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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this...
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
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In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...
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Changing Liquid Crystal Helical Pitch with a Reversible Rotaxane Switch.

Sujun Chen1, Nathalie Katsonis2, David A Leigh1,3

  • 1School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200062, P. R. China.

Angewandte Chemie (International Ed. in English)
|March 6, 2024
PubMed
Summary
This summary is machine-generated.

Researchers demonstrate how changing a molecule's (rotaxane) chirality dynamically alters the pitch of a liquid crystal. This bridges molecular chirality to macroscopic properties in soft matter systems.

Keywords:
Mechanical BondRotaxanesSoft MatterStereochemistry

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Area of Science:

  • Supramolecular Chemistry
  • Soft Matter Physics
  • Chirality Studies

Background:

  • Understanding the transmission of chiral information across different length scales is a significant challenge in both biology and artificial systems.
  • Mimicking and controlling chirality in molecular and macroscopic systems remains an active area of research.

Purpose of the Study:

  • To demonstrate the transduction of dynamic chirality changes in a rotaxane molecule into macroscopic changes in a soft matter system.
  • To investigate the mechanism by which molecular chirality influences larger-scale structures.

Main Methods:

  • Utilized a rotaxane molecule with a movable macrocycle and a point-chiral center on its axle.
  • Investigated the effect of macrocycle position on the expression of chirality at the molecular and macroscopic levels.
  • Observed changes in the pitch length of a chiral nematic liquid crystal phase.

Main Results:

  • Shuttling the macrocycle on the rotaxane axle dynamically altered the expression of chiral information.
  • This molecular-level change in chirality was successfully transmitted to the macroscopic scale.
  • A significant change in the pitch length of the chiral nematic liquid crystal structure was observed.

Conclusions:

  • The study successfully demonstrated the dynamic control of macroscopic chirality through molecular design.
  • Rotaxane systems can act as effective transducers of chiral information across multiple length scales.
  • This work provides insights into controlling soft matter properties via molecular-level chiral switching.