Receptors for Oxo Acids: Effects of Intra-Ion-Pair Hydrogen Bonding on Acid-Base Equilibria

Article abstract of DOI:10.1021/jo00076a031

Receptor molecules 1-3 containing a quinoline unit and hydroxyl groups were synthesized, and their properties in salt formation with oxo acids such as p-toluenesulfonic acid (TsOH) and methyl phenylphosphonate (MPP) were studied by using 1H NMR spectroscopy.In CDCl3 solution the salt formation of the receptors with TsOH or MPP is enhanced by hydrogen bonds between the hydroxyl groups and the counteranion.The salt-formation enhancement of 1 over 2 suggests the presence of multiple interactions.The X-ray structure of 1*MPP*(acetone)2 confirms that threehydrogen-bonding interactions stabilize the salt.In the salt formation of 5-(dimethylamino)quinoline derivative 3, the hydroxyl groups also affect the relative basicity of the basic nitrogens in 3.The results described here emphasize that acid-base equilibria in apolar solvents are significantly affected by intra-ion-pair hydrogen bonding (i.e., hydrogen bonding within a contact ion pair).