Electrochemical Epoxidation Catalyzed by Manganese Salen Complex and Carbonate with Boron-Doped Diamond Electrode

At a Glance

Metadata	Details
Publication Date	2023-02-14
Journal	Molecules
Authors	Pijush Kanti Roy, Keisuke Amanai, Ryosuke Shimizu, Masahito Kodera, Takuya Kurahashi
Institutions	Daicel (Japan), Doshisha University
Citations	7
Analysis	Full AI Review Included

Executive Summary

This research establishes a novel, cleaner electrochemical route for alkene epoxidation using Boron-Doped Diamond (BDD) electrodes and carbonate ions as the mediator, addressing limitations found in previous chloride-based systems.

Core Value Proposition: Successful in situ electrochemical generation of percarbonate ions from sodium carbonate (Na₂CO₃) to drive epoxidation, eliminating the unwanted allylic halogenation observed when using chloride ions (hypochlorite generation).
Electrode Performance: BDD anodes are critical, achieving an 18.5% epoxide yield for trans-beta-methylstyrene. This significantly outperforms traditional electrodes like platinum, graphite, and glassy carbon (all yielding less than 2.2%).
Catalyst Optimization: Jacobsen’s catalyst (Mn(L1-t-Bu), a manganese salen complex) proved superior to other manganese salen and porphyrin complexes, yielding the highest epoxide percentage (18.5%).
Mechanism Insight: The high performance of the bulky Jacobsen’s catalyst is tentatively attributed to its structure preventing the formation of inactive μ-oxo dimers, thereby maintaining a higher effective concentration of the active manganese-oxo species.
Optimal Conditions: The reaction requires a low bath temperature (-5 °C) and a specific applied voltage (2.50 V vs. Ag/AgCl) in a dichloromethane/1 M Na₂CO₃ biphasic system.
Future Outlook: While yields require further improvement, this study validates the BDD/carbonate system as a promising, environmentally benign platform for electrocatalytic organic synthesis.

Technical Specifications

Parameter	Value	Unit	Context
Optimal Anode Material	Boron-Doped Diamond (BDD)	N/A	Required due to high overpotential for water oxidation, favoring percarbonate generation.
Optimal Applied Voltage	2.50	V	Applied potential vs. Ag/AgCl reference electrode.
Optimal Bath Temperature	-5	°C	Temperature yielding highest epoxide conversion for trans-beta-methylstyrene.
Anode Surface Area	3	cm²	Electrode size used for comparative tests (BDD, Pt, Graphite).
Electrolyte Concentration	1	M	Sodium Carbonate (Na₂CO₃) concentration in the aqueous phase.
Solvent Ratio (Organic:Aqueous)	1:9	Ratio	Dichloromethane (CH₂Cl₂) to 1 M Na₂CO₃ aq. biphasic system.
Epoxide Yield (trans-beta-methylstyrene)	18.5	%	Achieved using BDD and Mn(L1-t-Bu) catalyst (30 min reaction).
Epoxide Yield (Cyclooctene)	6.8	%	Achieved using BDD and Mn(L1-t-Bu) catalyst (90 min reaction).
Epoxide Yield (Platinum Anode)	1.8	%	Comparative yield for trans-beta-methylstyrene under optimal conditions.
Cis/Trans Epoxide Ratio	85:15	Ratio	Epoxide product derived from cis-beta-methylstyrene substrate.
Catalyst Loading	0.05	mmol	Used for all metal complex tests (0.25 mmol substrate).

Key Methodologies

Electrochemical Setup: Experiments were conducted in a simple, undivided 5 mL vial using an IKA ElectraSyn2.0 system, facilitating rapid screening of reaction conditions.
Electrode Selection: A Boron-Doped Diamond (BDD) plate (3 cm²) was utilized as the working anode, paired with a Platinum (Pt) plate as the cathode.
Biphasic Reaction Mixture: The system consisted of an organic phase (0.5 mL dichloromethane containing the olefin substrate and the manganese catalyst) and an aqueous phase (4.5 mL of 1 M sodium carbonate solution).
Temperature Control: The reaction vessel was immersed in a cooled alcohol bath maintained at -5 °C to suppress unwanted side reactions and maximize epoxide yield.
Constant Voltage Electrolysis: A constant voltage of 2.50 V was applied across the electrodes (referenced against Ag/AgCl) for reaction times ranging from 30 to 90 minutes under gentle stirring.
Product Quantification: Epoxide yields and cis/trans ratios were determined by Gas Chromatography (GC) and High-Performance Liquid Chromatography (HPLC) analysis using authentic samples and an internal standard (p-nitrotoluene or nitrobenzene).

Commercial Applications

Pharmaceutical and Fine Chemical Synthesis: Provides a scalable, electrocatalytic method for generating epoxide functional groups, critical intermediates in drug synthesis and specialty chemical production.
Green Manufacturing and Sustainability: The use of carbonate ions and electricity replaces hazardous, stoichiometric peroxyacids (like m-CPBA) and avoids the formation of halogenated by-products, aligning with modern green chemistry principles.
Advanced Oxidation Processes (AOPs): The reliance on BDD electrodes for generating highly oxidative species (percarbonate) is directly applicable to industrial AOPs, including water purification and pollutant degradation.
Electrochemical Reactor Technology: Validates BDD as the preferred anode material for anodic organic synthesis requiring high overpotentials, informing the design and material selection for industrial electrochemical flow reactors.
Epoxy Precursor Production: Enables the cleaner synthesis of precursors for high-performance epoxy resins used in aerospace, automotive, and construction industries.

View Original Abstract

Epoxides are essential precursors for epoxy resins and other chemical products. In this study, we investigated whether electrochemically oxidizing carbonate ions could produce percarbonate to promote an epoxidation reaction in the presence of appropriate metal catalysts, although Tanaka and co-workers had already completed a separate study in which the electrochemical oxidation of chloride ions was used to produce hypochlorite ions for electrochemical epoxidation. We found that epoxides could be obtained from styrene derivatives in the presence of metal complexes, including manganese(III) and oxidovanadium(IV) porphyrin complexes and manganese salen complexes, using a boron-doped diamond as the anode. After considering various complexes as potential catalysts, we found that manganese salen complexes showed better performance in terms of epoxide yield. Furthermore, the substituent effect of the manganese salen complex was also investigated, and it was found that the highest epoxide yields were obtained when Jacobsen’s catalyst was used. Although there is still room for improving the yields, this study has shown that the in situ electrochemical generation of percarbonate ions is a promising method for the electrochemical epoxidation of alkenes.

Tech Support

Original Source

DOI: https://doi.org/10.3390/molecules28041797

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