Binding of Neutral Substrates by Calix[4]pyrroles

摘要

Molecular recognition of neutral compounds presents a challenge in the area of supramolecular chemistry. Binding of substrates such as short-chain alcohols and simple monoa-jnides is particularly difficult because these molecules have few functionalized sites available for hydrogen bonding, and they lack the large hydrocarbon surfaces necessary to participate in efficient hydrophobic or pi--pi stacking interactions. Association constants for neutral substrate/synthetic receptor complexes are thus generally modest, even though the architectural complexity of the receptors is often high. Previously, we found that the Rg-calix[4]pyrroles (i.e., meso-octaalkylpor-phyrinogens), readily made by acid-catalyzed condensation of pyrrole with symmetrical ketones,5 act as receptors for anionic substrates such as fluoride, chloride, dihydro'gen phosphate, and carboxylate. In this paper, we report that these macrocycles also bind neutral substrates, both in the solid state and in solution.