Undivided Cell Research Articles

ELECTROSYNTHESIS OF CATALYTIC ACTIVE Pd-Cu AND Pd-Au BIMETALLIC NANOPARTICLES NANOCOMPOSITES WITH POLY(N-VINYLPYRROLIDONE) AND NANOCELLULOSE

It was investigated the preparation in an undivided cell of Pd-Cu and Pd-Au bimetallic nanoparticles (NPs) by methylviologen (MV2+) -mediated electrochemical reduction of equimolar amounts of Cu(II), Pd(II) and Au(I) in the presence of poly(N-vinylpyrrolidone) (PVP) and nanocellulose (NC) at controlled potential of generating MV cation radical in aqueous medium at room temperature. Electrosyntheses were performed by sequential or joint reduction of metal ions by passing a theoretical amount of electricity. When Pd(II) ions are added to CuNPs, as well as Au(I) ions are added to PdNPs, a galvanic replacement process is observed, namely oxidation of Cu0 by Pd(II) and Pd0 Au(I) ions. The results of complete reduction are nanocomposites of mainly spherical MNPs, dispersed in the solution bulk, and stabilized by PVP on the surface of the NC. In the sequential synthesis of CuNPs and then PdNPs, the nanocomposite is presented as Cu2O nanoroses coated with fine PdNPs. Nanocomposites of Pd NPs with Cu2O or Au shows the mainly formation of spherical particles with the size of 4 to 50 nm depending on the production method. X-ray powder diffraction (XRD) data of nanocomposites confirm the formation of a mixture of PdNPs (0.8 - 10 nm) with large gold crystallites (until 24 nm), as well as the oxidation of CuNPs to cuprite (Cu2O). The size of metal crystallites and copper oxide varies in the range from 0.8 to 24 nm. In the test reaction of p-nitrophenol reduction with sodium borohydride in aqueous medium, all tested nanocomposites showed time-increasing catalytic activity. When Cu is added to Pd, the catalytic reduction reaction is maintained, while the addition of Au to Pd decreases the catalytic activity of PdNPs by an order of magnitude.

Electrochemical Synthesis of Isoxazoles and Isoxazolines via Anodic Oxidation of Oximes

AbstractIsoxazol(in)es are widely featured structural motifs in natural products, agrochemicals, and pharmaceuticals. The first intermolecular approach for a direct electrochemical synthesis from readily available aldoximes is reported. Isoxazoles and isoxazolines were obtained in yields up to 81 %. The synthesis is carried out in an undivided cell as the simplest electrochemical set‐up and requires only the use of electric current as traceless oxidizing agent. The application of inexpensive and widely available electrode materials in combination with recyclable supporting electrolytes and solvents paves the path for translation of the presented reaction onto preparative scale. This is underlined by successful scale‐up to multi‐gram runs.

Electrochemical Synthesis of Sulfonamides in Single‐Pass Flow

AbstractAn electrochemical multicomponent synthesis of sulfonamides at room temperature in single‐pass flow is presented. In contrast to batch‐type electrolysis, an undivided flow cell setup with a stainless‐steel cathode and either a boron‐doped diamond (BDD) anode or a glassy carbon anode can be employed. Simply by using SO2 stock solutions, less atom‐economic sulfur dioxide surrogates can be avoided. Moreover, no additional supporting electrolytes are required due to the in‐situ generation of amidosulfinates, which also serve as intermediate for this transformation. This protocol allows sulfonamides to be synthesized directly from non‐prefunctionalized electron‐rich arenes with amines and SO2. In total, 10 examples are demonstrated with isolated yields up to 92 %. The robust scalability of this electrosynthesis with an easy downstream processing was also proven.

Electrochemical Oxidative Sulfenocyclization of Alkenoic Acids with Disulfides for the Synthesis of Lactones

AbstractA direct electrochemical oxidative sulfenocyclization of alkenoic acids with disulfides for the synthesis of substituted lactone derivatives has been developed under metal‐free and oxidant‐free conditions. The simple undivided cell with platinum/platinum electrode setups offers broad substrate scope and good functional group compatibility, affording a series of thio‐substituted lactones in moderate to good chemical yields. Furthermore, the mechanism has been investigated by control experiments and cyclic voltammetry.

