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[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.

[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement

The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

Diels–Alder Reaction Forming Cyclic Products: Stereochemistry

The Diels–Alder reaction is one of the robust methods for synthesizing unsaturated six-membered rings. The reaction involves a concerted cyclic movement of six π electrons: four π electrons from the diene and two π electrons from the dienophile.

Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products.

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.

Diels–Alder Reaction: Characteristics of Dienes

Diels–Alder Reaction: Characteristics of Dienes

The Diels–Alder reaction brings together a diene and a dienophile to form a six-membered ring. Both components have unique characteristics that influence the rate of the reaction.
Characteristics of the diene
Conformation
The simplest example of a diene is 1,3-butadiene, an acyclic conjugated π system. At room temperature, the molecule exists as a mixture of s-cis and s-trans conformers by virtue of rotation around the carbon–carbon single bond. Although the s-trans isomer is...

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Exploring the Skeletal Rearrangement/Self-Ring-Opening Reaction of Dibenzo[b,f][1,5]diphosphacines.

Shuangchen Lv^1,2,3, Haiyang Huang^2,4, Hongli Bao^2,5,3

¹College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350117, P. R. China.

Organic Letters

|April 15, 2025

Summary

Researchers discovered a novel self-ring-opening reaction in an eight-membered heterocyclic compound, dibenzo[b,f][1,5]diphosphacyclooctatetraene (PCOT). This process yields (Z)-alkenyldiphosphines through phosphonium-ylide bond cleavage and C=C bond formation.

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Area of Science:

Organophosphorus Chemistry
Heterocyclic Chemistry
Organic Synthesis

Background:

Eight-membered heterocyclic skeletons are less explored compared to smaller rings.
Phosphonium ylides are versatile intermediates in organic synthesis.
Understanding ring-opening reactions provides insights into novel chemical transformations.

Purpose of the Study:

To report the first self-ring-opening reaction of dibenzo[b,f][1,5]diphosphacyclooctatetraene (PCOT).
To investigate the mechanism and stereoselectivity of the ring-opening process.
To explore the chemical driving forces behind the observed rearrangement.

Main Methods:

Synthesis of the novel eight-membered heterocyclic skeleton, PCOT.
Experimental observation of the self-ring-opening reaction.
Theoretical analyses (e.g., DFT calculations) to rationalize reaction pathways and stereoselectivity.

Main Results:

Successful demonstration of the first self-ring-opening reaction of PCOT.
Formation of (Z)-alkenyldiphosphines as the primary products.
Identification of sequential phosphonium-ylide bond cleavage and C=C bond formation as key steps.
Theoretical analysis confirmed the stereoselective formation of Z-isomers and the reaction's driving force.

Conclusions:

This study introduces a novel class of π-conjugated eight-membered phosphacycles with internal phosphonium-ylide bonds.
The self-ring-opening rearrangement provides a new synthetic route to (Z)-alkenyldiphosphines.
The findings advance the understanding of reactivity in medium-sized phosphorus-containing heterocycles.