Ring inversion pathways of exo- and endo-calix[4]arenes studied by means of the MM3 force field

Investigations on the dynamic properties of 25,26,27,28-tetraalkoxycalix[4]arenes: para-substituent- and solvent-dependent properties of paco conformers and determination of thermodynamic parameters of the pinched cone/pinched cone conversion

“…5 are of general validity. This is also implied by the MM3 calculations of Thondorf et al 25 who showed that the ring inversion of exo-calix [4]arene and of 4-tert-butylcalix [4]arene proceeds also via a paco and 1,2-alt intermediate. [15][16][17]26,27 Upon successive cooling of 1 in CD 2 Cl 2 a broadening of the Ar᎐H and the methoxy signals of the cone conformer is observed.…”

Solvent Effect on the Isomerization Rate of Calix[4]arene Studied by Molecular Dynamics Simulations

“…Among the properties of calix[4]arenes that have extensively been measured are the isomerization rates in solvents. These rates were found to strongly depend on the lower rim substituant, and to a lesser extent on the solvent and the upper rim substituant. ,,, Calculations in vacuum, especially of minimum energies and saddle point energies, , may yield a reasonably good estimate of the reaction rate, but, of course, they cannot account for the solvent effect. Grootenhuis et al , performing simulations of calix[4]arenes in water, observed several transitions within 50 ps, but these astonishingly high rates cannot be confirmed experimentally since calix[4]arene does not dissolve in water.…”

“…Conformations of calix[4]arenes , and of numerous derivatized calix[4]arenes have been studied in a series of molecular mechanics studies in vacuum, using various force fields. ,− In the case of the calix[4]arene, all calculations agree on the order of stability of the four possible conformations, but they do not agree on the energies of these conformations, nor on the symmetry of the cone conformation. Wipff, Varnek, and co-workers , published a series of articles on simulations of derivatized calix[4]arenes in several solvents, concentrating on the complexation with neutral guests and cations.…”

Conformation, Inversion Barrier, and Solvent-Induced Conformational Shift inExo- andEndo/Exo-Calix[4]arenes

“…These groups are involved in a circular array of hydrogen bonds in which each hydroxyl group serves both as a donor and as an acceptor. In the resorcarenes the hydroxyl groups are present in extra annular ( exo ) positions of the macrocycle, and therefore these and related compounds possessing a similar disposition of the hydroxyls may be designated by the general term “ exo -calixarenes” as opposed to the “conventional” endo -calixarenes. − Although intramolecular hydrogen bonds may be formed between adjacent OH groups, no circular array of hydrogen bonds is possible in resorcarenes. The all - cis resorcarenes and the alkanediyl derivatives of 1a (e.g., 1b ) adopt cone conformations, but they differ in the conformational preferences of the alkyl groups: the alkanediyl group of 1b prefers the equatorial position, , while a tetraaxial disposition of the alkyl substituents is exclusively found in all - cis - 2a …”

“…The preparation of exo -calixarenes possessing only four exo hydroxyl groups ( 3 ) was recently described. − , X-ray diffraction studies of 3a ·CH 3 CN and 3a ·CH 2 Cl 2 indicate that the exo -calixarene exists in a cone conformation 3,5a whereas 3c and a bis(dioxamethylene) derivative exist in the crystal in a 1,2-alternate conformation. 4c,5b In solution all the exo -calixarenes 3 were found to be flexible on the NMR time scale, even at −70 °C, and since the dynamic processes could not be frozen, the solution conformation could not be unambiguously determined 4b …”