Undivided Cell Research Articles

Electrochemical oxidative radical cascade cyclization of dienes and diselenides towards the synthesis of seleno-benzazepines.

Selenium-containing compounds are important scaffolds owing to their value in medicinal chemistry, biochemistry and material chemistry. Herein, we report an electrochemical approach to access seleno-benzazepines through an oxidative radical cascade cyclization of dienes with diselenides under metal-free, external oxidant-free and base-free conditions. In a simple undivided cell, various dienes and diselenides were suitable for this transformation, generating the desired products in up to 84% yields. This method provides a green and convenient route for the synthesis of valuable selenium-containing seven-membered N-heterocycles from simple starting materials.

Effect of the pressure on the cathodic production of H2O2 and on electro-Fenton in undivided and divided cells

The key step for both the electrochemical H2O2 production and the electro-Fenton (EF) process is the cathodic reduction of O2 which is adversely affected by the low oxygen solubility in water contacted at atmospheric pressure of air. In this work, we studied the effect of current density and air pressure on the H2O2 production and on the EF process using both a divided and an undivided cell. It was shown that the use of pressures between 5 and 15 bar allows to enhance drastically the H2O2 production in both undivided and divided cells and that the best results were achieved in the presence of the separator. Moreover, the coupled use of pressurized air and divided cells made it possible to accelerate the organics removal achieved by the EF process. Eventually, the advantages given by the utilization of divided and undivided cells and of pressurized air were analyzed from both technical and economic points of view.

Selected Electrosynthesis of 3-Aminopyrazoles from α,β-Alkynic Hydrazones and Secondary Amines.

An available and simple electromediated cyclization method for 3-amino-substituted pyrazoles by using α,β-alkynic hydrazone and secondary amine is described. The strategy utilizes KI as an electrolyte in an undivided cell with a constant current, generating the desired products in moderate-to-good yield. The method features selective amination at the 3-position of the pyrazole skeleton. The results indicate that α,β-alkynic hydrazones functionalized with aromatic groups and secondary amines functionalized with electron-rich groups were better tolerated in this transformation.

Electrochemical Oxidative Ring‐Opening Reactions of 2H‐Indazoles with Alcohols to Obtain Ortho‐alkoxycarbonylated Azobenzenes

AbstractA strategy for synthesis of azo‐derived compounds by electrochemical oxidative ring‐opening reactions of 2H‐indazoles with alcohols is reported. The reactions are carried out under galvanostatic electrolysis conditions in an undivided cell without the use of transition metal catalysts. On the basis of cyclic voltammetry and control experiments, a plausible mechanism for this ring‐opening reaction involving the radical pathway is proposed. Furthermore, various 2H‐indazoles and alcohols are suitable in this system, giving the desired ortho‐alkoxycarbonylated azobenzenes in 9–84% yields.

Electrochemically driven regioselective organoselenation for selective synthesis of β- hydroxy substituted selanylated ketones

A simple yet efficient approach towards the synthesis of β-hydroxy selanylated ketones was developed from easily accessible chalcones and diphenyl diselenide through an electrochemical pathway. In this technique, diphenyl diselenide and water are the source of PhSe. and hydroxyl (–OH) groups that can be inserted at the α and β positions respectively producing β-hydroxy selanylated ketones in moderate to good yields. The reaction features difunctionalization of ketones, catalyst free, mild reaction conditions, stereo and regioselectivity in an undivided electrochemical cell at room temperature. For this transformation, a plausible radical mechanism has been put out.

Membrane-Free Selective Semi-Hydrogenation of Alkynes Over an In Situ Formed Copper Nanoparticle Electrode.

Selective semi-hydrogenation of alkynes is a significant reaction for preparing functionalized alkenes. Electrochemical semi-hydrogenation presents a sustainable alternative to the traditional thermal process. In this research, affordable copper acetylacetonate is employed as a catalyst precursor for the electrocatalytic hydrogenation of alkynes, using MeOH as the hydrogen source in an undivided cell. Good to excellent yields for both aromatic and aliphatic internal/terminal alkynes are obtained under constant current conditions. Notably, up to 99% Z selectivity is achieved for various internal alkynes. Mechanistic investigations revealed the formation of copper nanoparticles (NPs) at the cathode during electrolysis, acting as the catalyst for the selective semireduction of alkynes. The copper NPs deposited cathode demonstrated reusable for further hydrogenation.

