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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

Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic SN1

Nucleophilic Aromatic Substitution of Aryldiazonium Salts: Aromatic S_N1

Treating arylamines with nitrous acid gives aryldiazonium salts that are effective substrates in nucleophilic aromatic substitution reactions. The diazonio group in these salts can be easily displaced by different nucleophiles, yielding a wide variety of substituted benzenes. The leaving group departs as nitrogen gas, and this easy elimination is the driving force for the substitution reaction.
In the Sandmeyer reaction, for example, the diazonio group is replaced by a chloro, bromo, or cyano...

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.

Diazonium Group Substitution with Halogens and Cyanide: Sandmeyer and Schiemann Reactions

Diazonium Group Substitution with Halogens and Cyanide: Sandmeyer and Schiemann Reactions

Arenediazonium substitution reactions occur when the diazonium group is substituted by various functional groups such as halides, hydroxyl, nitrile, etc. For instance, arenediazonium salts react with copper(I) salts of chloride, bromide, or cyanide to form corresponding aryl chlorides, bromides, and nitriles. These reactions are named Sandmeyer reactions. Although the mechanism of this reaction is complicated, as illustrated in Figure 1, they are believed to progress via an aryl copper...

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 with both...

Aryldiazonium Salts to Azo Dyes: Diazo Coupling

Aryldiazonium Salts to Azo Dyes: Diazo Coupling

The reaction of weakly electrophilic aryldiazonium (also called arenediazonium) salts with highly activated aromatic compounds leads to the formation of products with an —N=N— link, called an azo linkage. This reaction, presented in Figure 1, is known as diazo coupling and occurs without the loss of the nitrogen atoms of the aryldiazonium salt. Highly activated aromatic compounds such as phenols or arylamines favor the diazo coupling reaction. The coupling generally occurs at the para position.

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Related Experiment Video

Updated: Jun 16, 2026

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Published on: July 17, 2020

Solarylations via 4-aminophenyl cations.

Valentina Dichiarante¹, Maurizio Fagnoni, Angelo Albini

¹Department of Organic Chemistry, University of Pavia, Viale Taramelli 10, 27100 Pavia, Italy.

The Journal of Organic Chemistry

|January 29, 2010

Summary

This summary is machine-generated.

This study demonstrates a metal-free arylation method using solar irradiation for 4-chloroanilines. The optimized photo-S(N)1 reaction offers an environmentally friendly and scalable alternative to metal-catalyzed processes.

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Area of Science:

Organic Chemistry
Photochemistry
Sustainable Chemistry

Background:

Traditional arylation methods often rely on metal catalysts, posing environmental and cost concerns.
Developing metal-free synthetic routes is crucial for greener chemical processes.

Purpose of the Study:

To explore the photo-S(N)1 reaction for metal-free arylation of 4-chloroanilines under solar irradiation.
To optimize reaction conditions for efficiency, scalability, and environmental friendliness.

Main Methods:

Investigated the photo-S(N)1 reaction of 4-chloroanilines under solar and UV irradiation.
Tested new trapping agents (alpha-methylstyrene) and halides (N,N-dimethyl-4-fluoroaniline).
Optimized halide concentration, trapping agent excess, and solvent composition, including water.

Main Results:

The photo-S(N)1 reaction proceeded effectively under solar irradiation with various substrates.
Optimized conditions allowed for increased halide concentration and reduced trapping agent excess.
Gram-scale photoarylation was achieved using solar irradiation with eco-friendly solvents.

Conclusions:

Solar-driven photo-S(N)1 reaction provides a convenient and scalable metal-free arylation method.
The optimized process offers a sustainable alternative to metal-catalyzed arylations with reduced environmental impact.