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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.

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 Photochemical Activation

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.

[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 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.

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

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Updated: Jul 1, 2025

Synthesis of a Borylated Ibuprofen Derivative Through Suzuki Cross-Coupling and Alkene Boracarboxylation Reactions

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Published on: November 30, 2022

Borylated cyclobutanes via thermal [2 + 2]-cycloaddition.

Kateryna Prysiazhniuk¹, Oleksandr Polishchuk¹, Stanislav Shulha¹

¹Enamine Ltd Winston Churchill St. 78 02094 Kyiv Ukraine Pavel.Mykhailiuk@gmail.com https://www.mykhailiukchem.org.

Chemical Science

|March 1, 2024

Summary

This study presents a novel one-step method for synthesizing borylated cyclobutanes using amides and vinyl boronates. The reaction utilizes a thermal [2 + 2]-cycloaddition pathway involving keteniminium intermediates.

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Efficient Construction of Drug-like Bispirocyclic Scaffolds Via Organocatalytic Cycloadditions of α-Imino γ-Lactones and Alkylidene Pyrazolones

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Published on: February 7, 2019

Area of Science:

Organic Chemistry
Synthetic Methodology

Background:

Cyclobutanes are valuable structural motifs in medicinal chemistry and materials science.
Efficient synthetic routes to functionalized cyclobutanes, particularly borylated derivatives, remain an area of active research.
Existing methods for borylated cyclobutane synthesis can be multi-step or lack broad substrate scope.

Purpose of the Study:

To develop a streamlined, one-step protocol for the synthesis of borylated cyclobutanes.
To explore the utility of readily available amides and vinyl boronates as starting materials.
To elucidate the reaction mechanism, specifically the role of keteniminium intermediates.

Main Methods:

A one-step reaction combining amides of carboxylic acids with vinyl boronates under thermal conditions.
In situ generation of keteniminium salts as key reactive intermediates.
Characterization of the resulting borylated cyclobutane products using standard spectroscopic techniques (NMR, Mass Spectrometry).

Main Results:

Successful synthesis of diverse borylated cyclobutanes in a single synthetic operation.
Demonstration of the reaction's efficiency and functional group tolerance.
Evidence supporting a thermal [2 + 2]-cycloaddition mechanism involving keteniminium intermediates.

Conclusions:

The developed one-step approach offers a practical and efficient route to borylated cyclobutanes.
This methodology expands the synthetic toolbox for accessing valuable cyclobutane derivatives.
The findings provide insights into the reactivity of keteniminium salts in cycloaddition reactions.