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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.

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.

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.

[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.

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...

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.

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Planar and Curved π-Extended Porphyrins by On-Surface Cyclodehydrogenation.

Miloš Baljozović¹, Joffrey Pijeat², Stéphane Campidelli²

¹Molecular Surface Science Group, Empa, 8600 Dübendorf, Switzerland.

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|December 4, 2024

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Synthesizing novel π-extended porphyrins on gold surfaces, this study uses controlled heating to achieve cyclodehydrogenation and create new molecular structures. These advanced materials show promise for catalysis and electronics.

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Area of Science:

Materials Science
Surface Chemistry
Organic Chemistry

Background:

On-surface synthesis enables precise low-dimensional materials preparation.
Porphyrins are tunable molecules studied in solution and on surfaces.
Synthesizing π-extended porphyrins with anthracenyl groups is challenging.

Purpose of the Study:

To report the synthesis of π-extended porphyrins via on-surface cyclodehydrogenation.
To explore the formation of fused anthracenyl porphyrin products.
To investigate the influence of molecular coverage and temperature on product formation.

Main Methods:

In vacuo temperature-controlled cyclodehydrogenation.
Using bis- and tetraanthracenyl Zn(II) porphyrins as precursors.
Substrate: Gold(111) surface.

Main Results:

Sequential dehydrogenation and formation of fused anthracenyl porphyrins achieved.
Bowl-shaped porphyrin formation observed at high molecular coverage.
Transmetalation of Zn with Au occurred during the process.

Conclusions:

On-surface cyclodehydrogenation is an effective method for creating π-extended anthracenyl porphyrins.
The process yields diverse products, including bowl-shaped structures and metal-exchanged porphyrins.
These novel materials have potential applications in catalysis and electronics.