Selective Electrochemical Oxidation of Benzylic C-H to Benzylic Alcohols with the Aid of Imidazolium Radical Mediators.

Selective oxidation of benzylic C-H to benzylic alcohols is a well-known challenge in the chemical community since benzylic C-H is more prone to be overoxidized to benzylic ketones. In this work, we report the highly selective electro-oxidation of benzylic C-H to benzylic alcohols in an undivided cell in ionic liquid-based solution. As an example, the selectivity toward xanthydrol could be as high as 95.7% at complete conversion of xanthene, a typical benzylic C-H compound, on gram-scale in imidazolium bromide/H2O/DMF. Mechanism investigation reveals that the imidazolium radical generated in situ participants in a proton-coupled electron transfer process and low-barrier hydrogen bonds stabilize the reaction intermediates, together steering the redox equilibrium, favoring benzylic alcohols over benzylic ketones.

Peracetic acid-based electrochemical treatment of sulfamethoxazole and real antibiotic wastewater: Different role of anode and cathode

Although has high oxidation capacity and low toxic by-product formation potential, the feasibility, mechanism, and antibiotic treatment performance of peracetic acid (PAA)-based electrochemical system remains unknown. This work systematically studied the electro-activation process of PAA, and distinguished the different mechanisms of anode and cathode. In the PAA-based electrochemical system, the anode mainly produces BDD(•OH), and the cathode is mainly the R-O• (especially CH3CO3•). These differences lead to different degradation pathway and toxicity evolution of sulfamethoxazole (SMX). The anode transformation products (TPs) show negative toxicity and are difficult to be further removed, while TPs from PAA-dominated cathode posed electron-donating effect and a tapering ecological risk. The BDD(•OH) can well mineralize the TPs produced from cathode. Moreover, the active chlorine produced by the anode can effectively avoid the accumulation of NH4+- N released by antibiotic degradation. In an undivided cell, PAA-based treatment for real antibiotic wastewater achieved 73.9%, 59.4%, 76.9%, and 31.7% of COD, TOC, NH4+- N, and TN removal, respectively. More importantly, when PAA existed in this system, the active chlorine and AOCl accumulation are inhibited (inhibition ratio 83.5% and 82.7%, respectively). This study provides theoretical and technical support for the practical application of PAA-based electrochemical system.

Rhodaelectro-Catalyzed Decarboxylative Cross-Dehydrogenative Coupling of Indole-3-carboxylic Acids and Olefins via Weakly Coordinating Carboxyl Groups.

A rhodaelectro-catalyzed C2-H selectively decarboxylative alkenylation of 3-carboxy-1H-indoles employing electricity as the traceless terminal oxidant has been accomplished. The weakly coordinating carboxyl group serves as the traceless directing groups. External oxidant-free in an undivided cell with constant current in aqueous solution ensures the decarboxylative C-H alkenylation to be viable and sustainable.

Degradation of the antibiotic ciprofloxacin in urine by electrochemical oxidation with a DSA anode

Ciprofloxacin (CIP) is a commonly prescribed fluoroquinolone antibiotic that, even after uptake, remains unmetabolized to a significant extent—over 70%. Unmetabolized CIP is excreted through both urine and feces. This persistent compound manages to evade removal in municipal wastewater facilities, leading to its substantial accumulation in aquatic environments. This accumulation raises concerns about potential risks to the health of various living organisms. Herein, we present a study on the remediation of CIP in synthetic urine by electrochemical oxidation in an undivided cell with a DSA (Ti/IrO2) anode and a stainless-steel cathode. Physisorbed hydroxyl radical formed at the anode surface from water discharge and free chlorine generated from Cl− oxidation were the main oxidizing agents. The effect of pH and current density (j) on CIP degradation was examined, and its total removal was easily achieved at pH ≥ 7.0 and j ≥ 60 mA cm−2 due to the action of free chlorine. The CIP decay always followed a pseudo-first-order kinetics. The components of the synthetic urine were also oxidized. The main nitrogenated species released was NH3. A very small concentration of free chlorine was quantified at the end of the treatment, thus demonstrating the good performance of electrochemical oxidation and its effectiveness to destroy all the organic pollutants. The present study demonstrates the simultaneous oxidation of the organic components of urine during CIP degradation, thus showing a unique perspective for its electrochemical oxidation that enhances the environmental remediation strategies.

