Conformationally immobile calixarenes

1 Minimum Structural Requirements for Conformational Immobility of Unbridged Calixarene Ethers and Esters 

Two pathways are available to a calixarene for conformational inversion. One involves the upper rim through the annulus and the other involves the lower 

In the calix[5]arenes it is not yet known whether the p-H compound can engage in upper rim through the annulus conformational inversion, but it seems quite certain that the p-tert-buty compound cannot do so at temperatures ordinarily encountered in laboratory procedures. As anticipated, larger OR groups are necessary to curtail conformational interconversion of calix[5]arenes than for the calix[4]arenes it has been determined that the threshold size requires a group slightly more bulky than n-butyl (AG of penta-n-butyl ether of 51-Bu jYioP for ethers and n-butanoyl (AG for penta-n-butanoate of 

St-Bu i7 5 fcal mol ) for esters. For example, the pentaisobutanoate of 5 (AG 20kcal mol ) and the pentakis(ethoxyearbonyl)methyl ether of 

Guelzim. A. Khrifi, S. Baert, F. Loeber, C. Asfari, Z. Matt, D. Vicens, J. Acta Crvstallogr. 1993, C49, 72. 

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Modified calixarenes that are conformationally immobile, fixed in either the cone or partial cone conformation, have been successfully synthesized by... 

For Ag+, complexation via the calixarene aromatic rings has been firmly established for both the partial cone and cone conformations of the immobile tetra/j-propyl ethers by X-ray crystallography.60 In both cases, the Ag+ cation is sandwiched between two distal (opposite sides of the molecule, as opposed to proximal, meaning adjacent to one another) calixarene rings, which are nearly mutually perpendicular giving a pinched cone conformation. The exposed face of the cation is coordinated to a triflate anion in both cases. For the partial cone case, the Ag+ ion also interacts with one of the anisole oxygen atoms, which is inverted with respect to the other three at the lower rim (Figure 3.85). 

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