CONFORMATIONAL COMPARISON OF A 7-MEMBERED RING IN TRICOORDINATED AND AND PENTACOORDINATED GEOMETRIES OF OXYGEN-CONTAINING PHOSPHORUS-COMPOUNDS - SOLUTION NMR BEHAVIOR OF 7-MEMBERED AND 8-MEMBERED RINGS IN OXYPHOSPHORANES

Reaction of cyclic phosphites with alcohols or a quinone in oxidative addition reactions led to the new spirocyclic oxyphosphoranes (C6H4O)2(C8H12O2)P(O-Xyl) (1), [(C6H4O)2CH2](C8H12O2)P(O-Xyl) (5), and (C6H4O)2(C12H8O2)P(S-Xyl) (3) and the monocyclic oxyphosphorane (C6H4O)2P(OC6F5)3 (4), containing rings varying in size from five- to eight-membered (Xyl is C6H3Me2). In addition, the cyclic phosphite (C6H4O)2P(S-Xyl) (2) was synthesized. The X-ray structure of 1 present in a trigonal bipyramidal (TBP) geometry and that of 2 showed similar "rowboat" conformations for the common seven-membered ring system. The latter contrasts with conformational change usually found for six-membered rings from chair to boat accompanying a change of coordination from tri- to pentacoordinate at phosphorus. Variable-temperature H-1 NMR spectra of 1, 3, and 5 provide activation energies for intramolecular ligand-exchange processes (pseudorotation). The spectra of 1 and 5 are consistent with ground-state trigonal bipyramids (in agreement with the axial-equatorial (a-e) ring orientations found for 1 in the X-ray analysis) that undergo ligand exchange via TBP intermediates with the seven- and eight-membered rings positioned diequatorially (e-e). Activation energies of 11.5-12.9 kcal/mol are interpreted to include a steric effect and to reflect the higher energy of e-e vs a-e ring placement, which is not significantly different for seven- and eight-membered rings. In contrast, this energy difference for saturated six-membered rings in TBP geometries in the absence of steric effects has been estimated at 7 kcal/mol. Oxyphosphorane 1 crystallizes in the triclinic space group P1BAR with a = 9.497 (2) angstrom, b = 11.159 (4) angstrom, c = 13.272 (6) angstrom, alpha = 110.50 (3)-degrees, beta = 97.19 (3)degrees, gamma = 107.87 (2)-degrees, and Z = 2. Cyclic phosphite 2 crystallizes in the monoclinic space group P2(1)/c with a = 8.857 (3) angstrom, b = 24.200 (5) angstrom, c = 8.285 (2) angstrom, beta = 93.27 (3)-degrees, and Z = 4. The final conventional unweighted residuals are 0.064 (1) and 0.053 (2).