Related Concept Videos
01:17Thermal 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.
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.
01:26Thermal 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.
01:26Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
01:28Diels–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.
![[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction](/_next/image?url=https%3A%2F%2Fcloudfront.jove.com%2Ffiles%2Fmedia%2Fscience-education%2Fscience-education-thumbs%2F12417.jpg&w=3840&q=75)
01:16[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.
01:28Stability 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.
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.
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Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes
Published on: April 1, 2013
Engineering a twist in 9,10-diethynylanthracenes by steric interactions.
Andrew Beeby1, Karen S Findlay, Andrés E Goeta
1Department of Chemistry, University of Durham, Durham, UK. andrew.beeby@durham.ac.uk
Summary
Bulky tert-butyl groups lock 9,10-bis(phenylethynyl)anthracene derivatives into an orthogonal conformation. This structure partially planarizes in the excited state at room temperature but not at low temperatures.
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Area of Science:
- Organic Chemistry
- Photophysics
- Materials Science
Background:
- 9,10-bis(phenylethynyl)anthracenes are fluorescent organic molecules with potential applications in optoelectronics.
- Molecular conformation significantly influences photophysical properties, including fluorescence and excited-state behavior.
- Steric hindrance can be used to control molecular geometry and, consequently, electronic properties.
Purpose of the Study:
- To synthesize and characterize novel 9,10-bis(phenylethynyl)anthracene derivatives incorporating bulky tert-butyl substituents.
- To investigate the impact of steric bulk on the ground-state conformation of these anthracene derivatives.
- To explore the excited-state dynamics and conformational changes using fluorescence spectroscopy.
Main Methods:
- Synthesis of substituted 9,10-bis(phenylethynyl)anthracenes.
- Spectroscopic characterization, including fluorescence spectroscopy.
- Variable temperature studies in solution and low-temperature solvent glasses.
Main Results:
- Successful preparation of 9,10-bis(phenylethynyl)anthracenes with sterically demanding tert-butyl groups.
- Demonstrated locking of the ground state into an orthogonal conformation due to ortho-substituted tert-butyl groups.
- Observed partial planarization in the excited state at ambient temperature, which is suppressed at low temperatures.
Conclusions:
- Sterically demanding substituents effectively control the ground-state conformation of 9,10-bis(phenylethynyl)anthracenes.
- Excited-state behavior is sensitive to temperature, with partial planarization occurring at room temperature but not in low-temperature glasses.
- These findings provide insights into structure-property relationships in fluorescent organic materials.