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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.

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

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but 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.

Benzene to 1,4-Cyclohexadiene: Birch Reduction Mechanism

Benzene to 1,4-Cyclohexadiene: Birch Reduction Mechanism

Birch reduction uses solvated electrons as reducing agents. The reaction converts benzene to 1,4-cyclohexadiene. The reaction proceeds by the transfer of a single electron to the ring to form a benzene radical anion. This anion is highly basic—it abstracts a proton from the alcohol to form a cyclohexadienyl radical. Another single electron transfer gives the cyclohexadienyl anion. A proton transfer from the alcohol forms 1,4-cyclohexadiene. Since this reduction occurs via radical anion...

Cycloaddition Reactions: Overview

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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Updated: Oct 18, 2025

[DPEPhosbcpCu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst

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Biphenyl Cyclobutenone Photoelectrocyclizations.

Changhang Dai¹, Xuchen Zhao¹, Changqing Song¹

¹Department of Chemistry, University of Utah, 315 South, 1400 East, Salt Lake City, Utah 84112, United States.

The Journal of Organic Chemistry

|September 29, 2021

Summary

Conjugated bis-aryl cyclobutenones transform into dihydrophenanthrene cyclobutanones via 350 nm light-induced photoelectrocyclization. This reaction reveals an equilibrium between cyclobutenone electrocyclizations and cycloreversions.

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Published on: July 17, 2020

Area of Science:

Organic Chemistry
Photochemistry
Synthetic Methodology

Background:

Cyclobutenones are versatile synthetic intermediates.
Photoelectrocyclization is a key reaction in organic synthesis.
Understanding reaction equilibria is crucial for synthetic control.

Purpose of the Study:

To investigate the photoelectrocyclization of conjugated bis-aryl cyclobutenones.
To explore the use of trifluoroacetic acid (TFA) and trimethylsilyl chloride (TMSCl) in this transformation.
To determine the reversibility of the cyclobutenone electrocyclization/cycloreversion.

Main Methods:

Irradiation of bis-aryl cyclobutenones with 350 nm light.
Inclusion of catalytic amounts of TFA and TMSCl.
Analysis of reaction products using spectroscopic methods.

Main Results:

Successful synthesis of dihydrophenanthrene cyclobutanones from bis-aryl cyclobutenones.
Demonstration of photoelectrocyclization under mild conditions (350 nm light, TFA, TMSCl).
Evidence for a reversible reaction pathway, with cyclobutenone electrocyclizations and cycloreversions in equilibrium.

Conclusions:

Conjugated bis-aryl cyclobutenones are effective precursors for dihydrophenanthrene cyclobutanones via photoelectrocyclization.
The reaction proceeds efficiently under specific photochemical and catalytic conditions.
The observed equilibrium highlights the dynamic nature of these cyclization and cycloreversion processes.