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

Three new calixarenes 3–5 featuring an alkyl residue of different chain lengths 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 comparison with the unsubstituted parent compound 1 is reported. The application of special 2D NMR techniques, 2D-EXSY and ROESY methods at various temperatures establishes that 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 well reproduced 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.