Conformational behaviour of methyl p-tert-butylcalix[6]arene ester: interconversions among 1,2,3-alternate conformations

Abstract

The conformational characteristics of methyl hexa(p-tert-butyl)hexabenzenacyclododecaphane (p-tert-butylcalix[6]arene) ester 1 have been studied by means of ¹H and ¹³C NMR spectroscopy. Methyl ester 1, in its solution state, was found to adopt one of several equivalent 1,2,3-alternate conformations that can undergo mutual interconversions over the observed range of temperatures (233–303 K). Thermodynamic and kinetic parameters for these interconversions in several organic solvents were successfully obtained by complete lineshape analysis of temperature dependent ¹H NMR spectra of 1. The results seem to suggest that the conformational interconversions in methyl ester 1 take place predominantly in such a manner that only a pair of benzene rings exchange their up–down orientations at a time. In contrast to 1, its homologues, ethyl and propyl ester 2 and 3, were found to have no such preferential conformations. To gain further insight into the role of the methyl group substituted into 2, we examined a methyl and ethyl mixed ester 4 and found this compound preferentially adopted the symmetrical 1,2,3-alternate conformation rather than other asymmetrical conformations.