DIEQUATORIAL AND AXIAL-EQUATORIAL ORIENTATIONS OF EIGHT-MEMBERED RINGS IN MONOCYCLIC PENTAOXYPHOSPHORANES CONTAINING TRIFLUOROETHOXY GROUPS

Abstract

Reaction of (CF₃CH₂O)₃P with diols yielded new monocyclic pentaoxyphosphoranes with eight-membered ring systems containing methylene or sulfur bridges. Trigonal-bipyramid (TBP) geometries were shown to be present by X-ray analysis for CH₂(C₆H₄O)₂P(OCH₂CF₃)₃ (2) and S(Me₃C₆H₂O)₂P(OCH₂CF₃)₃ (3) with the ring in 2 located in axial-equatorial (a–e) positions and the ring in 3 in diequatorial (e–e) sites. These structures are compared with the first pentaoxyphosphorane derivative to exhibit a diequatorial ring in a TBP, CH₂[Me(t-Bu)C₆H₂O]₂P(OCH₂CF₃)₃ (1), recently prepared in our laboratory. A pseudooctahedral structure was revealed by X-ray analysis for S[(t-Bu)₂C₆H₂O]₂P(OCH₂CF₃)₃ (4). ¹H, ³¹P, and ¹⁹F solution NMR data indicated rapid intramolecular ligand exchange for 2 and 3 at room temperature. In contrast, NMR data on 1 and 4, which contained tert-butyl groups, did not indicate exchange behavior. Here, retention of the basic solid-state TBP in solution was consistent with the observed spectra. Comparison of solid-state ³¹P NMR spectra with solution ³¹P NMR data of 3 and 4 suggested retention of their basic solid-state geometries in solution. Comparisons with related pentaoxyphosphoranes having six- and seven-membered rings suggest that seven- and eight-membered rings are more easily accommodated in diequatorial positions compared to positioning saturated six-membered rings in this location of a TBP. Oxyphosphorane 2 crystallizes in the triclinic space group P 1 with a = 9.712 (2) Å, b = 11.039 (3) Å, c = 12.521 (2) Å, a = 97.48 (2)°, β = 111.77 (1)°, γ = 110.56 (2)°, and Z = 2. Derivative 3 crystallizes in the triclinic space group P 1 with a = 9.245 (2) Å, b = 12.464 (2) Å, c = 12.891 (2) Å, α = 76.77 (1)°, β = 74.69 (2)°, γ = 70.60 (1)°, and Z = 2, Oxyphosphorane 4 crystallizes in the triclinic space group P 1 with a = 11.202 (2) Å, b = 11.307 (4) Å, c = 16.372 (8) Å, α = 99.66 (3)°, β = 94.39 (3)°, γ = 103.42 (2)°, and Z = 2. The final conventional unweighted residuals are 0.050 (2), 0.075 (3). and 0.076 (4).