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Related Concept Videos

Pericyclic Reactions: Introduction01:17

Pericyclic Reactions: Introduction

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 rearrangements are...
Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

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...
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.
Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

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.
Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

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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Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds
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Characterization of pericyclic reactions using multicenter electron delocalization analysis.

Marcos Mandado1, María J González-Moa, Ricardo A Mosquera

  • 1Departamento de Química Física, Facultade de Química, Universidade de Vigo, 36310 Vigo, Galicia, Spain. mandado@uvigo.es

Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry
|March 6, 2007
PubMed
Summary

Multicenter delocalization analysis offers a reliable method for characterizing pericyclic reactions, revealing aromatic transition states and concerted mechanisms. This approach provides quantitative insights into electron delocalization, surpassing limitations of magnetic-based indices.

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Area of Science:

  • Quantum Chemistry
  • Theoretical Chemistry
  • Chemical Reaction Dynamics

Background:

  • Pericyclic reactions are fundamental in organic chemistry, often proceeding through concerted mechanisms.
  • Characterizing transition states and reaction pathways is crucial for understanding reaction mechanisms.
  • Existing methods for analyzing electron delocalization have limitations, particularly in complex concerted processes.

Purpose of the Study:

  • To investigate the utility of multicenter delocalization analysis for characterizing pericyclic reactions.
  • To compare the effectiveness of multicenter delocalization indices with magnetic-based indices.
  • To elucidate the nature of concerted mechanisms in various pericyclic reactions.

Main Methods:

  • Application of multicenter delocalization analysis to study pericyclic reactions.
  • Quantitative assessment of electron delocalization along reaction pathways.
  • Comparison with established magnetic-based indices for reaction characterization.

Main Results:

  • Multicenter delocalization indices effectively characterize aromatic transition states in concerted processes.
  • These indices show a significant increase in electron delocalization at transition states.
  • The analysis revealed distinct electron delocalization patterns for Diels-Alder, cyclobutene ring-opening, and [2+2] cycloaddition reactions.

Conclusions:

  • Multicenter delocalization analysis is a powerful and reliable tool for studying concerted pericyclic reactions.
  • It provides quantitative and robust insights into electron delocalization, overcoming limitations of magnetic-based methods.
  • This method aids in understanding the detailed mechanisms of complex chemical transformations.