Conformational studies of calix[5]arenes containing a single alkanediyl bridge

Six new calix[5]arenes substituted at one of the methylene bridges by methyl, ethyl, isopropyl, tert-butyl p-tolyl and p-nitrophenyl have been synthesised by heat induced (3+2) fragment condensation of a linear trimer with the corresponding bishydroxymethylated alkanediyl diphenols in boiling xylene. These conditions give the calix[5]arenes more reliably in about 20-30% yield, while the corresponding condensation with bisbromomethylated alkanediyl diphenols leads to complex mixtures from which sometimes only a calix[8]arene could be isolated. In agreement with molecular mechanics calculations the calix[5]arenes prefer the cone conformation with an equatorial position of the alkyl or aryl substituent at the bridge. From variable temperature ¹H NMR spectroscopy the conformational equilibria and the energy barriers for the cone-to-cone ring inversion have been determined. Single crystal X-ray analyses have been performed for the calix[5]arene substituted by tert-butyl and for the corresponding tert-butyl substituted dinuclear precursor.