Undivided Cell Research Articles

Regioselective Electrooxidative [3+2] Annulation between Indole and Aniline Derivatives to Construct Functionalized Indolo[2,3-b]indoles.

A protocol for the electrooxidative [3+2] annulation to generate indolo[2,3-b]indoles in an undivided cell is reported. It exhibits good yields with excellent regioselectivities and tolerates various functional groups without external chemical oxidants. Cyclic voltammetry and density functional theory calculations indicate that the [3+2] annulation is initiated by the simultaneous anodic oxidation of indole and aniline derivatives, and the step to determine the rate relies on the combination of radical cations.

Electrochemically‐Enabled Construction of N‐Acyl Iminophosphoranes

AbstractN‐acyl iminophosphoranes have shown significant potentials in various areas. This paper presents two electrochemical strategies for the successful construction of a series of substituted N‐acyl iminophosphoranes using electrochemical activation of hydroxylamines or dioxazolones to generate unprotected amide radicals and radical anions. The simple undivided cell with inexpensive electrode setups (graphite felts, carbon rods, and copper sheets) provides excellent substrates tolerance under mild conditions. This simple electrochemical construction of N‐acyl iminophosphoranes also has good scalability, practicability and extensibility. Furthermore, synthetic transformations of the N‐acyl iminophosphorane products have demonstrated the application potentials of the P=N bond.

Atroposelective N-N Axes Synthesis via Electrochemical Cobalt Catalysis.

Here, we disclosed an unprecedented cobalt electrocatalyzed atroposelective C-H activation and annulation for the efficient construction of diversely functionalized N-N axes in an undivided cell. A broad range of allene substrates and benzamides bearing different functionalities are compatible with generating axially chiral products with good yields and excellent enantioselectivities (up to 92% yield and 99% ee). A series of synthetic applications and control experiments were also performed, which further expanded the practicality of this strategy.

Electrochemical ammonia oxidation on terephthalic acid framework (BDC-NH2) derived core-shell nickel oxide/nickel-enriched 2D carbon nanoflakes (NiO/Ni/CNF)

A terephthalic framework, Ni-BDC-NH2, was used to synthesize the nickel nanoparticles and nanoclusters on-site encapsulated within mesoporous carbon nanoflakes (NiO/Ni/CNF) for ammonia electrolysis. Ni2+ nodes recrystallized to form a core-shell oxide film covering the Ni metal, with the dispersity and particles size influenced by the pyrolysis temperature in a N2 atmosphere. Electrochemical characterization and Raman spectra identified the solid-state Ni(II)⇌Ni(III) transition as the electron mediator, while its lowered onset potential (+1.3 V vs. RHE), compared to that of oxygen evolution reaction (+1.7 V) enhanced the Faradaic efficiency. The performance of electrodes in ammonia oxidation was assessed, which revealed a N2 selectivity of at least 80 % under electrolytic conditions of current density = 0.5 – 1.5 mA cm−2, pH > 10, and initial NH3 level = 50–300 mg-N L−1 using NiO/Ni/CNF(700). An undivided cell equipped with NiO/Ni/CNF(700) anode and Cu/Ni cathode was utilized to treat a real wastewater (∼500 mg-N L−1, pH 4.5). Consequently, both the removal efficiency of NH3 and N2 selectivity surpassed 90 % in a continuous flow mode for 4 days, indicating the durability of electrode system.

Selective Hydro‐ and Deuterodechlorination of Trichloroacetamides under Controlled Electrochemical Conditions To Prepare Mono‐, Di‐, and Deuterochloroacetamides

AbstractMono‐ and dichloro acetamides were prepared via a hydrodechlorination reaction of trichloroacetamides under controlled electrochemical conditions. The reactions were carried out in an undivided cell with constant current flow using n‐Bu4NI as supporting electrolyte and methanol as solvent and proton source. Deuterodechlorination of trichloroacetamide was also successful when methanol‐d4 was used as solvent under identical electrochemical conditions.

