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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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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.
Selection Rules: Photochemical Activation
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Pericyclic Reactions: Introduction01:17

Pericyclic Reactions: Introduction

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Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic...
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Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation01:27

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Robinson annulation is a base-catalyzed reaction for the synthesis of 2-cyclohexenone derivatives from 1,3-dicarbonyl donors (such as cyclic diketones, β-ketoesters, or β-diketones) and α,β-unsaturated carbonyl acceptors. Named after Sir Robert Robinson, who discovered it, this reaction yields a six-membered ring with three new C–C bonds (two σ bonds and one π bond).
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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

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Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
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Updated: Nov 9, 2025

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
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[10]annulene: Electrocyclization mechanisms.

Raúl Palmeiro1, Obis Castaño1

  • 1Analytical Chemistry, Physical Chemistry and Chemical Engineering Department, University of Alcalá, Alcalá de Henares, Spain.

Journal of Computational Chemistry
|April 12, 2021
PubMed
Summary
This summary is machine-generated.

Quantum-chemical methods reveal the 6-π electrocyclization mechanism of [10]annulene isomers. Mono-trans [10]annulene directly forms trans-4a,8a-dihydronaphthalene, while all-cis requires a bond shift before cyclization.

Keywords:
6π-electrocyclizationCASPT2[10]annulenebond shiftingkinetics

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Area of Science:

  • Organic Chemistry
  • Computational Chemistry

Background:

  • [10]annulene isomers undergo 6-π electrocyclization to form 4a,8a-dihydronaphthalene.
  • Understanding the reaction mechanism and stereochemical outcomes is crucial for synthetic chemistry.

Purpose of the Study:

  • To elucidate the detailed quantum-chemical mechanism of 6-π electrocyclization for all-cis, mono-trans, and double-trans [10]annulene isomers.
  • To compare computational predictions with experimental findings for these cyclization reactions.

Main Methods:

  • Utilized various quantum-chemical methods to explore reaction pathways.
  • Analyzed transition states and energy profiles for different [10]annulene configurations.

Main Results:

  • Mono-trans [10]annulene preferentially cyclizes to trans-4a,8a-dihydronaphthalene, aligning with experimental data.
  • All-cis [10]annulene requires a rate-limiting bond shift to a double-trans naphthalene-like conformation before electrocyclization to cis-4a,8a-dihydronaphthalene.
  • Computed rate coefficients for all-cis cyclization match experimental values, supporting the proposed mechanism.

Conclusions:

  • The study confirms the distinct cyclization pathways for different [10]annulene isomers.
  • Computational results validate the stereochemical assignments of isolated isomers by Masamune et al.