Synthesis, structure, and binding properties of lipophilic cavitands based on a calix[4]pyrrole-resorcinarene hybrid scaffold.

We report the synthesis, structural characterization, and binding properties of a series of unprecedented cavitands based on a meso-dodecyl-calix[4]pyrrole-resorcin[4]arene hybrid scaffold. The reported structural and conformational features of the prepared cavitands are derived from results obtained in solution, solid state, and molecular modeling studies. In the solid state, these cavitands are exclusively observed in the kite C4 structure and as a racemic mixture of two cyclochiral conformers, which are interconverting fast on the (1)H NMR time scale, according to solution studies. In agreement, molecular modeling studies assign an energy preference for the kite conformer of the cavitands. The polar interior of the synthesized containers allows for the inclusion of a series of pyridine N-oxide derivatives. This results in the formation of 1:1 complexes that are kinetically and thermodynamically highly stable. The putative switching process between the vase and kite forms of these cavitands is investigated in solution by means of variable temperature (1)H NMR experiments. N-Oxide guests that are size and shape complementary to the volume of the cavity of the vase form are also employed to facilitate its emergence. All of the results obtained indicate the existence of a remarkable preference toward the kite conformation both in free and bound calix[4]pyrrole-based cavitands.