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Pericyclic Reactions: Introduction

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...

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

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: Overview

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.

Cycloaddition Reactions: MO Requirements for Thermal Activation

Cycloaddition Reactions: MO Requirements for Thermal Activation

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.

Thermal Electrocyclic Reactions: Stereochemistry

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.

Photochemical Electrocyclic Reactions: Stereochemistry

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

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Quantum Tunneling Instability in Pericyclic Reactions.

Alexander Frenklach¹, Hila Amlani¹, Sebastian Kozuch¹

¹Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 841051, Israel.

Journal of the American Chemical Society

|April 18, 2024

Summary

Quantum tunneling significantly impacts molecular stability, revealing many supposedly stable molecules are unsynthesizable. Computational chemistry is crucial for predicting reactivity and guiding experimental efforts, especially at cryogenic temperatures.

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Area of Science:

Computational chemistry
Chemical kinetics
Quantum mechanics

Background:

Retro-Diels-Alder reactions are crucial for understanding chemical stability.
Quantum tunneling (QT) effects can influence reaction rates and molecular stability.
Exothermic reactions involving N2, CO, and other leaving groups were investigated.

Purpose of the Study:

To computationally assess quantum tunneling reactivity in cycloreversion reactions.
To understand the impact of QT on molecular stability and decomposition rates.
To explore the role of tunneling in activation enthalpy and kinetic isotope effects.

Main Methods:

Computational assessment of cycloreversion reactions.
Analysis of quantum tunneling (QT) decomposition rates.
Investigation of thermodynamic and kinetic stabilities.

Main Results:

Quantum tunneling significantly reduces molecular stability and half-lives, even at cryogenic temperatures.
Many molecules considered stable are predicted to be unsynthesizable or unisolable due to QT.
Tunneling is essential for accurately understanding activation enthalpy and kinetic isotope effects.

Conclusions:

Computational chemistry is vital for guiding experimental synthesis and detection of molecules.
QT decomposition rates must be considered for accurate stability predictions.
The study highlights the importance of in silico methods to resolve experimental uncertainties in chemistry.