Conformational preferences of spirocyclic pentaoxyphosphoranes varying in ring size

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-membered rings. Phosphorane 1 crystallizes in the monoclinic space group P2₁//nwith a = 10.633 (3), b = 17.648 (3). c = 13.601 (1) Å, β = 102.47 (1)° and Z = 4. The bicyclic 2 crystallizes in the monoclinic space group P2₁//n with a = 10.459 (2), b = 12.712 (1). c = 19.949 (2) Å, β = 95.28 (1)°, and Z = 4. Bicyclic 3 crystallizes in the monoclinic space group P2₁//n with a = 9.655 (4), b = 11.662 (4), c = 22.720 (6) Å, β 94.28 (3)° and Z = 4. The bicyclic phosphorane 4 crystallizes in the orthorhombic space group Pbcn with a = 13.922 (4), b = 11.050 (2), c = 11.020 (3) Å, and Z = 4. The final conventional unweighted residuals are 0.037 (1), 0.093 (2), 0.050 (3), and 0.062 (4).