NMR and X-ray evidence for the chair conformation of six-membered rings attached diequatorially to five-coordinate phosphorus. Implications for reported transition-state analogs of nucleoside cyclic 3′,5′-monophosphate hydrolysis

Article abstract of DOI:10.1021/ja00063a024

A series of phosphoranes, 5, 6a, 6b, and 7a, has been prepared and structurally characterized by ¹H NMR spectroscopy and/or X-ray crystallography. In the crystalline state, 5a, 6a, and 6b feature five-coordinate phosphorus bonded in a somewhat distorted, trigonal bipyramidal fashion. The phosphorus-containing six-membered ring is attached to phosphorus diequatorially and is in the chair conformation. ¹H NMR coupling constants reveal that for all four phosphoranes a chair-form ring, rather than a boat or twist conformation, also is populated in solution. The six-membered ring for 5 in solution is primarily in conformation 5a (ca. 90%) in equilibrium with 5b (ca. 10%). Arguments are presented to explain the greater stability of 5a in terms of reduced steric repulsion between its axial-like four-membered ring oxygen with the axial hydrogens of the phosphorus-containing ring compared to that of the analogous oxygen of the five-membered ring in 5b. The trans-fused ring structures of 6a, 6b, and 7a are closely related to the previously reported phosphoranes 1-3, prepared as transition-state analogs for the hydrolysis of cAMP. The results of the present paper render highly unlikely the assertion that thymidine-based phosphorane 3 populates in solution measurable amounts of a permutational isomer with its ring attached to phosphorus in diequatorial fashion. Structural aspects of the five-membered rings of phosphoranes 6a, 6b, and 7a also are discussed.