoranes

Phosphoranes were prepare by allowing 2,4,8,10-tetra-tert-butyl-6-(2,2,2,trifluoroethoxy)-12H-dibenzo[d,g][1,3,2]dioxaphosphocin, the analogous ethoxy compound, 2,4,8,10-tetra-tert-butyl-6-(2,2,2,-trifluoroethoxy)-dibenzo[d,f][1,3,2]dioxaphosphepin and the analogous compound without tert-butyl groups to react with trifluoroethyl benzenesulfenate. The first three phosphoranes showed significant barriers to intramolecular ligand reorganization, 14–16 kcal/mole. The phosphorane without the tert-butyl groups had a barrier too low to measure. These observations are discussed in terms of steric inhibition of pseudorotation.