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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.

Thermal and Photochemical Electrocyclic Reactions: Overview

Thermal and Photochemical Electrocyclic Reactions: Overview

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.

Stability of Conjugated Dienes

Stability of Conjugated Dienes

Introduction
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.

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...

[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.

Structure of Conjugated Dienes

Structure of Conjugated Dienes

Introduction
Conjugated dienes are compounds characterized by the presence of alternating double and single bonds. In a conjugated system like 1,3-butadiene, the unhybridized 2p orbital on each carbon overlaps continuously, allowing the π electrons to be delocalized across the entire molecule. In contrast, this type of overlap does not occur in cumulated and isolated dienes, such as 2,3-pentadiene and 1,4-pentadiene, respectively. Instead, the π electrons remain localized between the...

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Related Experiment Video

Updated: Apr 26, 2026

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

Published on: November 29, 2018

Dithienylethene-based rotaxanes: synthesis, characterization and properties.

Fang Hu¹, Juanyun Huang, Meijiao Cao

¹Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China. yinj@mail.ccnu.edu.cn.

Organic & Biomolecular Chemistry

|August 2, 2014

Summary

Photochromic ammoniums were used to create novel photo-responsive rotaxanes and pseudorotaxanes. These molecular machines exhibit reversible photoisomerization, with cucurbituril-based systems showing enhanced performance.

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Last Updated: Apr 26, 2026

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

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Area of Science:

Supramolecular Chemistry
Materials Science
Organic Chemistry

Background:

Photochromic materials are essential in various applications.
Dithienylethene derivatives serve as key photochromic units.
Mechanically interlocked molecules (MIMs) offer unique functionalities.

Purpose of the Study:

To synthesize novel photochromic ammoniums and utilize them as templates.
To construct photo-responsive rotaxanes and pseudorotaxanes.
To investigate the photoisomerization behavior of these MIMs.

Main Methods:

Template-directed clipping reactions and threading approaches were employed.
Synthesis and structural characterization of novel rotaxanes, including single crystal X-ray diffraction.
UV-Vis spectroscopy to study photoisomerization and reversibility.

Main Results:

Photochromic ammoniums successfully templated the formation of rotaxanes and pseudorotaxanes.
N-hetero crown ether-based rotaxanes demonstrated reversible photochromism similar to their ammonium precursors.
Cucurbit[6]uril-based pseudorotaxanes exhibited superior photoisomerization compared to ammoniums and N-hetero crown ether rotaxanes.

Conclusions:

The study successfully demonstrates the construction of photo-responsive MIMs using photochromic ammonium templates.
The photoisomerization properties can be tuned by the choice of host molecule.
Cucurbituril hosts show promise for enhancing photochromic performance in pseudorotaxanes.