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Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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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.
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The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
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Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

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Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
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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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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.
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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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Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
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Light-controlled interconversion between a [c2]daisy chain and a lasso-type pseudo[1]rotaxane.

Chih-Wei Chu1, Daniel L Stares1, Christoph A Schalley1

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This summary is machine-generated.

This study introduces a light-responsive molecule that changes its structure between a daisy chain and a lasso form using light. This novel photoswitchable conjugate offers new possibilities in molecular machinery and supramolecular chemistry.

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Area of Science:

  • Supramolecular Chemistry
  • Photochemistry
  • Organic Chemistry

Background:

  • Crown ethers and ammonium ions form host-guest complexes.
  • Photoswitches enable light-controlled molecular changes.
  • Arylazopyrazole derivatives are effective photoswitches.

Purpose of the Study:

  • To design and synthesize a novel light-responsive conjugate.
  • To investigate the structural transformation of the conjugate upon light irradiation.
  • To explore the potential of this system in molecular switches and machines.

Main Methods:

  • Synthesis of a crown ether/ammonium conjugate incorporating an arylazopyrazole photoswitch.
  • Photoisomerization studies using UV-Vis spectroscopy and NMR.
  • Structural characterization of the E- and Z-isomers.

Main Results:

  • The conjugate exists as a [c2]daisy chain in the E-isomer form.
  • Upon Z-isomerization of the photoswitch, the conjugate transforms into a lasso-type pseudo[1]rotaxane.
  • The structural switching is reversible and light-controllable.

Conclusions:

  • A light-responsive molecular system capable of reversible structural transformation was developed.
  • The arylazopyrazole photoswitch effectively controls the self-assembly of the crown ether/ammonium conjugate.
  • This work demonstrates a new strategy for creating light-switchable supramolecular architectures.