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Related Concept Videos

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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Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in...
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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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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
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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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Pericyclic Reactions: Introduction01:17

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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.
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Going Beyond Woodward and Hoffmann's Electrocyclizations and Cycloadditions: Sigmatropic Rearrangements.

Jeffrey I Seeman1

  • 1Department of Chemistry, University of Richmond, Richmond, Virginia, 23173, USA.

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|November 13, 2024
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Summary

Orbital symmetry control, a concept developed by Woodward and Hoffmann, explained valence isomerizations, now known as sigmatropic reactions. Their 1965 research trajectory utilized quantum chemical tools, detailed in surviving manuscripts.

Keywords:
Conservation of Orbital SymmetryCope reactionFrontier Molecular Orbital TheoryWoodward-Hoffmann rulessigmatropic reactions

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Area of Science:

  • Organic Chemistry
  • Quantum Chemistry
  • Chemical History

Background:

  • R. B. Woodward and Roald Hoffmann published seminal work in 1965.
  • Their research focused on explaining valence isomerization mechanisms.

Purpose of the Study:

  • To detail the research trajectory of Roald Hoffmann.
  • To analyze the application of orbital symmetry control to sigmatropic reactions.
  • To examine the manuscript-writing process of Nobel laureates.

Main Methods:

  • Application of orbital symmetry control.
  • Utilized five quantum chemical tools.
  • Based on extended Hückel theory and frontier molecular orbital theory.

Main Results:

  • Provided a mechanistic explanation for sigmatropic reactions.
  • Demonstrated the utility of quantum chemical calculations in understanding reaction mechanisms.
  • Revealed the research and writing process of Hoffmann and Woodward.

Conclusions:

  • Orbital symmetry control is a key principle in understanding valence isomerizations.
  • The surviving documents offer insights into the scientific discovery and communication process.
  • Hoffmann's work on this problem was foundational.