Metal-Free Electrochemical Reduction of Disulfides in an Undivided Cell under Mass Transfer Control.

Electroorganic synthesis is generally considered to be a green alternative to conventional redox reactions. Electrochemical reductions, however, are less advantageous in terms of sustainability, as sacrificial metal anodes are often employed. Divided cell operation avoids contact of the reduction products with the anode and allows for convenient solvent oxidation, enabling metal free greener electrochemical reductions. However, the ion exchange membranes required for divided cell operation on a commercial scale are not amenable to organic solvents, which hinders their applicability. Herein, we demonstrate that electrochemical reduction of oxidatively sensitive compounds can be carried out in an undivided cell without sacrificial metal anodes by controlling the mass transport to a small surface area electrode. The concept is showcased by an electrochemical method for the reductive cleavage of aryl disulfides. Fine tuning of the electrode surface area and current density has enabled the preparation of a wide variety of thiols without formation of any oxidation side products. This strategy is anticipated to encourage further research on greener, metal free electrochemical reductions.

Electrochemical Cyclization of α,β‐Alkynic Hydrazones and Primary Amines for the Synthesis of 3‐Alkynyltriazoles

AbstractAn environmentally friendly iodine‐catalyzed electrochemical synthetic protocol has developed to access 3‐alkynyltriazoles through the cyclization of α,β‐alkynic hydrazones and primary amines. This reaction is carried out without the addition of electrolyte with excellent functional group tolerance in an undivided cell under mild conditions, affording substituted 3‐alkynyltriazoles with moderate to good yields. The gram scale reaction demonstrates the potential of this protocol in industrial synthesis.

Electrosynthesis of Spiro-indolenines via Dearomative Arylation of Indoles in Batch and Continuous Flow.

An electrosynthesis of spiro-indolenines in batch and continuous flow was achieved through dearomative arylation of indoles with good functional group compatibility. User-friendly undivided cells were used under catalyst- and oxidant-free conditions. Moreover, the use of a flow electrolysis cell gave high daily productivity and excellent scale-up potential under less supporting electrolyte and higher substrate concentration conditions.

CYCLOBIS(PARAQUAT-P-PHENYLENE) - MEDIATED ELECTROSYNTHESIS OF SILVER NANOPARTICLES

Silver nanoparticles (Ag-NP) were obtained in MeCN/0.05 M Bu4NPF6 medium by сyclobis(paraquat-p-phenylene) (CBPQT4+) – mediated reduction of the silver ions generated by anodic oxidation of metallic silver during the electrolysis in an undivided cell. Due to multipoint donor-acceptor interaction CBPQT4+ binds the resulting electron-donor Ag-NP to each other, which leads to their enlargement, aggregation and adsorption. This property of the macrocycle allows to call it a “molecular glue” for NP-Ag. In the absence of stabilizers, aggregated polydisperse Ag-NP of indefinite shape are formed with sizes ranging from 20 to 500 nm. Electrosynthesis in the presence of a stabilizer, polyvinylpyrrolidone (PVP), also leads to the formation of aggregated smaller metal particles of 55 ± 26 nm, which have, in addition to the quasi-spherical shape, the shape of a flat triangle and hexagon. Ag-NP stabilized by PVP are partially bound on the surface of nanocellulose (NC). In the presence of NC, larger Ag-NP with an average size of 97 ± 29 nm are formed, the main shape of which is quasi-spherical; cubic, tetrahedral, and rod-shaped Ag-NP are also formed; the formation of Ag-NP with a flat structure is excluded. The catalytic activity of the obtained particles in the reduction of p-nitrophenol with sodium borohydride is extremely low due to the large size, aggregation, and coating of the NP-Ag surface with the stabilizer PVP and marcocycle.

