Conformational studies of calixarenes containing a single alkanediyl bridge†
Six new calixarenes 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 calixarenes more reliably in about 20–30% yield, while the corresponding condensation with bisbromomethylated alkanediyl diphenols leads to complex mixtures from which sometimes only a calixarene could be isolated. In agreement with molecular mechanics calculations the calixarenes 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 calixarene substituted by tert-butyl and for the corresponding tert-butyl substituted dinuclear precursor.