COMBINED NMR-SPECTROSCOPY AND MOLECULAR MECHANICS STUDIES OF OH-DEPLETED CALIX[4]ARENES - ON THE INFLUENCE OF OH GROUPS ON THE RELATIVE STABILITY OF CALIX[4]ARENE CONFORMERS

The effects of OH groups on the relative stabilities and the structures of four conformers of OH-depleted calix[4]arenes are discussed on the basis of a computational method using molecular mechanics (MM3(92)) calculations. The results are compared with various NMR spectroscopic experiments. From these studies, it is found that the calix[4]arene framework with no OH group in the lower rim (tetra-OH-depleted calix[4]arenes: 4a and 4b) favors a cone conformation with C-4 symmetry and the changes in the relative stabilities in calix[4]arenes with OH-groups are due to the formation of stable hydrogen bonds and/or the relaxation of the steric crowding around OH-substituents in a lower rim site. NMR studies suggest that mono-OH-depleted calix[4]arene 1b forms intra- and intermolecular hydrogen bonds among phenolic OH groups, while di- and tri-OH-depleted calix[4]arenes (2b and 3b) scarcely form it. They also show that the ring inversion barriers of calix[4]arenes increase with the number of OH groups in 25, 26, 27 or 28-position.