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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

Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene

Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene

Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.

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 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.

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.

The Z-Scheme of Electron Transport in Photosynthesis

The Z-Scheme of Electron Transport in Photosynthesis

The light reactions of photosynthesis assume a linear flow of electrons from water to NADP+. During this process, light energy drives the splitting of water molecules to produce oxygen. However, oxidation of water molecules is a thermodynamically unfavorable reaction and requires a strong oxidizing agent. This is accomplished by the first product of light reactions: oxidized P680 (or P680+), the most powerful oxidizing agent known in biology. The oxidized P680 that acquires an electron from the...

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Updated: Jan 8, 2026

Synthesis of Antiviral Tetrahydrocarbazole Derivatives by Photochemical and Acid-catalyzed C-H Functionalization via Intermediate Peroxides CHIPS

Synthesis of Antiviral Tetrahydrocarbazole Derivatives by Photochemical and Acid-catalyzed C-H Functionalization via Intermediate Peroxides CHIPS

Published on: June 20, 2014

Harnessing Photoenzymatic Systems for Intermolecular C-H Fluorination.

Yu Zhou¹, Danielle Lawson², Zihan Zhang¹

¹Department of Chemistry, The University of Texas at Austin, Austin, TX, 78712, USA.

Angewandte Chemie (International Ed. in English)

|December 22, 2025

Summary

This summary is machine-generated.

Scientists developed a novel radical photoenzymatic system for direct C-H fluorination. This breakthrough enables precise, selective enzymatic fluorination of organic compounds, crucial for pharmaceutical development.

Keywords:

Biocatalysis Biosynthesis Fluorination Photoenzymes Protein engineering

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Light-driven Enzymatic Decarboxylation

Light-driven Enzymatic Decarboxylation

Published on: May 22, 2016

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Synthesis of Antiviral Tetrahydrocarbazole Derivatives by Photochemical and Acid-catalyzed C-H Functionalization via Intermediate Peroxides CHIPS

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Synthesis of pH Dependent Pyrazole, Imidazole, and Isoindolone Dipyrrinone Fluorophores using a Claisen-Schmidt Condensation Approach

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Light-driven Enzymatic Decarboxylation

Light-driven Enzymatic Decarboxylation

Published on: May 22, 2016

Area of Science:

Biocatalysis and Organic Chemistry
Enzyme Engineering and Protein Design

Background:

Organofluorine compounds are essential in pharmaceuticals, yet efficient enzymatic fluorination methods are limited.
Nature's enzymes offer precise catalysis, but enzymatic intermolecular C-H fluorination remained an unmet challenge.

Purpose of the Study:

To develop the first enzymatic strategy for intermolecular C-H fluorination.
To engineer a novel radical photoenzymatic system for selective fluorination.

Main Methods:

Designed a de novo protein scaffold incorporating an unnatural amino acid.
Utilized a radical photoenzymatic approach driven by photoexcited amino acid hydrogen atom transfer.
Employed Selectfluor as the fluorinating agent in aqueous solutions.

Main Results:

Achieved chemoselective benzylic monofluorination of various aromatic compounds.
Demonstrated successful biosynthesis of fluorinated polyketides.
Enabled the synthesis of chiral fluorinated alcohols.

Conclusions:

Established the first radical photoenzymatic system for intermolecular C-H fluorination.
This biocatalytic approach offers efficient and selective fluorination relevant to pharmaceutical synthesis.
The developed system opens new avenues for enzymatic synthesis of organofluorine compounds.