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Related Concept Videos

Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

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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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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

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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
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)01:16

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Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring...
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Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

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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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Photochemical Electrocyclic Reactions: Stereochemistry01:26

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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
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Chair Conformation of Cyclohexane02:02

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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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Reversible polyphenylacetylene helix conversion driven by a thermoresponsive rotaxane switch in the solid state.

Nan Zhu1, Kazuko Nakazono2, Toshikazu Takata2

  • 1Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan. ttakata@polymer.titech.ac.jp.

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Thermoresponsive rotaxane switches enable reversible helical pitch and color changes in polyphenylacetylenes. This polymer modification offers potential for novel smart materials and sensors.

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Area of Science:

  • Polymer Chemistry
  • Supramolecular Chemistry
  • Materials Science

Background:

  • Polyphenylacetylenes are known for their unique helical structures.
  • Rotaxanes are mechanically interlocked molecules with potential for responsive applications.
  • Controlling polymer conformation and properties is crucial for advanced materials.

Purpose of the Study:

  • To demonstrate reversible helical pitch change in polyphenylacetylenes using a thermoresponsive rotaxane switch.
  • To investigate the correlation between helical pitch alteration and observable color changes.
  • To explore the utility of rotaxane moieties in side-chain functionalized polymers.

Main Methods:

  • Synthesis of polyphenylacetylene with a rotaxane moiety in its side chain.
  • Induction of conformational changes using a thermoresponsive rotaxane switch triggered by trichloroacetic acid (TCA) treatment and heating.
  • Characterization of helical pitch and color changes in both solution and solid states.

Main Results:

  • The polyphenylacetylene derivative exhibited reversible changes in helical pitch.
  • A distinct color change accompanied the helical pitch alteration.
  • The thermoresponsive rotaxane switch effectively controlled the polymer's conformation.
  • These changes were observed in both solution and solid states.

Conclusions:

  • Thermoresponsive rotaxane switches can be successfully integrated into polyphenylacetylene side chains.
  • This integration allows for external control over polymer helical pitch and color.
  • The findings suggest potential applications in smart materials, sensors, and responsive systems.