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A racemic mixture, or racemate, is an equimolar mixture of enantiomers of a molecule that can be separated using their unique interaction with chiral molecules or media. Racemic mixtures are denoted by the (±)- prefix. This ‘optical rotation descriptor’ applies to the whole solution of a racemic mixture rather than a specific stereoisomer. Enantiomers typically have the same physical and chemical properties. Hence, they are not easily separable. However, enantiomers can exhibit...
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Hexahelicenophanes and Their Racemization.

Herbert Meier1, Manfred Schwertel1, Dieter Schollmeyer1

  • 1Institut für Organische Chemie der Universität, J. J. Becherweg 18-22, D-55099 Mainz (Germany), Fax: (+49) 6131-395396.

Angewandte Chemie (International Ed. in English)
|May 2, 2018
PubMed
Summary

Bridged hexahelicenophanes exhibit significantly faster racemization rates compared to [6]helicene. Steric hindrance from polymethylenedioxy chains alters ground and transition state energies, accelerating this process.

Keywords:
CyclophanesHelical structuresKineticsRacemization

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Area of Science:

  • Organic Chemistry
  • Stereochemistry
  • Molecular Dynamics

Background:

  • Chiral molecules like helicenes are crucial in various scientific fields.
  • Understanding racemization mechanisms is key to controlling stereochemistry.
  • Helicenophanes offer a unique structural platform for studying molecular properties.

Purpose of the Study:

  • To investigate the impact of bridged polymethylenedioxy chains on the racemization rate of [6]helicene derivatives.
  • To elucidate the conformational and energetic factors influencing the stereochemical stability of hexahelicenophanes.
  • To compare the racemization dynamics of substituted hexahelicenophanes with the parent [6]helicene.

Main Methods:

  • Synthesis of bridged hexahelicenophanes (n=8, 10).
  • Computational analysis of ground and transition state energies.
  • Spectroscopic characterization of the synthesized compounds.

Main Results:

  • A drastic increase in the rate of racemization was observed for bridged hexahelicenophanes (1, n=8, 10) compared to unsubstituted [6]helicene.
  • Steric interactions between polymethylenedioxy chains and terminal rings were identified as the primary cause.
  • These interactions lead to a destabilization of the ground state and a stabilization of the transition state.

Conclusions:

  • Bridged hexahelicenophanes demonstrate significantly enhanced racemization rates due to specific steric effects.
  • The findings provide insights into the relationship between molecular structure and stereochemical dynamics in complex helical systems.
  • This study contributes to the understanding of chiral molecular behavior and the design of novel stereochemically controlled compounds.