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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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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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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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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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Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

4.3K
Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
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CASSCF Computational Study of Pseudopericyclic Character in Electrocyclic Rearrangements Involving Heteroatoms.

Irena R Bierzynski1, Cassandra A Settle1, Henry W Kreiman1

  • 1Department of Chemistry, Lewis & Clark College , Portland, Oregon 97219-7899, United States.

The Journal of Organic Chemistry
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Summary

Computational chemistry reveals four of six heteroatom-containing 1,2,4,6-heptatetraene electrocyclic rearrangements are pseudopericyclic, while two are pericyclic. This study clarifies reaction mechanisms and revises prior classifications for specific rearrangements.

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Area of Science:

  • Organic Chemistry
  • Computational Chemistry
  • Quantum Chemistry

Background:

  • Electrocyclic rearrangements are fundamental organic reactions.
  • Distinguishing between pericyclic and pseudopericyclic pathways is crucial for predicting reactivity.
  • Heteroatoms (oxygen, nitrogen) can significantly influence reaction mechanisms.

Purpose of the Study:

  • To computationally investigate six electrocyclic rearrangements of 1,2,4,6-heptatetraene systems with heteroatoms.
  • To classify these rearrangements as either pericyclic or pseudopericyclic.
  • To compare computational findings with previous theoretical studies.

Main Methods:

  • Utilizing the Complete Active Space Self-Consistent Field (CASSCF) method.
  • Employing the 6-31G* basis set for calculations.
  • Analyzing active space orbitals, transition structure geometries, and activation energies.

Main Results:

  • Four rearrangements (3 → 4, 5 → 6, 7 → 8, 9 → 10) were identified as pseudopericyclic.
  • Two rearrangements (13 → 14, 15 → 16) were classified as pericyclic.
  • Discrepancies were found with previous density functional theory studies for rearrangements 5 → 6 and 15 → 16.

Conclusions:

  • The study provides detailed rationales for the pericyclic and pseudopericyclic classifications.
  • Findings contribute to a deeper understanding of reaction mechanisms in heteroatom-substituted systems.
  • This work refines the mechanistic understanding of electrocyclic reactions involving complex unsaturated systems.