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01:26Photochemical Electrocyclic Reactions: Stereochemistry
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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
Selection Rules: Photochemical Activation
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02:38Electrophilic Addition to Alkynes: Halogenation
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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.
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.
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01:26Thermal and Photochemical Electrocyclic Reactions: Overview
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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.
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02:33Preparation and Reactions of Thiols
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Thiols are prepared using the hydrosulfide anion as a nucleophile in a nucleophilic substitution reaction with alkyl halides. For instance, bromobutane reacts with sodium hydrosulfide to give butanethiol.
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01:19Diazonium Group Substitution: –OH and –H
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Nitrous acid, a weak acid, is prepared in situ via the reaction of sodium nitrite with a strong acid under cold conditions. This nitrous acid prepared in situ reacts with primary arylamines to form arenediazonium salts. Such reactions are known as diazotization reactions. As shown in Figure 1, the formation of arenediazonium salts begins with the decomposition of nitrous acid in an acidic solution to give nitrosonium ions.
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01:11Aryldiazonium Salts to Azo Dyes: Diazo Coupling
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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...
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Thianthrenium Salts in Photochemistry.
1Max-Planck-Institut für Kohlenforschung, D45470 Mülheim an der Ruhr, Germany.
Accounts of Chemical Research
|March 4, 2026
Summary
Thianthrenium (TT) salts are versatile reagents for photochemistry and catalysis. They enable efficient radical generation through single-electron transfer or energy transfer, offering distinct advantages over traditional methods.
Area of Science:
- Organic Chemistry
- Photochemistry
- Catalysis
Background:
- Thianthrenium (TT) salts are emerging reagents with broad applications in catalysis and photochemistry.
- They offer advantages over traditional aryl (pseudo)halides, especially in single-electron transfer processes under visible light.
- Their unique redox properties facilitate radical generation and avoid back-electron transfer issues.
Purpose of the Study:
- To explore the evolution and diverse applications of thianthrenium chemistry in photochemistry.
- To highlight advances in photocatalysis, including single-electron transfer (SET) and energy transfer (EnT) mechanisms.
- To discuss direct photolysis and biocompatible applications of TT analogues.
Main Methods:
- Investigated thianthrenium salts in various catalytic systems: transition-metal catalysis, photoredox catalysis, and energy transfer catalysis.
![Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions](/_next/image?url=https%3A%2F%2Fcloudfront.jove.com%2FCDNSource%2Fteasers%2F60786.jpg&w=3840&q=75)
Main Results:
- Thianthrenium salts exhibit favorable redox potentials for photoredox catalysis and low triplet energies for efficient EnT.
- Direct photolysis of TT and selenium salts enables radical generation with visible light.
- Selenonium-based TT analogues allow for photochemical late-stage modification of biomolecules under physiological conditions.
Conclusions:
- Thianthrenium chemistry provides practical and mechanistically distinct solutions for modern radical chemistry.
- These reagents offer versatile activation modes, expanding the scope of photochemical transformations.
- The development of TT analogues, including selenonium-based systems, opens new avenues in catalysis and bioconjugation.