The macrocycle 2-oxo-p-tert-butyltetramethoxycalix[4]arene is conformationally mobile in solution (CDCl3, C2D2Cl4) as judged by NMR spectroscopy (2D-EXSY). Of six possible limiting conformations which differ in relative orientation of ring units, NMR spectra reveal the presence of three of these: partial cone (paco), 1,2-alternate (1,2-alt), and 1,3-alternate (1,3-alt). Each of these have rings flanking the carbonyl (proximal rings) in an anti orientation and are distinguished by the relative orientations of the other two rings (distal rings). A single distal-ring flip converts the paco into either of the other two species, while two distal-ring flips suffice to exchange NMR signals within the paco itself. In short, all observed data is explained by distal ring flips. An overlooked feature of the conformers is their chirality; each exists as a racemate as shown for the paco form. To seek evidence of proximal-ring flips we turned to spectroscopy in a chiral environment created by a large excess of a chiral shift reagent. 1H NMR spectroscopy with ca. 85 equiv of (R)-(−)-1-(9-anthryl)-2,2,2-trifluoroethanol resolved some signals from enantiomeric conformers. Preliminary exchange spectroscopy experiments demonstrate that conformer conversions involving proximal-ring flips do occur, with a higher energy barrier than seen for distal ring flip.

Distal and proximal ring flips in 2-oxo-p-tert-butyltetramethoxycalix[4]arene: NMR spectroscopic studies of chiral conformers

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