Electrochemical Synthesis of Unnatural Amino Acids via Anodic Decarboxylation of N-Acetylamino Malonic Acid Derivatives.

Broad application of α,α-disubstituted cyclic amino acid derivatives in medicinal chemistry urges for analogue design with improved pharmacokinetic properties. Herein, we disclose an electrochemical approach toward unnatural THF- and THP-containing amino acid derivatives that relies on anodic decarboxylation-intramolecular etherification of inexpensive and readily available N-acetylamino malonic acid monoesters under Hofer-Moest reaction conditions. The decarboxylative cyclization proceeds under constant current conditions in an undivided cell in an aqueous medium without any added base. A successful bioisosteric replacement of the 1-aminocyclohexane-1-carboxylic acid subunit by the THP-containing amino acid scaffold in cathepsin K inhibitor balicatib helped to reduce lipophilicity while retaining low nanomolar enzyme inhibitory potency and comparable microsomal stability.

Electrochemical 1,3-Alkyloxylimidation of Arylcyclopropane Radical Cations: Four-Component Access to Imide Derivatives.

Herein, a general electrochemical radical-cation-mediated four-component ring-opening 1,3-alkyloxylimidation of arylcyclopropanes, acetonitrile, carboxylic acids, and alcohols is described, providing a facile and sustainable approach to quickly construct structurally diverse imide derivatives from easily available raw materials in an operationally simple undivided cell. This metal-catalyst- and oxidant-free single-electron oxidation strategy offers a green alternative for the formation of highly reactive cyclopropane-derived radical cations, and this protocol features a broad functional group tolerance.

Reduction of bromate formation in electrochemical production of hypobromite for water disinfection

Hypohalites are powerful water disinfectants for many applications. Continuous production of hypobromite was studied using an undivided electrochemical cell operating at 20 A cell current (28.9 mA cm−2) and an electrolyte flow of 50 L h−1. With 34 mM NaBr as electrolyte current efficiency is between 25 and 42 %. Anodic oxidation and disproportionation of hypobromite led to formation of substantial concentrations of BrO3− in the electrolyte. BrO3− concentrations up to 1 mM (22.6 mg L−1) were formed during anodic Br− oxidation. Replacement of NaBr by mixtures of NaCl and NaBr as electrolyte led to reduction of BrO3− concentration to 0.5 mM (11.3 mg L−1). By combination of NaCl electrolysis with following dosage of NaBr solution the BrO3− concentration could be reduced below the detection limit of the analytical method which was used to quantify BrO3− and hypobromite. In a freshly prepared 5 mM hypobromite solution the BrO3− concentrations remained below 0.1 mM (2.2 mg L−1). When applied in typical dosage for halogen based disinfectants (10−3 mM and 10−2 mM hypohalite) the BrO3− concentration dilutes to 0.02 μM – 0.2 μM. The optimised process thus leads to BrO3− concentrations below the legal limit for BrO3− in drinking water (10 μg L−1 BrO3−).

Electrochemical reduction of aqueous nitrate using metallic silver particles as spatially suspended catalyst

Increasing nitrate concentrations in water sources is a serious concern. The current study demonstrates the electrochemical reduction of nitrate in the presence of silver metallic particles (AgMPs). Using Ti/RuO2 anode and a Ti cathode, nitrate removal in undivided and divided cells was limited to 18% and 77%, respectively, in the absence of AgMPs. Total Nitrogen (TN) removal in undivided and divided cells was nil and 29.5%, respectively, in the absence of AgMPs. Under identical conditions, the presence of 8 mM AgMPs increased nitrate removal to 63% and 99%, respectively, with significant improvement in nitrate conversion to N2. The effect of various parameters on nitrate removal and selectivity of products was studied using a divided cell: concentration of AgMPs, inter-electrode distance, cathode material, current density, initial nitrate concentration, and reusability of AgMPs. In the presence of 8 mM AgMPs, nitrate removal using different cathodes was Ti ≈ Fe > Cu > SS > Fe/Sn. The cathode type also affected the selectivity, with the maximum conversion to N2 achieved with the Ti cathode. Nitrate removal and conversion to N2 improved with the increase in current density up to 10 mA/cm2. Overall, the presence of AgMPs substantially improved nitrate removal and conversion to N2. The production of reductive species (AgHx) by the intercalation of nascent hydrogen on metallic silver appears to promote catalytic nitrate reduction. AgMPs could be reused up to 10 times with consistent performance. This work offers a new approach to nitrate reduction using an efficient and reusable catalyst.

