Conformational analysis of p-tert-butylcalix[4]arene derivatives with trans-alkyl substituents on opposite methylene bridges: Destabilization of the cone form by axial alkyl substituents

Samah Simaan, Silvio E. Biali*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

23 Scopus citations

Abstract

The stereochemistry of calix[4]arenes substituted by a pair of identical alkyl substituents in a trans fashion at two distal bridges is analyzed. MM3 calculations suggest that increasing the bulk of the alkyl group at the bridges destabilizes those conformations possessing an axial disposition of the substituent. In contrast to the 1,3-dimethyl ether of p-tert-butylcalix[4]arene, which adopts a cone conformation, solution NMR data indicate that the 1,2-alternate conformation is preferred in the dimethyl ether derivatives 5b (alkyl = i-Pr) and 5c (alkyl = t-Bu). In the derivative substituted by the less bulky methyl substituent (5a), both the cone and 1,2-alternate forms coexist in CDCl3. Increasing the polarity of the solvent increases the relative population of the cone form of 5a and 5b. The steric destabilization ensuing from the presence of the axial substituent is so large in the cone conformation of 5c that the 1,2-alternate conformer is the major form even in polar solvents. The cone → 1,2-alternate interconversion barrier of 5a is 18.2 kcal mol-1, indicating that the presence of an axial methyl group both destabilizes the cone conformation and decreases its rigidity.

Original languageAmerican English
Pages (from-to)7685-7692
Number of pages8
JournalJournal of Organic Chemistry
Volume68
Issue number20
DOIs
StatePublished - 3 Oct 2003

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