PENTACOORDINATED MOLECULES .85. INFLUENCE OF HYDROGEN-BONDING ON THE FORMATION OF BOAT AND CHAIR CONFORMATIONS OF 6-MEMBERED RINGS IN SPIROCYCLIC TETRAOXYPHOSPHORANES

Oxidative addition reactions of the monocyclic phosphite, (Me2C3H4O2P(O-Xyl), with aminophenol and aminocresol derivatives yielded the new bicyclic tetraoxyphosphoranes containing saturated six-membered rings, (C5H10O2)(Xyl-O)P(1-O-2-NHC6H4) (1) and (C5H10O2)(Xyl-O)P(1-O-2-NH-X-MeC6H3) (X = 4 (2), 5 (3)). Hydrogen bonding was introduced via N-H---O intermolecular interactions to ascertain their importance as a structural determinant. X-ray analysis revealed phosphoranes in a trigonal-bipyramidal framework with the rings positioned in axial-equatorial sites. Hydrogen bonding between molecular units gave rise to helical chain form 1 and dimeric forms 2 and 3. The phosphorinane rings were present in a twist-chair conformation for 1 and in both twist-chair and twist-boat conformations for 2 and 3. Variable-temperature solution-state H-1 and C-13 NMR supported retention of the solid-state structures and the presence of hydrogen bonding. Activation energies for intramolecular ligand exchange suggested at least a 10-kcal/mol requirement for reorientation of the six-membered ring in these phosphoranes to diequatorial locations in a TBP. The relevance of these results to pentaoxyphosphorane intermediates in c-AMP enzyme reactions is discussed. 1 crystallizes in the tetragonal space group P4(1) with a = b = 10.772 (3) angstrom, c = 16.917 (3) angstrom, and Z = 4. 2 crystallizes in the triclinic space group P1BAR with a = 12.529 (2) angstrom, b = 14.638 (4) angstrom, c = 14.842 (3) angstrom, alpha = 60.78 (2)o, beta = 76.21 (2)o, gamma = 59.25 (2)o, and Z = 4. 3 crystallizes in the monoclinic space group P2(1)/c with a = 21.607 (5) angstrom, b = 14.901 (4) angstrom, c = 13.679 (3) angstrom, beta = 108.30 (2)o, and Z = 8. The final conventional unweighted residuals are 0.035 (1), 0.041 (2), and 0.049 (3).