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

Abstract

Calixarenes 4a and 4b having hydroxyl groups in endo and exo positions and the ethanediyl-bridged exo-calixarene 5a were synthesized by a stepwise strategy. Single-crystal X-ray structures were obtained for 4a and for the exo-calixarene 3d, showing the molecules to exist in the 1,2-alternate conformation which is also found for 4a,b in solution. The inversion barriers of 4a and 4b (10.3 and 10.8 kcal mol(-1)) are similar to that determined for the endo-dihydroxycalixarene 12, indicating that the additional intramolecular hydrogen bond between the exo OH groups does not decrease the flexibility of the molecule. In CDCl(3) solution exo-calixarene 5a adopts a 1,2-alternate conformation with the methyl group at the bridge located in an axial position, while in DMSO-d(6) the conformation adopted is the partial cone. Similar solvent-induced conformational shifts were found for the exo-calixarenes 3b and 3d. MM3 calculations predict that the cone form is the lowest energy conformation of 4 and the exo-calixarenes 3 and 5. The calculations suggest that the conformational preferences of the methyl group at the bridge for either the axial or equatorial positions are in large part determined by the repulsive steric interactions with the hydroxyl groups. The inversion barrier of 4b is satisfactorily reproduced by calculations, which indicate that the rotation of the exo rings is less energetically demanding than the rotation of the endo rings.

C–Me Bond Formation at All Methylene Bridges of the Calix[4]arene Scaffold

“…Notably, whereas in 6 all the methyls are located at equatorial positions, all the alkyl groups are located at axial positions in the all-cis calix[4]­resorcinarenes, adopting a cone (“bowl”) conformation . Assuming that these arrangements represent the lower energy conformations of each compound, these conformational preferences are consistent with the proposal that the axial/equatorial preferences of an alkyl group are, in large part, determined by the need to avoid repulsive interactions with the oxygens . In the calixarene family, the OR are located in the lower rim, and thus in the cone conformation the equatorial positions are preferred, whereas alkyl groups in the calix[4]­resorcinarenes favor the axial positions since the OR groups are located at the upper rim (Scheme ).…”

Quantum Mechanical Calculations of Conformationally Relevant 1H and 13C NMR Chemical Shifts of N‐, O‐, and S‐Substituted Calixarene Systems

“…To investigate these aspects, we have very recently undertaken a quantum mechanical (QM) study10 on the 13 C and 1 H chemical shift values of ArCH 2 Ar groups of some representative calixarene systems using GIAO (gauge‐including atomic orbital)11 calculations with the hybrid DFT functional MPW1PW9112 and a 6‐31G(d,p) basis set 13. This study10 showed that the OR groups on the lower rim influenced both the magnetic non‐equivalence of the methylene protons and the shielding variations of the corresponding 13 C nucleus 14. These observations prompted us to extend the QM GIAO studies to mercapto‐15 and aminocalixarene derivatives,16 in which hydroxyl groups are replaced by mercapto and amino groups, to verify the influence of these groups on the conformationally relevant NMR data of ArCH 2 Ar groups.…”

Calixrotanes: Calixarenes Incorporating Spiro‐Linked Cyclopropyl Groups

“…On this basis, the two low-field cyclopropyls ignals are assigned to the pair of cyclopropyl protons (in a trans relationship) that, in the 1,2,3-alternate conformation,a re in close proximity to ap air of aromatic protons, whereas the upfield cyclopropyls ignal is assigned to the protons in steric proximity to methoxy groups. [24] Upon raising the temperature, the aromatic, methylene, methoxy, tert-butyl, and cyclopropyl signals broadened and coalesced, and at 243 K, as ignal pattern consistent with a1 ,2,3-alternate form of averaged C 2v symmetry was observed ( Figure 5). Ap air of doublets was observed for the methylene protons at d = 4.25 and 3.42 ppm.…”

“…The NOESY spectrum recorded at 183 K displayed a NOE cross peak between the high‐field cyclopropyl signal and the methoxy group signal at δ =3.10 ppm. On this basis, the two low‐field cyclopropyl signals are assigned to the pair of cyclopropyl protons (in a trans relationship) that, in the 1,2,3‐ alternate conformation, are in close proximity to a pair of aromatic protons, whereas the upfield cyclopropyl signal is assigned to the protons in steric proximity to methoxy groups …”