Calix[4]pyrroles Containing Deep Cavities and Fixed Walls. Synthesis, Structural Studies, and Anion Binding Properties of the Isomeric Products Derived from the Condensation of p-Hydroxyacetophenone and Pyrrole

Abstract

The acid-catalyzed condensation of 4-hydroxyphenylmethyl ketone (p-hydroxyacetophenone) with pyrrole affords a new type of calix[4]pyrrole derivative that contains deep cavities and fixed walls. These new systems, which can be readily converted into the corresponding 4-methoxyphenyl species, can exist in the form of four different configurational isomers, as well as in different conformational forms. Three of the configurational isomers, namely the ????, ????, and ???? isomers, where the terms “?” and “?” indicate whether the bulky functionalized phenyl substituent faces “up” or “down” relative to the mean calixpyrrole plane, were isolated by column chromatography. Proof of structure and assignment of configuration was then effected using X-ray diffraction analysis. The affect of the meso-aryl substituent on the anion binding affinity of the receptor was investigated in solution (acetonitrile-d3:D2O; 99.5:0.5 v./v.) using standard 1H NMR titration techniques. Surprisingly, it was found that both the 4-hydroxyphenyl- and 4-methoxyphenyl-derived systems show lower affinities for small anions, viz. Cl-, and H2PO4-, than do simple unsubstituted calix[4]pyrroles such as meso-octamethylcalixpyrrole. On the other hand, increased selectivities and other binding effects ascribable to the ‘walls’ of the cavities were observed.