The influence of heteroatoms on the conformational properties of heteracalix[4]arenes. Comparison of oxygen-, sulfur-, and nitrogen-bridged [14]metacyclophanes

Oxygen- and sulfur-bridged calix4]arenes have been synthesized by the nucleophilic aromatic substitution of 1,5-difluoro-2,4-dinitrobenzene with 4,6-diisopropylresorcinol or 4,6-diisopropyl-1,3-benzenedithiol. X-ray crystal structure analyses revealed that the oxacalix4]arene 2 adopts an unsymmetrical 1,3-alternate conformation and that the dinitrobenzene rings strongly conjugate with the bridging oxygen atoms. On the other hand, the thiacalix4]arene 3 adopts a heavily twisted unsymmetrical 1,3-alternate conformation, and the conjugation is very weak. In the ¹H NMR spectrum (CDCl₃, 30 °C), both compounds display a pair of diastereotopic methyl signals for the isopropyl groups in agreement with the frozen 1,3-alternate conformation on the NMR time scale. The free energies of activation of the macrocyclic inversion for 2 and 3 were determined to be 69.5 kJ mol⁻¹ and 74.9 kJ mol⁻¹, respectively, by variable temperature NMR spectroscopy.