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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.

Cycloaddition Reactions: MO Requirements for Thermal Activation

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 Reaction

[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.

Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control

Electrophilic Addition of HX to 1,3-Butadiene: Thermodynamic vs Kinetic Control

The addition of a hydrogen halide to 1,3-butadiene gives a mixture of 1,2- and 1,4-adducts. Since more substituted alkenes are more stable, the 1,4-adduct is expected to be the major product. However, the product distribution is strongly influenced by temperature; low temperature favors the 1,2-adduct, whereas the 1,4-adduct is predominant at high temperature.

Conjugate Addition to α,β-Unsaturated Carbonyl Compounds

Conjugate Addition to α,β-Unsaturated Carbonyl Compounds

α,β-Unsaturated carbonyl compounds are molecules bearing a carbonyl and alkene functionality in conjugation with each other. The conjugation in the molecule leads to three resonance structures. The hybrid form exhibits two probable electrophilic sites: the carbonyl carbon and the β carbon.

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

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

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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Related Experiment Video

Updated: May 28, 2026

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Published on: April 19, 2019

Dynamics of carbene cycloadditions.

Lai Xu¹, Charles E Doubleday, K N Houk

¹Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States.

Journal of the American Chemical Society

|October 4, 2011

Summary

This summary is machine-generated.

This study reveals the detailed dynamics of carbene cycloadditions to alkenes. It classifies these reactions based on bond formation timing, differentiating concerted versus complex mechanisms for dichlorocarbene (CCl2) and difluorocarbene (CF2).

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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

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Area of Science:

Chemical Dynamics
Computational Chemistry
Reaction Mechanisms

Background:

Understanding the dynamics of carbene cycloaddition reactions is crucial for predicting reaction outcomes.
Previous studies have lacked detailed dynamical insights into these processes.
Computational methods offer a powerful tool to explore reaction pathways at the molecular level.

Purpose of the Study:

To provide a detailed dynamical picture of singlet carbene (CCl2 and CF2) cycloadditions to alkenes.
To explore the range of geometries and bond formation timing in reactive trajectories.
To propose a quantitative dynamical classification of cycloaddition mechanisms.

Main Methods:

Quasiclassical trajectory calculations utilizing quantum mechanical energies and forces from Venus and Gaussian programs.
Employing the B3LYP/6-31G* density functional theory method.
Utilizing a modified B3LYP functional with reduced exact exchange for CF2 calculations to enhance accuracy.

Main Results:

All trajectories followed a nonlinear approach to cycloaddition.
The reaction of CCl2 with ethylene was found to be dynamically concerted, with a 50 fs bond formation time gap.
The reaction of CF2 with ethylene exhibited complex dynamics, characterized by biexponential decay of the intermediate diradical species.

Conclusions:

The timing of bond formation is a key factor in classifying carbene cycloaddition mechanisms.
Dynamical differences exist between CCl2 and CF2 cycloadditions, leading to distinct reaction pathways.
This work provides a foundational dynamical framework for understanding carbene-alkene cycloadditions.