A NEW STRUCTURAL FORM FOR A PENTAOXYPHOSPHORANE. DIEQUATORIAL RING FORMATION IN A TRIGONAL BIPYRAMID

Abstract

Previous studies^2–8 of structural preferences of cyclic pentaoxyphosphoranes have led to trigonal bipyramidal geometries with the ring systems, which varied from five- to eight-membered, occupying axial–equatorial positions. Although inferences in mechanistic studies of reactions of cyclic phosphates and variable-temperature NMR studies of ligand-exchange behavior of oxyphosphoranes support intermediates with diequatorial ring placement, no X-ray investigations have yielded this structural form. For example, the activation of protein kinases by c-AMP has been proposed^9.10 to result from attack by a functional group of the enzyme to yield a covalent complex, A. Diequatorial ring formation has also been proposed^11.12

in the course of hydrolysis of epimeric phosphoranes. Only in the case of a NMR solution study by Denney and co-workers¹³ of monocyclic pentaoxyphosphoranes containing seven- and eight-membered rings has it been concluded that these rings reside in diequatorial position.¹⁴ This is indicated to occur in the ground state at reduced temperature where pseudorotaton has stopped, e.g., in formulations B and C, respectively.