CONFORMATIONAL PREFERENCES OF SPIROCYCLIC PENTAOXYPHOSPHORANES VARYING IN RING SIZE

Abstract

Abstract New bicyclic pentaoxyphosphoranes 1–3 containing ring sizes varying from five to seven membered were synthesized by oxidative addition of a quinone or a diol to a cyclic phosphite. Variable-temperature solution 1H and 13C NMR studies revealed the presence of dynamic intramolecular ligand exchange processes, one in which apical-equatorial ring interchange occurred between trigonal bipyramidal ground states and a higher temperature process supporting an exchange intermediate with the ring located diequatorially in a trigonal bipyramid. Activation energies for the latter process were determined. X-ray analysis supported the interpretation of the solution-state behavior and showed that saturated six-membered rings prefer a boat conformation occupying apical-equatorial positions in trigonal bipyramidal structures. The stability of the six-membered ring in this conformation is supported by the shorter P-O bond lengths found for this ring size compared to that for phosphoranes having five- and seven-mem...