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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.

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

Radical Substitution: Halogenation of Alkanes and Alkyl Substituents

Radical Substitution: Halogenation of Alkanes and Alkyl Substituents

In the presence of heat or light, alkanes react with molecular halogens to form alkyl halides by a substitution reaction called radical halogenation. This reaction has three steps: initiation, propagation, and termination, as seen in the radical chlorination of methane to produce methyl chloride.
In the initiation step of the reaction, the chlorine molecule undergoes homolytic cleavage in the presence of light or heat, forming two highly reactive chlorine radicals. Propagation occurs in two...

Halogenation of Alkenes

Halogenation of Alkenes

Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.

Electrophilic Addition to Alkynes: Halogenation

Electrophilic Addition to Alkynes: Halogenation

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.

Reactions at the Benzylic Position: Halogenation

Reactions at the Benzylic Position: Halogenation

Benzylic halogenation takes place under conditions that favor radical reactions such as heat, light, or a free radical initiator like peroxide.

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Updated: Apr 14, 2026

Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst

Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst

Published on: April 22, 2016

No photocatalyst required--versatile, visible light mediated transformations with polyhalomethanes.

Johannes F Franz¹, Wolfgang B Kraus, Kirsten Zeitler

¹Institut für Organische Chemie, Universität Leipzig, D-04103 Leipzig, Germany. kzeitler@uni-leipzig.de.

Chemical Communications (Cambridge, England)

|April 17, 2015

Summary

A new photocatalyst-free method uses visible light to functionalize tertiary amines and perform Kharasch-type additions. This approach offers mild conditions for diverse chemical transformations.

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Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst

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Published on: April 22, 2016

Facile Synthesis of Worm-like Micelles by Visible Light Mediated Dispersion Polymerization Using Photoredox Catalyst

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[DPEPhosbcpCu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst

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Published on: May 21, 2019

Area of Science:

Organic Chemistry
Photochemistry
Synthetic Methodology

Background:

Tertiary amines are crucial building blocks in pharmaceuticals and materials.
Oxidative functionalization of C-H bonds offers efficient synthetic routes.
Visible light photoredox catalysis has emerged as a powerful tool in organic synthesis.

Purpose of the Study:

To develop a photocatalyst-free method for oxidative α-CH functionalization of tertiary amines.
To explore the use of visible light for activating polyhalomethanes.
To investigate the application of this method for Kharasch-type additions to olefins.

Main Methods:

Visible light irradiation.
Use of polyhalomethanes as halogen sources and activators.
Reactions involving tertiary amines and various nucleophiles.
Kharasch-type addition reactions to non-activated olefins.

Main Results:

Successful oxidative α-CH functionalization of tertiary amines with diverse nucleophiles.
Demonstration of visible light-triggered activation of polyhalomethanes.
Efficient Kharasch-type additions to non-activated olefins under mild conditions.
Preliminary mechanistic investigations were conducted.

Conclusions:

A novel, photocatalyst-free visible light-mediated method for amine functionalization and olefin addition has been established.
The method provides a mild and efficient pathway for complex molecule synthesis.
Polyhalomethanes serve as key reagents activated by visible light.