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-bipyramidal geometries with the rings spanning apical-equatorial positions. Retention of these structures in solution is indicated by ¹H, ¹³C, and ³¹P 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 ¹H and ¹³C 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) Å, β = 92.22 (1)°, and Z = 8. Compound 2 crystallizes in the monoclinic space group P2₁ with 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)°, γ = 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).