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Related Concept Videos

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.
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
Introduction to Electrophilic Addition Reactions of Alkenes02:24

Introduction to Electrophilic Addition Reactions of Alkenes

The double bond in a simple, unconjugated alkene is a region of high electron density that can act as a weak base or a nucleophile. The filled π orbital (HOMO) of the double bond can interact with the empty LUMO of an electrophile. A bonding interaction occurs when the electrophile attacks between the two carbons; the electrophile then accepts a pair of electrons from the π bond and undergoes addition across the double bond, yielding a single product.
Addition and elimination reactions can be...
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.
Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

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.
Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

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.

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Related Experiment Video

Updated: Jun 1, 2026

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization
12:19

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

Published on: November 29, 2018

The arene-alkene photocycloaddition.

Ursula Streit1, Christian G Bochet

  • 1Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland.

Beilstein Journal of Organic Chemistry
|June 8, 2011
PubMed
Summary

Benzene derivatives undergo three photocycloaddition reactions with alkenes: [2+2] (ortho), [3+2] (meta), and [4+2] (para). This review highlights the synthetic utility of these powerful photochemical transformations.

Keywords:
Diels–Alderbenzene derivativescycloadditionsphotochemistry

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
07:12

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

Published on: July 17, 2020

Related Experiment Videos

Last Updated: Jun 1, 2026

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization
12:19

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

Published on: November 29, 2018

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions
07:12

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 Chemistry

Background:

  • Photocycloaddition reactions are crucial in organic synthesis.
  • Benzene derivatives and alkenes are common substrates in photochemical reactions.
  • Understanding reaction modes is key to controlling synthetic outcomes.

Purpose of the Study:

  • To review the synthetic applications of photocycloaddition reactions involving benzene derivatives and alkenes.
  • To highlight the versatility of ortho, meta, and para photocycloaddition pathways.
  • To showcase the synthetic power of these photochemical methods.

Main Methods:

  • Literature review of photocycloaddition reactions.
  • Analysis of reaction mechanisms and outcomes.
  • Compilation of synthetic examples demonstrating utility.

Main Results:

  • Identified three distinct modes of photocycloaddition: [2+2] (ortho), [3+2] (meta), and [4+2] (para).
  • Demonstrated the regioselectivity and stereoselectivity achievable in these reactions.
  • Showcased the formation of complex cyclic structures from simple precursors.

Conclusions:

  • Photocycloaddition reactions offer powerful and versatile routes for constructing complex organic molecules.
  • The ability to control ortho, meta, and para pathways enhances their synthetic applicability.
  • These photochemical transformations are valuable tools for synthetic chemists.