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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.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical 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

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.

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.

meta-Directing Deactivators: –NO2, –CN, –CHO, –&NoBreak;CO2R, –COR, –CO2H

meta-Directing Deactivators: –NO₂, –CN, –CHO, –⁠CO₂R, –COR, –CO₂H

All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzene because of the deactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for...

Aromatic Hydrocarbon Cations: Structural Overview

Aromatic Hydrocarbon Cations: Structural Overview

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...

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Unlocking Structurally Nontraditional Naphthyridine-Based Electron-Transporting Materials with C-H

Anping Luo¹, Yuanyuan Bao¹, Xiaoyu Liu¹

¹Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064, People's Republic of China.

Journal of the American Chemical Society

|February 5, 2024

Summary

Researchers developed new 2,6-naphthyridine derivatives for organic light-emitting diodes (OLEDs). These electron transport materials (ETMs) offer high performance and stability, advancing OLED technology.

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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

Area of Science:

Organic electronics
Materials science
Synthetic chemistry

Background:

2,6-naphthyridine scaffolds show theoretical promise for electron transport materials (ETMs) in organic light-emitting diodes (OLEDs).
Existing synthetic methods are inadequate for creating complex, highly substituted 2,6-naphthyridine derivatives.

Purpose of the Study:

To develop an efficient synthetic strategy for novel 2,6-naphthyridine-based electron transport materials.
To investigate the performance of these new materials in organic light-emitting diodes.

Main Methods:

Rhodium-catalyzed consecutive C-H activation-annulation of fumaric acid with alkynes.
Design and synthesis of 2,6-naphthyridine framework-based ETMs.
Characterization of material properties, including glass-transition temperature (Tg) and electron mobility (μe).

Main Results:

Successful synthesis of 2,6-naphthyridine derivatives using the novel C-H activation strategy.
Achieved a high glass-transition temperature (Tg) of 282 °C.
Demonstrated high electron mobility (μe) exceeding 10⁻² cm² V⁻¹ s⁻¹, a new benchmark for ETMs.

Conclusions:

The developed rhodium-catalyzed method provides a viable route to complex 2,6-naphthyridine derivatives.
The synthesized ETMs exhibit exceptional thermal stability and electron mobility, suitable for advanced OLED applications.
These materials are versatile for red, green, and blue phosphorescent OLED devices.