Photocatalyst-Free Carbon-Sulfur Cross-Coupling via Visible Light
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Solution Overview
Problem
Current methods for forming carbon-sulfur bonds, such as those in aromatic thioethers, often require strong bases, specific ligands, high temperatures, and expensive catalysts, limiting scalability and sustainability, and existing photoredox catalysis methods rely on costly metals like ruthenium and iridium, with UV-induced reactions prone to side reactions.
Innovation Solution
A method involving irradiation of a system comprising an (hetero)aryl halide and a thiol in the absence of transition metal or organic photocatalysts, using mild bases like K2CO3 and Cs2CO3, and visible light or sunlight to form carbon-sulfur bonds, allowing for wide functional group tolerance and reducing costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If transition metal catalysts are used for C-S bond formation, then the reaction efficiency is improved, but the cost and scalability are worsened
Solution Approach 1:
The patent removes transition metal catalysts from the reaction system entirely, replacing them with a photocatalytic system using organic dyes or semiconductor particles under light irradiation. This extraction of harmful/expensive elements while maintaining reaction functionality directly resolves the contradiction between productivity and ease of manufacture.
Solution Approach 2:
The invention employs inexpensive organic photocatalysts (such as eosin, rose bengal, or simple semiconductor particles like TiO2) that can be easily synthesized or obtained, replacing expensive precious metal catalysts. These photocatalysts operate under mild light conditions and can be used in catalytic amounts, making the process both cost-effective and scalable.
2Productivity
If strong bases and high temperatures are used for C-S bond formation, then the reaction rate is improved, but the functional group tolerance and sustainability are worsened
Solution Approach 1:
The patent fundamentally changes the reaction parameters by replacing strong bases with mild bases (such as K2CO3, Cs2CO3, or even base-free conditions) and replacing high temperatures with ambient or mild temperatures enabled by photoexcitation. The light energy provides the activation needed for the reaction to proceed under gentle conditions, thereby improving functional group tolerance while maintaining acceptable reaction rates.
3Productivity
If UV irradiation is used for photoinitiated coupling, then the reaction is promoted, but side reactions increase
Solution Approach 1:
The invention applies selective wavelength irradiation (visible light or specific UV ranges) that matches the absorption characteristics of the photocatalyst used, rather than broad-spectrum high-energy UV. This localized energy input at specific wavelengths promotes the desired reaction while minimizing excessive energy that would cause side reactions or decomposition.
4Adaptability or versatility
If expensive photocatalysts like ruthenium and iridium are used, then photoredox reactions are enabled, but sustainability and scalability are worsened
Solution Approach 1:
The patent replaces expensive, rare earth photocatalysts (ruthenium, iridium complexes) with abundant, inexpensive alternatives such as organic dyes (eosin, rose bengal, methylene blue) or simple semiconductor particles (TiO2, ZnO). These substitutes maintain photoredox functionality under visible or near-UV light while being environmentally sustainable and suitable for large-scale production.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables the efficient formation of aromatic thioethers under mild conditions without expensive catalysts, promoting scalability and sustainability while tolerating various substituents, thus overcoming the limitations of existing methods.
Implementation Method 1
Irradiation causes an intermolecular charge transfer whereby an electron is transferred from the thiolate to the (hetero)aryl halide
Implementation Method 2
The base deprotonates the thiol to generate the thiolate anion
Implementation Method 3
the resulting (hetero)aryl radical and thiolate radical couple to form the C—S bond
Data Source
AI summary
In one aspect, the invention provides a method of promoting a carbon-sulfur bond forming reaction. In certain embodiments, the reaction comprises cross-coupling of a(n) (hetero)aryl halide with a thiol to form the carbon-sulfur bond, wherein the method is promoted by light irradiation in the absence of a photocatalyst. In other embodiments, the cross-coupling reaction can be promoted through visible light irradiation, including sunlight.