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 1H 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 1H 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.