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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

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: 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.

Loss of Carboxy Group as CO2: Decarboxylation of β-Ketoacids

Loss of Carboxy Group as CO₂: Decarboxylation of β-Ketoacids

Carboxylic acids, upon heating, undergo a decarboxylation reaction by releasing carbon dioxide gas. Monocarboxylic acids do not undergo decarboxylation easily. However, a silver salt of carboxylic acid reacts with bromine or iodine under high temperature to release carbon dioxide gas and forms halide with one less carbon. This reaction is called the Hunsdiecker reaction.

Loss of Carboxy Group as CO2: Decarboxylation of Malonic Acid Derivatives

Loss of Carboxy Group as CO₂: Decarboxylation of Malonic Acid Derivatives

Just like β-keto acids—which upon thermal decarboxylation form ketones—β-dicarboxylic acids undergo decarboxylation to generate monocarboxylic acids with the liberation of carbon dioxide.

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Related Experiment Video

Updated: Jan 11, 2026

Light-driven Enzymatic Decarboxylation

Light-driven Enzymatic Decarboxylation

Published on: May 22, 2016

Decarboxylative photocatalytic transformations.

Francisco Foubelo^1,2, Carmen Nájera², M Gracia Retamosa^1,2

¹Departamento de Química Orgánica and Instituto de Síntesis Orgánica (ISO), Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain.

Chemical Society Reviews

|November 10, 2025

Summary

Photocatalytic decarboxylation transforms carboxylic acids into valuable carbon radicals for organic synthesis. This visible light-driven method enables diverse bond formations and functionalizations under mild conditions.

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Area of Science:

Organic Chemistry
Photocatalysis
Radical Chemistry

Background:

Photocatalytic decarboxylation of carboxylic acids is a key strategy in modern organic synthesis.
It utilizes visible light and photocatalysts to generate carbon-centered radicals from carboxylic acids or their esters.

Purpose of the Study:

To review the diverse applications of photocatalytic decarboxylation in forming C-C and C-heteroatom bonds.
To highlight the efficiency and mild conditions of these photoredox transformations.

Main Methods:

Employing catalytic amounts of metal-based or organic photocatalysts.
Utilizing visible light irradiation to initiate the reaction.
Applying the generated carbon radicals to various synthetic transformations.

Main Results:

Successful formation of C-C bonds through addition, hydroalkylation, and cross-coupling reactions (arylation, alkylation, etc.).
Efficient synthesis of C-heteroatom bonds (C-halogen, C-O, C-N, etc.).
Regioselective hydro- and deuterodecarboxylation, olefination, and C-C bond cleavage reactions.

Conclusions:

Photocatalytic decarboxylation offers a versatile and efficient route to functionalized molecules from readily available carboxylic acids.
These photoredox reactions represent a significant advancement in organic synthesis due to mild conditions and broad applicability.
The method provides a clean energy input for synthesizing diverse compounds relevant to nature and the pharmaceutical industry.