Synthesis of N,N‐Disubstituted Hydrazines by Electrocatalytic Addition of Hydrazines to α,β‐Unsaturated Carbonyl Compounds

AbstractA strategy for the synthesis of N,N‐disubstituted hydrazines via the electrocatalytic addition of hydrazine to α,β‐unsaturated carbonyl compounds is reported. The reaction was carried out under constant current electrolytic conditions in an undivided cell. Using this methodology, various N,N‐disubstituted hydrazines were prepared in 57–94% yield in 40 minutes. By adding cyclohexanone and acetic acid to the reaction mixture of N,N‐disubstituted hydrazines and heating for 2 hours, a one‐pot synthesis of N‐substituted indoles was also developed. Furthermore, based on cyclic voltammetry and control experiments, a plausible mechanism involving a radical pathway is proposed.

Combined anodic and cathodic peroxide production in an undivided carbonate/bicarbonate electrolyte with 144% combined current efficiency

In recent years, the electrochemical synthesis of peroxides has attracted renewed interest as a potential environmentally friendly production compared to the established anthraquinone process. In addition, it is possible to produce the peroxides directly on site, eliminating the need for expensive and hazardous transportation and storage. Cathodic production of hydrogen peroxide from oxygen is already quite well developed. Anodic production from water, on the other hand, is still facing significant challenges, despite its historic pioneering role. In this manuscript we show that anodic and cathodic synthesis of peroxides can even be combined to achieve greater than 100% current efficiency (CE) due to the combined effect of both half-reactions. So far, similar devices have always employed different electrolytes for each, which necessitated the use of a membrane and posed contamination risk. However, herein we show that both half-reactions can also employ the same electrolyte. This enables even an undivided cell, omitting the need for the expensive membranes. Despite its simplicity, this setup yielded an outstanding performance with a combined CE of 144%.

Inside Back Cover

The cover displays a 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. This selective synthesis is like interstellar racing, reaching distant planets from the same starting point through different paths. Following control experiments, a plausible mechanism is proposed to rationalize the experimental results. More details are discussed in the article by Cao et al. on page 1367—1372.image

A two-step electrochemical method for separating Mg(OH)2 and CaCO3: Application to RO reject and polluted groundwater

The process of removing Ca2+ and Mg2+ ions typically results in the co-precipitation of Ca2+ and Mg2+ along with other salt waste. To improve water treatment efficiency towards a zero-waste goal, it is crucial to separate Ca2+ and Mg2+, and recover them in their purified form. This study proposes a two-step electrochemical approach that separately recovers Ca2+ as CaCO3 and Mg2+ as Mg(OH)2. The first step uses an undivided cell with 3D electrodes and controlled flow directions to selectively precipitate CaCO3 on the electrode, keeping the cell removal efficiency. The second step employs a two-compartment cell with a cationic exchange membrane to recover Mg(OH)2. This approach was evaluated on RO reject water with high Ca2+ to Mg2+ ratio and industrial effluent-polluted groundwater with a low ratio. Treatment of domestic RO reject water using undivided cell specifically recovered 64% of CaCO3, although the low conductivity of the RO reject water limited further Mg2+ recovery. Conversely, treating industrial effluent-polluted groundwater with this two-step process successfully recovered 80% of CaCO3 and 94% of Mg(OH)2. SEM, EDAX, and XRD analysis confirmed the quality of the recovered products.

Electrocatalytic, Sm‐Promoted Synthesis of Aminoarenes from Nitroaromatic Derivatives in MeOH

A new mild and chemoselective method for the preparation of various aminoarenes is described, based on the electrochemical catalytic reduction of nitroaryl derivatives promoted by samarium salts in methanol. The reactions are carried out in air, in an undivided electrochemical cell, at room temperature and tolerate a wide range of functional groups, including some reducible ones, without leading to by‐products. This procedure is also effective for the complete reduction of challenging substrates, namely those capable of coordinating metals, traditionally leading to the need for drastic conditions in terms of pressure, reaction time or temperature. 47 examples are described, including one nitroalkane, and all lead to the targeted reduced product in yields that are almost always over 90%.

