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

초록

The conformational characteristics of methyl hexa(p-tert-butyl)hexabenzenacyclododecaphane (p-tert-butylcalix[6]arene) ester 1 have been studied by means of H-1 and C-13 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 H-1 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.

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