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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.
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.
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.
Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation01:27

Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation

Robinson annulation is a base-catalyzed reaction for the synthesis of 2-cyclohexenone derivatives from 1,3-dicarbonyl donors (such as cyclic diketones, β-ketoesters, or β-diketones) and α,β-unsaturated carbonyl acceptors. Named after Sir Robert Robinson, who discovered it, this reaction yields a six-membered ring with three new C–C bonds (two σ bonds and one π bond).
Preparation of Alkynes: Alkylation Reaction02:27

Preparation of Alkynes: Alkylation Reaction

Introduction
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.

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Related Experiment Video

Updated: Jun 16, 2026

Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
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Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate

Published on: June 21, 2017

Exploiting [2+2] cycloaddition chemistry: achievements with allenes.

Benito Alcaide1, Pedro Almendros, Cristina Aragoncillo

  • 1Departamento de Química Orgánica I, Facultad de Química, Universidad Complutense de Madrid, 28040 Madrid, Spain. alcaideb@quim.ucm.es

Chemical Society Reviews
|January 30, 2010
PubMed
Summary

Allenes readily undergo [2+2] cycloaddition reactions with alkenes and alkynes, forming cyclobutane and cyclobutene rings. This review highlights recent advances in transition metal catalysis and intramolecular cycloadditions for synthesizing complex polycyclic compounds.

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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

Published on: September 18, 2016

Area of Science:

  • Organic Chemistry
  • Synthetic Chemistry
  • Catalysis

Background:

  • Allenes are versatile building blocks in organic synthesis.
  • The [2+2] cycloaddition reaction is a key method for forming cyclic structures.
  • Previous studies focused on thermal, photochemical, and microwave-assisted conditions.

Purpose of the Study:

  • To review recent developments in [2+2] cycloaddition reactions involving allenes.
  • To discuss the role of transition metal catalysis in activating allenes.
  • To highlight the application of intramolecular cycloadditions for polycyclic compound synthesis.

Main Methods:

  • Literature review of recent and early works on allene cycloadditions.
  • Analysis of reaction mechanisms, regio-, and diastereoselectivity.
  • Focus on transition metal-catalyzed and intramolecular reactions.

Main Results:

  • Allenes effectively participate in [2+2] cycloadditions with alkenes and alkynes.
  • Transition metal catalysis offers an alternative activation strategy for allenes.
  • Intramolecular cycloadditions enable regio- and stereoselective synthesis of polycyclic systems.

Conclusions:

  • The [2+2] cycloaddition of allenes is a powerful tool for constructing cyclobutane and cyclobutene frameworks.
  • Recent advancements, particularly in catalysis, have expanded the scope and efficiency of these reactions.
  • Intramolecular variants are crucial for accessing complex polycyclic architectures with high selectivity.