Conformational analysis of 2-substituted alkylphosphoryl compounds. 4. Substituent effect on the nature of the hydrogen bonding in (2-hydroxyalkyl)phosphoryl systems

The extent to which solid state conformational and hydrogen bonding preferences of five (2-hydroxyalkyl)phosphoryl compounds 2a-e [(R2P)-P-1(O)CH2CH(OH)R-2; 2a, R-1 = MeO, R-2 = Ph; 2b, R-1 = R-2 = Ph; 2c, R-1 = Ph, R-2 = Bu-t; 2d, R-1 = Me, R-2 = Ph; 2e, R-1 = Me, R-2 = Bu-t] change upon dissolution has been investigated using X-ray crystallography, IR, and NMR spectroscopy. Intermolecular hydrogen bonding involving inversion-center-related cyclic dimers is shown to be a common solid state arrangement, with relatively small structural changes producing infinite chains. Intramolecular hydrogen bonded monomers are uncommon. Conflicting X-ray and IR evidence on the equivalence of hydrogen bonding in the dimeric structure of 2b in the solid state is discussed. The changes in hydrogen bonding occurring upon dissolution have been studied and evidence presented that an equilibrium between dimers and monomers is usually set up, which can become extensively shifted to a predominance of intramolecular hydrogen bonded monomers when bulky groups (R-1 and R-2) are present. Conformational preferences are determined mainly by steric factors (the size of R2) and little influenced by changes in the nature of the hydrogen bonding or donation of electrons from the hydroxyl oxygen to vacant orbitals of the phosphorus atom. The extent of distortion from perfectly staggered geometries, upon which most conformational analyses are based, has been estimated to be 4 to 12 degrees for two of the compounds. Ab initio and molecular mechanics solvation studies have been used to explain NMR chemical shift trends of the a-methylene protons and the phosphorus nucleus. Large chemical shift differences between the a-methylene protons of diphenylphosphine oxides 2b and 2c in acetone-d(6) are attributed to the specific orientation of acetone-d(6) molecules toward one of the protons in the major conformer.