Coupled electrochemical crystallization-electrocoagulation-flocculation process for efficient removal of hardness and silica from reverse osmosis concentrate

The benign electrochemical technologies are the promising methods for the reuse of reverse osmosis concentrate (ROC) by softening and removing silica, whereas its low treatment performance and high energy consumption limit its wide industrial application. In this study, an undivided electrochemical crystallization cell with the extraction of acid anode boundary layer was developed for the production of the alkaline effluent, where the removal of hardness and silica occurred by the homogeneous crystallization reactions. This process was further improved in the electrocoagulation cell because coagulants enhance the removal of crystalline particles and accelerate the settling of the crystals. Under optimal conditions, the system showed a removal efficiency of 90 % for total hardness, 76 % for Mg hardness, and 82 % for silica, with an energy consumption of only 2.4 kWh m−3. The removal of Ca hardness in the forms of calcite and aragonite was impeded by the presence of Mg2+. But the coexistence of silica promoted Ca hardness removal since the reaction between Mg2+ and silica with the formation of sepiolite mitigated the inhibitory effect of Mg2+ on CaCO3 precipitation. The coupled electrochemical crystallization-electrocoagulation-flocculation technique provides an energy-efficient solution for the removal of hardness and silica from ROC, therefore improving the water utilization in industry.

Cascade Electrochemical Aerobic Oxygenation of 2-Substituted Indoles and Electrochemical [5 + 3] Annulation with Amidines: Access to Eight-Membered Benzo[1,3,5]triazocin-6(5H)-ones.

The cascade electrochemical C3-selective aerobic oxygenation of 2-substituted indoles and electrochemical [5 + 3] annulation with amidines through an undivided cell galvanostatic method employing molecular oxygen and "electricity" as green oxidants was developed. This protocol provides an efficient and direct approach to eight-membered benzo[1,3,5]triazocin-6(5H)-ones. Mechanistic studies suggested that two subsequent electrochemical processes both proceeded through radical pathways.

Controlling the morphology and properties of electrodeposited polyaniline layer on TiO2 nanotubes

Polyaniline (PANI) layers were electrodeposited, as the conductive emeraldine salt phase (PANI (ES)), on the surface of titanium dioxide nanotubes (TiO2 NTs) to evaluate the protective and photo(electro)chemical performance. TiO2 NTs plates were obtained by anodization, in a two-electrode in the undivided cell. The adjustment of the electropolymerization parameters (e.g. aniline concentration, cycles number in cyclic voltammetry, TiO2 cathodic polarization) allowed the control of the PANI (ES) morphology and film thickness, changing the charge recombination dynamics on the PANI (ES)/TiO2 and PANI (ES)/liquid interfaces. It was observed that lower aniline concentrations resulted in the best control of the electropolymerization process, which ensured the deposition of thin films. The TiO2 NTs/PANI (ES) interface was characterized by absorption and vibrational spectroscopies, and scanning electron microscopy. Electronic and photo(electro)chemical properties of the TiO2 NTs/PANI (ES) films were characterized by photoelectrochemical measurements and electrochemical impedance spectroscopy.

Energy-Saving Electrochemical Hydrogen Production Coupled with Biomass-Derived Isobutanol Upgrading.

The widespread application of electrochemical hydrogen production faces significant challenges, primarily attributed to the high overpotential of the oxygen evolution reaction (OER) in conventional water electrolysis. To address this issue, an effective strategy involves substituting OER with the value-added oxidation of biomass feedstock, reducing the energy requirements for electrochemical hydrogen production while simultaneously upgrading the biomass. Herein, we introduce an electrocatalytic approach for the value-added oxidation of isobutanol, a high energy density bio-fuel, coupled with hydrogen production. This approach offers a sustainable route to produce the valuable fine chemical isobutyric acid under mild condition. The electrodeposited Ni(OH)2 electrocatalyst exhibits exceptional electrocatalytic activity and durability for the electro-oxidation of isobutanol, achieving an impressive faradaic efficiency of up to 92.4 % for isobutyric acid at 1.45 V vs. RHE. Mechanistic insights reveal that side reactions predominantly stem from the oxidative C-C cleavage of isobutyraldehyde intermediate, forming by-products including formic acid and acetone. Furthermore, we demonstrate the electro-oxidation of isobutanol coupled with hydrogen production in a two-electrode undivided cell, notably reducing the electrolysis voltage by approximately 180 mV at 40 mA cm-2. Overall, this work represents a significant step towards improving the cost-effectiveness of hydrogen production and advancing the conversion of bio-fuels.

