Influence of laterally attached alkyl groups on the conformational behaviour of a basic calix[4]arene: Combined NMR, Molecular Mechanics and X-ray study

Edwin Weber (1) , Margit Gruner (2) , Tobias Gruber (3) , Conrad Fischer (4)

1 Institut für Organische Chemie [Freiberg]
2 Chemistry
3 Pharmaceutical Sciences
4 Institute of Organic Chemistry [Freiberg]

Three new calixarenes 3-5 featuring an alkyl residue of different chain length attached to one of the central ring methylene groups of the basic calix[4]arene 1 have been prepared. A systematic study that includes also the lower homologous compound 2, showing the effect of the alkyl substitution on the conformational behaviour of the calixarene framework in comparision with the unsubstituted parent compound 1 is reported. The application of special 2D NMR techniques, 2D-EXSY and ROESY method, at various temperatures establishes that the calixarenes 2-5 adopt the partial cone conformation of lower symmetry and far less the symmetric cone and 1,2-alternate conformations. In solution, they undergo a fast interconversion with relatively low activation energies of about 15 kcal/mol at room temperature. The conformer distribution is reproduced well by molecular mechanistic calculations (MMFF94) indicating the present conformers to assume the lowest steric energies. A single-crystal X-ray structure of the lateral ethyl derivative 2 corroborates these results, showing the molecule in a sterically favourable partial cone conformation.

Mots clés: Physical Sciences

Edwin Weber, Margit Gruner, Tobias Gruber, Conrad Fischer. Influence of laterally attached alkyl groups on the conformational behaviour of a basic calix[4]arene: Combined NMR, Molecular Mechanics and X-ray study. Supramolecular Chemistry, 2010, 22 (04), pp.256-266. ⟨10.1080/10610270903437044⟩. ⟨hal-00576633⟩