Electrochemical hydrodefluorination of an aromatic trifluoromethyl group with ammonia as the hydrogen source

The aromatic difluoromethyl group (Ar-CF2H) is an emerging functional group found in pharmaceutical compounds. For synthesis, the hydrodefluorination of Ar-CF3 molecules is a straightforward approach in terms of atom economy and substrate scope. The reported hydrodefluorination reaction focused on the Ar-CF3 substrate bearing electron-withdrawing groups. In this report, we utilized electricity as the driving force for the hydrodefluorination of ArCF3, which involves electron-donating groups. By using ammonia as a traceless hydrogen and electron donor, the reaction could tolerate a variety of labile groups, including unprotected amine, pyrrole, furan, thioether, and silyl group, without sacrificial anodes in undivided cells. In this work, tBuOLi was found to be an essential additive for achieving good reactivity and chemoselectivity. Several Ar-CF2H pharmaceutical and intermediate products were synthesized via this approach.

Electrochemical Oxidative Cross-Coupling Reactions of Ketene Dithioacetals and Dichalcogenides to Construct C−Se/C−S Bonds

A green protocol for construction of C−Se bonds from ketene dithioacetals and diselenides through direct electrochemical oxidative cross-coupling has been developed. This reaction was carried out in an undivided cell system with NaBF4 as the electrolyte and CH3CN as the solvent through galvanostatic electrolysis. A series of substituted ketene dithioacetals and diselenides were tolerant and the desired tetrasubstituted alkenyl selenides were obtained in moderate to excellent yields. In addition, construction of C−S bond from ketene dithioacetals and disulfides through electrochemical method in the presence of KI was also successfully realized. It exhibited high efficiency and broad functional group tolerance.

Electrochemical Oxidative (4 + 2) Cyclization of Anilines and o-Phenylenediamines for the Synthesis of Phenazines.

Phenazines, crucial constituents of nitrogen-containing heterocycles, widely exist in functional compounds. Herein, we report an anodic oxidative (4 + 2) cyclization between anilines and o-phenylenediamines for the uniform construction of phenazines in a simple undivided cell. Dual C-H amination followed by oxidation represents an outstanding step and atom efficiency. A sequence of phenazines is produced with excellent functional group tolerance at room temperature.

Redox Relay-Induced C-S Radical Cross-Coupling Strategy: Application in Nontraditional Site-Selective Thiocyanation of Quinoxalinones.

Oxidative cross-coupling is a powerful strategy to form C-heteroatom bonds. However, oxidative cross-coupling for constructing C-S bond is still a challenge due to sulfur overoxidation and poisoning transition-metal catalysts. Now, electrochemical redox relay using sulfur radicals formed in situ from inorganic sulfur source offers a solution to this problem. Herein, electrochemical redox relay-induced C-S radical cross-coupling of quinoxalinones and ammonium thiocyanate with bromine anion as mediator is presented. The electrochemical redox relay comprised initially the formation of sulfur radical via indirect electrochemical oxidation, simultaneous electrochemical reduction of the imine bond, electro-oxidation-triggered radical coupling involving dearomatization-rearomatization, and the reformation of the imine bond through anodic oxidation. Applying this strategy, various quinoxalinones bearing multifarious electron-deficient/-rich substituents at different positions were well compatible with moderate to excellent yields and good steric hindrance compatibility under constant current conditions in an undivided cell without transition-metal catalysts and additional redox reagents. Synthetic applications of this methodology were demonstrated through gram-scale preparation and follow-up transformation. Notably, such a unique strategy may offer new opportunities for the development of new quinoxalinone-core leads.

A Recyclable Electrochemical Reduction of Aldehydes and Ketones to Alcohols Using Water as the Hydrogen Source and Solvent.

There are several challenging problems such as the usage of combustible and hazardous hydrogen sources and severe environmental pollution in the conventional reduction of aldehydes/ketones to alcohols. We report here a practical, safe, and green electrochemical reduction, which solves these problems to a large extent. Through an undivided cell, Zn(+) and Sn(-) as the electrode, tetrabutylammonium chloride (TBAC) as the electrolyte, water as the solvent and hydrogen source, a wide range of aldehydes and ketones are converted into the corresponding alcohols in mild conditions. Furthermore, the electrolytes and water can be recycled, and reductive deuteration can be achieved by simply using D2O as the solvent. Finally, the reduction can be smoothly scaled up to a kilogram level.

Electrochemical Synthesis of a Sitagliptin Precursor.

