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Stabilizing a different cyclooctatetraene stereoisomer.

Longfei Li1,2, Ming Lei3, Yaoming Xie2

  • 1State Key Laboratory of Chemical Resource Engineering, Institute of Materia Medica, College of Science, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China.

Proceedings of the National Academy of Sciences of the United States of America
|August 30, 2017
PubMed
Summary

Researchers explored strategies to stabilize an unconventional cyclooctatetraene (COT) isomer. By using steric and aromatic clamping, the (Z,Z,Z,E) COT structure can be made more stable than the common tub-shaped isomer.

Keywords:
cyclooctatetraeneelectronic effectsfrustrated Lewis pairmolecular orbital theorysteric effects

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Area of Science:

  • Organic Chemistry
  • Computational Chemistry
  • Isomerism

Background:

  • Cyclooctatetraene (COT) exists primarily in a tub-shaped (Z,Z,Z,Z) isomer.
  • An unconventional (Z,Z,Z,E) isomer of COT is energetically close to the tub-shaped form.

Purpose of the Study:

  • To investigate methods for stabilizing the unconventional (Z,Z,Z,E) isomer of cyclooctatetraene.
  • To explore substituent effects on the relative stability of COT isomers.

Main Methods:

  • Computational calculations to predict energy differences between isomers.
  • Proposal of steric, clamping, and electronic strategies for isomer stabilization.

Main Results:

  • The (Z,Z,Z,E) COT isomer is calculated to be only 23 kcal/mol above the tub-shaped isomer, with a low interconversion barrier.
  • Steric strategy with bulky tert-butyl groups predicts the (Z,Z,Z,E) isomer to be 21 kcal/mol more stable.
  • Aromatic clamping, particularly with fused rings, can confer persistence on the (Z,Z,Z,E) isomer.
  • Push-pull electronic substitution was less effective for stabilizing the (Z,Z,Z,E) isomer but led to amine-borane COTs for H2 activation.

Conclusions:

  • The (Z,Z,Z,E) cyclooctatetraene isomer can be stabilized relative to the tub-shaped isomer through strategic substitution.
  • Steric and aromatic clamping are promising approaches for isolating the less common COT isomer.
  • Amine-borane substituted COTs show potential in frustrated Lewis pair chemistry for H2 activation.