Controllable Construction of Vinyl Sulfones and β‐Keto Selenosulfones via Selective Oxidative Sulfonylation of Alkenes

Comprehensive SummaryThe selective oxidative sulfonylation of alkenes with selenium sulfonate depended on the reaction conditions. The electrochemical C—H sulfonylation proceeded smoothly to afford (E)‐vinyl sulfones with good selectivity in an undivided cell without external oxidant. While aerobic trifunctionalization of alkenes occurred in the presence of KI in the air, which provides β‐keto selenosulfones via the formation of C—O, C—S, and C—Se bonds in one‐pot. Following control experiments, a plausible mechanism is proposed to rationalize the experimental results.

Electrochemical Dearomatizing Methoxylation of Phenols and Naphthols: Synthetic and Computational Studies.

The electrochemical oxidative dearomatizing methoxylation of phenols and naphthols was developed. It provides an alternative route for the preparation of methoxycyclohexadienones, important and versatile synthetic intermediates, that eliminates the need for stoichiometric high-energy chemical oxidants and generates hydrogen as a sole by-product. The reaction proceeds in a simple constant current mode, in an undivided cell, and it employs standardized instrumentation. A collection of methoxycyclohexadienones derived from various 2,4,6-tri-substituted phenols and 1-substituted-2-naphthols was obtained in moderate to excellent yields. These include a complex derivative of estrone, as well as methoxylated dearomatized 1,1'-bi-2-naphthols (BINOLs). The mechanism of the reaction was subject to profound investigations using density functional theory calculations. In particular, the reactivity of two key intermediates, phenoxyl radical and phenoxenium ion, was carefully examined. The obtained results shed light on the pathway leading to the desired product and rationalize experimentally observed selectivities regarding a side benzylic methoxylation and the preference for the functionalization at the para over the ortho position. They also uncover the structure-selectivity relationship, inversely correlating the steric bulk of the substrate with its propensity to undergo the side-reaction. Moreover, the loss of stereochemical information from enantiopure BINOL substrates during the reaction is rationalized by the computations.

Electrooxidative Rhodium(III)/Chiral Carboxylic Acid-Catalyzed Enantioselective C-H Annulation of Sulfoximines with Alkynes.

The combination of achiral Cp*Rh(III) with chiral carboxylic acids (CCAs) represents an efficient catalytic system in transition metal-catalyzed enantioselective C-H activation. However, this hybrid catalysis is limited to redox-neutral C-H activation reactions and the adopt to oxidative enantioselective C-H activation remains elusive and pose a significant challenge. Herein, we describe the development of an electrochemical Cp*Rh(III)-catalyzed enantioselective C-H annulation of sulfoximines with alkynes enabled by chiral carboxylic acid (CCA) in an operationally friendly undivided cell at room temperature. A broad range of enantioenriched 1,2-benzothiazines are obtained in high yields with excellent enantioselectivities (up to 99 % yield and 98 : 2 er). The practicality of this method is demonstrated by scale-up reaction in a batch reactor with external circulation. A crucial chiral Cp*Rh(III) intermediate is isolated, characterized, and transformed, providing rational support for a Rh(III)/Rh(I) electrocatalytic cycle.

Electrooxidative Rhodium(III)/Chiral Carboxylic Acid‐Catalyzed Enantioselective C−H Annulation of Sulfoximines with Alkynes

AbstractThe combination of achiral Cp*Rh(III) with chiral carboxylic acids (CCAs) represents an efficient catalytic system in transition metal‐catalyzed enantioselective C−H activation. However, this hybrid catalysis is limited to redox‐neutral C−H activation reactions and the adopt to oxidative enantioselective C−H activation remains elusive and pose a significant challenge. Herein, we describe the development of an electrochemical Cp*Rh(III)‐catalyzed enantioselective C−H annulation of sulfoximines with alkynes enabled by chiral carboxylic acid (CCA) in an operationally friendly undivided cell at room temperature. A broad range of enantioenriched 1,2‐benzothiazines are obtained in high yields with excellent enantioselectivities (up to 99 % yield and 98 : 2 er). The practicality of this method is demonstrated by scale‐up reaction in a batch reactor with external circulation. A crucial chiral Cp*Rh(III) intermediate is isolated, characterized, and transformed, providing rational support for a Rh(III)/Rh(I) electrocatalytic cycle.