Electrochemically‐Driven Organocatalytic Enantioselective Oxidative Coupling of Tetrahydroisoquinolines and Acrylaldehyde

AbstractAn electrochemically‐driven organocatalytic enantioselective oxidative coupling of tetrahydroisoquinolines and acrylaldehyde was developed. Various chiral C1‐alkenyl tetrahydroisoquinolines derivatives were obtained with 69–86% yields and 93:7–96:4 er. Notable features of this reaction include asymmetric organocatalysis (5.0 mol% β‐ICD as catalyst), electricity as the oxidant, air atmosphere, and undivided cell. This synthetic route offers access to various optically active C1‐substituted tetrahydroisoquinolines derivatives.

Electrochemical Dearomatizing Spirolactonization and Spiroetherification of Naphthols and Phenols

AbstractAn electrochemical oxidative ortho-dearomatization of naphthols and phenols with an intramolecular C–O bond formation has been developed. A careful optimization of the reaction parameters allowed for the application of free phenols as the starting materials, in contrast to the existing alternative procedures necessitating aryl methyl ether substrates. The reaction delivers an array of spirolactones and spiroethers in yields up to 97%, under simple experimental conditions: in a constant current mode, using an undivided cell, and without an inert atmosphere. The method avoids using catalysts or stoichiometric oxidants (e.g., hypervalent iodine reagents), generating hydrogen as the sole byproduct.

Electrocatalytic Reduction of Dinitrogen to Ammonia with Water as Proton and Electron Donor Catalyzed by a Combination of a Tri-ironoxotungstate and an Alkali Metal Cation.

The electrification of ammonia synthesis is a key target for its decentralization and lowering impact on atmospheric CO2 concentrations. The lithium metal electrochemical reduction of nitrogen to ammonia using alcohols as proton/electron donors is an important advance, but requires rather negative potentials, and anhydrous conditions. Organometallic electrocatalysts using redox mediators have also been reported. Water as a proton and electron donor has not been demonstrated in these reactions. Here a N2 to NH3 electrocatalytic reduction using an inorganic molecular catalyst, a tri-iron substituted polyoxotungstate, {SiFe3W9}, is presented. The catalyst requires the presence of Li+ or Na+ cations as promoters through their binding to {SiFe3W9}. Experimental NMR, CV and UV-vis measurements, and MD simulations and DFT calculations show that the alkali metal cation enables the decrease of the redox potential of {SiFe3W9} allowing the activation of N2. Controlled potential electrolysis with highly purified 14N2 and 15N2 ruled out formation of NH3 from contaminants. Importantly, using Na+ cations and polyethylene glycol as solvent, the anodic oxidation of water can be used as a proton and electron donor for the formation of NH3. In an undivided cell electrolyzer under 1 bar N2, rates of NH3 formation of 1.15 nmol sec-1 cm-2, faradaic efficiencies of ∼25%, 5.1 equiv of NH3 per equivalent of {SiFe3W9} in 10 h, and a TOF of 64 s-1 were obtained. The future development of suitable high surface area cathodes and well solubilized N2 and the use of H2O as the reducing agent are important keys to the future deployment of an electrocatalytic ammonia synthesis.

Selective Electrochemical Halogenation of Functionalized Quinolone.

This work describes an effective C3-H halogenation of quinoline-4(1H)-ones under electrochemical conditions, in which potassium halides serve as both halogenating agents and electrolytes. The protocol provides expedient access to different halogenated quinoline-4(1H)-ones with unique regioselectivity, broad substrate scope, and gram-scale synthesis employing convenient, environmentally friendly electrolysis, in an undivided cell. Mechanism studies have shown that halogen radicals can promote the activation of N-H bonds in quinolones.