Enantiomerization Barrier of all-cis C-Me-tetrahydroxy p-tert-butylcalix[4]arene and Atropisomeric Equilibrium of its Tetraacetoxy Derivatives

Galon Israeli, Ori Shalev, Silvio E. Biali*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

All-cis C–Me-tetrahydroxy p-tert-butylcalix[4]arene (5) was prepared by exhaustive demethylation (iodocyclohexane/DMF) of its tetramethyl ether derivative 4. The molecule crystallized as a 2:1 adduct of chlorobenzene and the calix[4]arene. The calix macrocycle adopts in the crystal a pinched-cone conformation where all methyls are located at equatorial positions of the macrocycle. The barrier for reversal of the sense of direction of the circular array of intramolecular hydrogen bonds of 5 (an enantiomerization process) is ΔG = 11.1 kcal mol–1 (197 K, CD2Cl2). Acetylation (acetic anhydride, conc. H2SO4) afforded a 7:3 mixture of partial-cone (10) and cone (11) tetraacetoxy derivatives. Both atropisomers were characterized by X-ray crystallography. In contrast to tetraacetoxy p-tert-butylcalix[4]arene, equilibration studies of the tetraacetoxy derivatives of 5 ([D6]DMSO, 423 K) indicate that the cone atropisomer 11 is populated at equilibrium conditions. The relative stabilization of 11 suggests that the other atropisomeric forms of the tetraacetoxy C–Me-calix[4]arene are destabilized by the presence of methyl substituents located at isoclinal positions of the macrocycle.

Original languageAmerican English
Pages (from-to)1968-1975
Number of pages8
JournalEuropean Journal of Organic Chemistry
Volume2020
Issue number13
DOIs
StatePublished - 7 Apr 2020

Bibliographical note

Publisher Copyright:
© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Keywords

  • Acylation
  • Atropisomerism
  • Calixarenes
  • Conformational analysis
  • Strained molecules

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