The article describes a simple and scalable preparation of 2-monothiacalix[4]arene 7, the simplest representative of the mixed-bridged (CH2 and S) calix[4]arenes. The synthesis is based on the condensation of linear building blocks (bisphenols), which are relatively readily available, and allows, depending on the conditions, the use of two alternative reaction routes that provide macrocycle 7 in high yield. The dynamic behavior of the basic macrocyclic skeleton was investigated using NMR spectroscopy at variable temperatures. High-temperature measurements showed that compound 7 undergoes a cone-cone equilibrium with activation free energy ΔG# of the inversion process of 63 kJ·mol-1. Interestingly, the same barrier for the oxidized sulfone derivative 14 shows a value of 60 kJ·mol-1, indicating weakened hydrogen bonds at the lower rim of the calixarene. The same was also confirmed at low temperatures, when barriers to changing the direction of the cyclic hydrogen bond arrays (flip-flop mechanism) were determined (compare ΔG# = 44 kJ·mol-1 for 7 vs. ΔG# = 40 kJ·mol-1 for 14).

• Keywords
• bisphenols, calixarene, conformation, flip-flop mechanism, fragment condensation, macrocyclization, synthesis, thiacalixarene, variable temperature NMR,
• Publication type
• Journal Article MeSH

A novel approach to calix[5-7]arenes possessing mixed (S and CH2) bridges within the skeleton is based on the reaction of thiacalix[4]arene monosulfoxide with BuLi leading to a linear phenolic tetramer in essentially quantitative yield. This key intermediate is then cyclized with suitable building blocks to give macrocyclic calixarene analogues. Compared to the traditional stepwise construction of similar systems, this procedure based on thiacalixarene cleavage represents a scalable, robust, and straightforward synthesis and enables the preparation of larger calixarenes on a gram scale. As shown by 1H NMR and UV-vis titration experiments, the mixed-bridge calix[7]arene is able to recognize fullerenes C60 and C70, thus showing possible applications of such systems. The structures of the mixed bridge systems were confirmed by single crystal X-ray analysis, and the behavior of novel macrocyclic skeletons in solution was studied using dynamic NMR techniques.

• Publication type
• Journal Article MeSH