POLYOL RECOGNITION BY A STEROID-CAPPED PORPHYRIN - ENHANCEMENT AND MODULATION OF MISFIT GUEST BINDING BY ADDED WATER OR METHANOL

A spacious semisynthetic receptor composed of a zinc porphyrin bridged by a steroidal diol is shown to complex alcohols and polyols in nonpolar solvents by a combination of Lewis acid coordination and hydrogen bonding, with negative free energies ranging from < 0 to > 45 kJ/mol (equilibrium constants ranging from K < 1 to K > 10(8) L/mol). The physical basis of polyol recognition is discussed in terms of the binding properties of the floor (zinc porphyrin) and roof (steroidal diol) components of the receptor. Lewis acid-induced polarization of the OH bond of a bound alcohol is found to promote hydrogen-bonded association of a second molecule of alcohol and to enhance cyclization of metal-bound diols. Organic-soluble pyranoside derivatives are complexed by the receptor more strongly than by the roof or floor components alone. A semiquantitative two-point binding model is developed to understand binding energetics and solvent effects. An inherent ''stickiness order'' for pyranosides based on the extent of intramolecular hydrogen bonding is proposed and used to rationalize binding selectivities. Following the observation that two or more molecules of an alcohol or small diol can bind cooperatively inside the receptor cavity, addition of water and methanol is shown to increase and modulate pyranoside binding by filling in the gaps between the receptor and a misfit ligand. A quantitative analysis of binding enhancements is presented, and parallels are drawn between synthetic receptors operating in nonpolar solution and natural receptors operating in water.