High-flux electrochemical phosphorus recovery in an undivided electrolytic cell coupled with microfiltration with low energy consumption

Electrochemical systems are promising for phosphorus (P) removal and recovery from wastewater. However, its industrial application still faces some challenges, including relatively low P recovery efficacy, high energy consumption, and complex configuration of the reactors. This work, developed an undivided electrolytic cell with H+ or OH− extraction from its corresponding electrode surface for P recovery. The experimental results and models validated the superiority of continuously and precisely extracting H+ from the tubular Ti porous anode surface for H+–OH− separation over OH− extraction from the cathode surface. In the electrolytic cell, alkaline effluent with H+ extraction, the P could be precipitated with Ca2+ in the form of insoluble calcium phosphate (Ca-phosphate), which could then be separated from the wastewater by microfiltration. Under the conditions of a current density of 4 mA cm−2, an initial P concentration of 0.6 mM, a influent flow rate of 500 mL min−1, and H+ extraction rate of 50 mL min−1, the P recovery efficiency reached 81 % with the alkaline effluent pH of 10.5. The calculated P recovery rate and treatment capacity were 18.3 g P (m2 h)-1 and 1194 L (m2 h)-1, respectively, at energy consumption of 8.75 kWh (kg P)-1, which were superior over the cutting-edge divided electrolytic systems and the conventional undivided electrolytic systems. The recovered precipitates are composed principally of hydroxyapatite in the minor presence of amorphous calcium phosphate, showing a Ca/P molar ratio of 1.8. Generally, the established undivided electrolytic cell coupled with microfiltration exhibited significant prospects for recovering P from wastewater without acid-base reagents.

A Facile Electrosynthesis of Polysubstituted Oxazoles from Aromatic Ketones and Benzylamines under Mild Conditions

AbstractWith KI as the supporting electrolyte and a mediator, DMSO as the solvent and KHCO3 as the base, a convenient and efficient electrochemical route was developed for the synthesis of polysubstituted oxazoles from aromatic ketones and benzylamines. The electrosynthesis was smoothly carried out in an undivided cell at room temperature, affording the target products in moderate to high yields.

Electrochemical-Induced C-N Bond Formation: A New Method to Synthesis (Z)-Quinazolinone Oximes Using Primary Amines and Quinazolin-4(3H)-one.

A novel and highly selective electrochemical method for the synthesis of diverse quinazolinone oximes via direct electrooxidation of primary amines/C(sp2)-H functionalization of oximes has been developed. The reaction is conducted in an undivided cell under constant current conditions and is oxidant-free, open-air, and eco-friendly. Notably, the protocol shows good functional group tolerance, providing versatile quinazolinone oximes in good yields. Moreover, the mechanism is investigated through control experiments and cyclic voltammogram (CV) experiments.

One-pot redox cascade paired electrosynthesis of gamma-butyrolactone from furoic acid

The catalytic valorisation of biomass to afford synthetically useful small molecules is essential for sustainable biorefinery processes. Herein, we present a mild cascaded electrochemical protocol for converting furoic acid, a common biomass-derived feedstock, into a versatile platform chemical, gamma-butyrolactone. In the platinum(+)|nickel(−) electrode paired undivided cell, furoic acid is electrochemically oxidised with 84.2% selectivity to 2(5H)-furanone, the olefin of which is then hydrogenated to yield gamma-butyrolactone with 98.5% selectivity. The final gamma-butyrolactone yield is 69.1% with 38.3% Faradaic efficiency and 80.1% carbon balance when the reaction is performed with 100 mM furoic acid at 80 °C at +2.0 VAg/AgCl. Mechanistic investigation revealed the critical temperature and electrolyte pH conditions that maximise the production and protection of the key intermediate, furan radical, promoting its transition to 2(5H)-furanone rather than self-polymerising. The reaction is scalable, as 2.1 g of 98.1% pure gamma-butyrolactone is isolated through a simple solvent extraction.

Electrochemical Stereoselective Synthesis of (Z)‐Allyl Aryl Selenides and Sulfides from Baylis‐Hillman Acetates

AbstractThis research presents a method for the oxidative selenylation and thiolylation of Morita‐Baylis‐Hillman adducts through constant‐current electrolysis in an undivided cell at room temperature. The reaction enables direct production of trisubstituted (Z)‐allyl aryl selenides and sulfides with yields of 27–98%. A wide variety of different functionalities are well tolerated under these reaction conditions. This strategy could be carried out on gram scale and several synthetic transformations were accomplished for the construction of other allyl selenide derivatives. Control experiments and mechanistic studies indicate a radical or an ionic addition pathway for this electrochemical transformation.

Direct electrochemical C-3 arylation of Quinoxalin-2(1H)- ones with cyanoaromatics via paired electrolysis

A novel and environmentally friendly electrochemical process is described for the direct C-3 arylation of quinoxalin-2(1H)-ones using readily available cyanoaromatics through paired electrolysis. The reaction features excellent atom economy, mild reaction conditions and simple operation (using undivided electrolytic cells at ambient temperature), without the need for oxidants, catalysts, metals, or sacrificial reagents. It offers a practical method of synthesizing 3-arylquinoxalin-2(1H)-one derivatives.