Conformational preferences of monocyclic pentaoxyphosphoranes varying in ring size

Abstract

New monocyclic pentaoxyphosphoranes 1-4, 6 and the furanosyl derivative, 7, were synthesized from the reaction of tris(2, 6-dimethylphenyl) phosphite (5) with a diol or a quinone. The pentacoordinated derivatives 1-4 were studied by X-ray analysis and represent the first structurally characterized monocyclic oxyphosphoranes that have six-, seven-, and eight-membered rings. All possess trigonal-bipy-ramidal geometries with the rings spanning apical-equatorial positions. Retention of these structures in solution is indicated by 1 H, 13 C, and 31 P NMR. Twist-boat, rowboat, and distorted-tub conformations are found for the six-(1), seven (2), and eight (4) membered ring derivatives, respectively. Phosphorane 3 has a more planar phosphorinane ring, a consequence of ring unsaturation. Variable-temperature 1 H and 13 C NMR establish nonrigid behavior supporting a simple Berry pseudorotation in which the rings exchange apical-equatorial positions. It is concluded that six-membered rings of pentaoxyphosphoranes prefer apical-equatorial sites of a TBP. The preferred conformation of saturated six-membered rings is generally that of a boat. Phosphorane 1 crystallizes in the monoclinic space group C2/c with a = 29. 392 (8), b = 11. 420 (5), c = 16.379 (2) Å2.22 (1)°, and Z = 8. Compound 2 crystallizes in the monoclinic space group P21with a = 12. 268 (2), b = 9. 916 (3), c = 12.625 (2) Å, β = 91. 79 (1)°, and Z = 2. The monocyclic derivative 3 crystallizes in the monoclinic space group P21/c with a=20. 114 (6), b = 9. 554 (2), c = 17.178 (3) Å, β = 114. 24 (2)°, and Z = 4. Phosphorane 4 crystallizes in the triclinic space group P1 with a = 9. 690 (2), b = 15. 414 (4), c = 21. 350 (5) Å, α = 93. 07 (2)°, β = 90. 17 (2)°, λ = 99. 97 (2)°, and Z = 4. The final conventional unweighted residuals are 0.056 (1), 0.039 (2), 0.038 (3), and 0.076 (4). © 1995, Taylor & Francis Group, LLC. All rights reserved.