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Related Concept Videos

Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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 with both...
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

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.
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

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In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...
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
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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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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Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
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Tetraphenylene ring flip revisited.

Steven M Bachrach1

  • 1Department of Chemistry, Trinity University, 1 Trinity Place, San Antonio, Texas 78212, USA. sbachrach@trinity.edu

The Journal of Organic Chemistry
|April 7, 2009
PubMed
Summary
This summary is machine-generated.

Benzannulated cyclooctatetraenes undergo ring flipping via a nonplanar transition state. Tetraphenylene exhibits a high energy barrier of 78.6 kcal mol(-1) for this process.

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Area of Science:

  • Organic Chemistry
  • Computational Chemistry

Background:

  • Benzannulated cyclooctatetraenes are a class of polycyclic aromatic hydrocarbons.
  • Understanding their conformational dynamics is crucial for predicting their reactivity and properties.

Purpose of the Study:

  • To investigate the ring flip barrier of benzannulated cyclooctatetraenes.
  • To determine the transition state geometry for the ring flip mechanism.

Main Methods:

  • Density Functional Theory (DFT) calculations using the B3LYP functional.
  • Second-order Møller–Plesset perturbation theory (MP2) calculations.

Main Results:

  • The ring flip of tetraphenylene proceeds through a nonplanar transition state.
  • A large energy barrier of 78.6 kcal mol(-1) was calculated for the ring flip of tetraphenylene.
  • Results contradict previous studies suggesting a planar transition state.

Conclusions:

  • The ring flip mechanism for tetraphenylene involves a nonplanar transition state.
  • The high energy barrier indicates significant conformational rigidity.