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 1 H and 13 C 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 P21ln with 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). © 1995, Taylor & Francis Group, LLC. All rights reserved.