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Related Concept Videos

Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

3.7K
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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Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

Cycloaddition Reactions: MO Requirements for Thermal Activation

5.1K
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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[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

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

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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.
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Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

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Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
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Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene01:14

Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene

3.9K
Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.
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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

2.9K
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.
2.9K

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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Preparation of a Corannulene-functionalized Hexahelicene by CopperI-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

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Alkenes in [2+2+2] Cycloadditions.

Gema Domínguez1, Javier Pérez-Castells2

  • 1Chemistry and Biochemistry department, Universidad San Pablo CEU, Urb. Montepríncipe. Boadilla del Monte., E28668, Madrid, Spain.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|February 27, 2016
PubMed
Summary
This summary is machine-generated.

Alkenes and allenes can participate in [2+2+2] cycloadditions, forming complex polycycles. Recent advances focus on metal-catalyzed reactions and efficient asymmetric synthesis for creating stereogenic centers.

Keywords:
alkenescyclotrimerizationheterocycleshomogeneous catalysistransition metals

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Area of Science:

  • Organic Chemistry
  • Catalysis
  • Synthetic Methodology

Background:

  • The [2+2+2] cycloaddition is a powerful tool for synthesizing cyclic compounds.
  • Traditionally, this reaction involves alkynes, but recent interest has focused on incorporating alkenes and allenes.
  • The use of double bonds offers new pathways to diverse polycyclic structures.

Purpose of the Study:

  • To review recent advancements in [2+2+2] cycloaddition reactions involving alkenes and allenes.
  • To highlight the mechanistic diversity and catalytic systems employed.
  • To discuss the development of asymmetric versions for stereoselective synthesis.

Main Methods:

  • Metal-catalyzed cycloaddition reactions.
  • Cascade processes involving alkenes and allenes.
  • Development of enantioselective catalytic systems.

Main Results:

  • Alkenes and allenes can effectively participate in [2+2+2] cycloadditions.
  • Diverse polycyclic compounds, including cyclohexadienes, are accessible.
  • Various metal catalysts enable different mechanistic pathways.
  • Efficient asymmetric protocols have been developed, controlling stereogenic centers.

Conclusions:

  • The incorporation of alkenes and allenes significantly expands the scope of [2+2+2] cycloadditions.
  • Metal catalysis plays a crucial role in controlling reactivity and selectivity.
  • Asymmetric synthesis is a key area of development for accessing chiral polycycles.