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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.
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.
[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 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).
C–C Bond Cleavage: Retro-Aldol Reaction00:57

C–C Bond Cleavage: Retro-Aldol Reaction

The reverse of the aldol addition reaction is called the retro-aldol reaction. Here, the carbon–carbon bond in the aldol product is cleaved under acidic or basic conditions to form two molecules of carbonyl compounds. The mechanism of the reaction consists of three steps.
In the first step, as depicted in Figure 1, the base deprotonates the β-hydroxy ketone at the hydroxyl group to form an alkoxide ion.

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Related Experiment Video

Updated: Jun 2, 2026

Solid-phase Synthesis of [4.4] Spirocyclic Oximes
05:15

Solid-phase Synthesis of [4.4] Spirocyclic Oximes

Published on: February 6, 2019

Mechanically facilitated retro [4+2] cycloadditions.

Kelly M Wiggins1, Jay A Syrett, David M Haddleton

  • 1Department of Chemistry and Biochemistry, The University of Texas, Austin, Texas 78712, USA.

Journal of the American Chemical Society
|April 21, 2011
PubMed
Summary
This summary is machine-generated.

Mechanical force triggers polymer chain scission via retro cycloaddition. Ultrasound applied to poly(methyl acrylate)s with embedded adducts initiated this reaction, demonstrating a new mechanochemical pathway.

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Area of Science:

  • Polymer Chemistry
  • Mechanochemistry
  • Materials Science

Background:

  • Polymer chains can be functionalized using cycloaddition reactions.
  • Mechanochemical reactions offer novel pathways for polymer modification.

Purpose of the Study:

  • To investigate the mechanical activation of retro [4+2] cycloaddition reactions in polymers.
  • To explore ultrasound-induced polymer chain scission.

Main Methods:

  • Synthesis of poly(methyl acrylate)s with embedded [4+2] cycloaddition adducts.
  • Ultrasound treatment of polymers at low temperatures.
  • Analysis using gel permeation chromatography (GPC) and UV-vis spectroscopy.
  • Labeling of liberated moieties with chromophores.

Main Results:

  • Ultrasound induced retro [4+2] cycloaddition in polymers with M(n) > 30 kDa.
  • Mechanically activated retro cycloaddition rate was measured using anthracene adducts.
  • Chain scission and moiety liberation were dependent on adduct location and polymer molecular weight.

Conclusions:

  • Embedded cycloaddition adducts within polymer chains are susceptible to mechanical force.
  • Vectorially opposed mechanical force is key to activating retro cycloaddition.
  • This study demonstrates a new method for polymer chain scission via mechanochemistry.