A novel synthesis of sitagliptin based on a redox-active ester derived from the chiral pool is reported. The key step is an electrochemical nickel-catalyzed sp2-sp3 cross-coupling reaction using inexpensive nickel foam in an undivided cell. It was successfully applied to 21 examples in up to 88% yield. These sitagliptin-analogue precursors could potentially interact with the DPP4 enzyme. A full synthesis based on our new reaction pathway provided sitagliptin in an overall yield of 33%.

Electrochemical Formation of α-Ketoamides from Ketoximes through Non-Beckmann Mechanism Pathway.

α-Ketoamides are highly valued in synthetic chemistry due to their incorporation into diverse natural products and drug molecules. Here, we present an innovative electrochemical approach for constructing α-ketoamides, utilizing a mild and environmentally friendly strategy in a user-friendly undivided cell setup under constant current. The excellent functional-group tolerance, convenient accessibility of reaction instruments and starting materials, and easy scalability collectively enhance the importance of this protocol compared to previous challenging methods. Additionally, mechanistic insight into this reaction is obtained through the investigation of cyclic voltammograms of the reactants.

An Electrocatalytic Cascade Reaction for the Synthesis of Ketones Using CO2 as a CO Surrogate

AbstractThe construction of carbonyl compounds via carbonylation reactions using safe CO sources remains a long‐standing challenge to synthetic chemists. Herein, we propose a catalyst cascade Scheme in which CO2 is used as a CO surrogate in the carbonylation of benzyl chlorides. Our approach is based on the cooperation between two coexisting catalytic cycles: the CO2‐to‐CO electroreduction cycle promoted by [Fe(TPP)Cl] (TPP=meso‐tetraphenylporphyrin) and an electrochemical carbonylation cycle catalyzed by [Ni(bpy)Br2] (2,2′‐bipyridine). As a proof of concept, this protocol allows for the synthesis of symmetric ketones from good to excellent yields in an undivided cell with non‐sacrificial electrodes. The reaction can be directly scaled up to gram‐scale and operates effectively at a CO2 concentration of 10 %, demonstrating its robustness. Our mechanistic studies based on cyclic voltammetry, IR spectroelectrochemistry and Density Functional Theory calculations suggest a synergistic effect between the two catalysts. The CO produced from CO2 reduction is key in the formation of the [Ni(bpy)(CO)2], which is proposed as the catalytic intermediate responsible for the C−C bond formation in the carbonylation steps.

An Electrocatalytic Cascade Reaction for the Synthesis of Ketones Using CO2 as a CO Surrogate.

The construction of carbonyl compounds via carbonylation reactions using safe CO sources remains a long-standing challenge to synthetic chemists. Herein, we propose a catalyst cascade Scheme in which CO2 is used as a CO surrogate in the carbonylation of benzyl chlorides. Our approach is based on the cooperation between two coexisting catalytic cycles: the CO2-to-CO electroreduction cycle promoted by [Fe(TPP)Cl] (TPP=meso-tetraphenylporphyrin) and an electrochemical carbonylation cycle catalyzed by [Ni(bpy)Br2] (2,2'-bipyridine). As a proof of concept, this protocol allows for the synthesis of symmetric ketones from good to excellent yields in an undivided cell with non-sacrificial electrodes. The reaction can be directly scaled up to gram-scale and operates effectively at a CO2 concentration of 10 %, demonstrating its robustness. Our mechanistic studies based on cyclic voltammetry, IR spectroelectrochemistry and Density Functional Theory calculations suggest a synergistic effect between the two catalysts. The CO produced from CO2 reduction is key in the formation of the [Ni(bpy)(CO)2], which is proposed as the catalytic intermediate responsible for the C-C bond formation in the carbonylation steps.

Electrochemical Cascade Annulation for the Synthesis of 3‐Sulfanylquinoline Derivatives Under Mild Conditions

Comprehensive SummaryAn efficient electrochemical approach has been developed for the construction of 3‐sulfanylquinoline derivatives by treating phenylethynylbenzoxazinanones with disulfides in an undivided cell. The protocol provided a convenient route to functionalized quinolines with good functional group tolerance. Moreover, it does not require any metal catalysts or additives, furnishing a series of biological quinolines in moderate to good yields.