Indirect Electrolysis Research Articles

Solid‐Liquid‐Gas Three‐Phase Indirect Electrolysis Enabled by Affinity Auxiliary Imparted Covalent Organic Frameworks

The design of efficient heterogenous redox mediators with favorable affinity to substrate and electrolyte are much desired yet still challenging for the development of indirect electrolysis system. Herein, for the first time, we have developed a solid‐liquid‐gas three‐phase indirect electrolysis system based on a covalent organic framework (Dha‐COF‐Cu) as heterogenous redox mediator for S‐S coupling reaction. Dha‐COF‐Cu with the integration of high porosity, nanorod morphology, abundant hydroxyl groups and active Cu sites is much beneficial for the adsorption/activation of thiols, uniform dispersion and high wettability in electrolyte, and efficient interfacial electron transfer. Notably, Dha‐COF‐Cu as solid‐phase redox mediator exhibits excellent electrocatalytic efficiency for the formation of value‐added liquid‐phase S‐S bond product (yields up to 99%) coupling with the generation of gas‐phase product of H2 (~1.40 mmol g‐1 h‐1), resulting in a powerful three‐phase indirect electrolysis system. This is the first work about COFs that can be applied in three‐phase indirect electrolysis system, which might promote the development of porous crystalline materials in this field.

Molecular Electro(Photo)Catalysis: Triarylamine and Triarylimidazole Derivatives Mediated Oxidation Systems in Organic Electrosynthesis

AbstractOrganic electrosynthesis has become a green, efficient and sustainable alternative to traditional organic transformation for the redox reactions. In this connection, indirect electrolysis employing redox mediators is attaining increasing significance as it offers a chance to improve reaction selectivity and avoids the over‐oxidation/reduction issues encountered in direct electrolysis. Triarylamines and triarylimidazoles, as representative organic molecular mediators, have attracted widespread attention in recent years due to the advantages of well‐defined easy structural modification and tunable redox ability. Hence, in this minireview, the recent growth of triarylamine and triarylimidazole derivatives mediated electrochemical oxidation reactions were presented, highlighting the structural optimization, electrochemical performance and reaction mechanism of these organic mediators. Moreover, the photocatalytic and photoelectrocatalytic cases with these molecular mediators were also highlighted. To conclude, the current challenges and future prospects of this field are also discussed.

Electrochemical synthesis of heterocyclic compounds via carbon–heteroatom bond formation: direct and indirect electrolysis

Abstract Electrochemical organic synthesis has attracted attention as an environmentally friendly method for constructing heterocyclic compounds via carbon–heteroatom bond formation. Herein, we describe the representative examples of electrochemical reactions to produce heterocycles and discuss them according to whether they involve direct or indirect electrolysis.

Manganese-Mediated Electrochemical Oxidation of Thioethers to Sulfoxides Using Water as the Source of Oxygen Atoms.

Oxygen-atom transfer reactions are a prominent class of synthetic redox reactions that often use high-energy oxygen-atom donor reagents. Electrochemical methods can bypass these reagents by using water as the source of oxygen atoms through pathways involving direct or indirect (mediated) electrolysis. Here, manganese porphyrins and related mediators are shown to be effective molecular electrocatalysts for selective oxidation of thioethers to sulfoxides, without overoxidation to the sulfone. The reactions proceed by proton-coupled oxidation of a MnIII-OH2 species to generate a MnIV-OH and MnV═O species. This methodology is compared to direct electrolysis methods initiated by single-electron oxidation of the thioether, and chloride-mediated electrochemical oxidation of thioethers. The Mn-mediated reactions operate at lower applied potential and exhibit improved substrate scope and functional group compatibility relative to direct electrolysis, and the tunability of the Mn-based mediators allows for improved performance relative to chloride-mediated electrolysis. An electrochemical parallel screening platform is developed and applied to a library of pharmaceutically relevant thioethers.

Electrophotocatalysis Versus Indirect Electrolysis: Electrochemical Selenocyclization of 3-Aza-1,5-dienes Facilitated by Energy Transfer, Direct Photolysis or N-Hydroxyphthalimide.

Three hybrid electrochemical protocols, which involve the energy transfer, direct photolysis and N-hydroxyphthalimide catalyst, respectively, are presented for the selenylation/cyclization of the fragile substrates of 3-aza-1,5-dienes with diorganyl diselenides to afford 3-selenomethyl-4-pyrrolin-2-ones. The two electrophotocatalytic reactions and the indirect electrolysis one are both regioselective and external-oxidant- and transition-metal-free, and are associated with a broad substrate scope and high Se-economy, and all three methods are amenable to gram-scale syntheses, late-stage functionalizations, sunlight-induced experiments and all-solar-driven syntheses.

Treatment of Organics in Wastewater Using Electrogenerated Gaseous Oxidants.

This work focuses on the comparison of the performance of direct electrochemical oxidation with indirect electrolysis mediated by gaseous oxidants in the treatment of diluted wastewater. To do this, energy consumptions of the electrolysis using mixed metal oxide (MMO) electrodes are compared with those required for the production and use of chlorine dioxide in the degradation of methomyl contained in aqueous solutions. Results demonstrate the feasibility of the mediated oxidation process and that this process is competitive with direct oxidation. The oxidants are produced under optimized conditions using the same anodic material applied for the direct degradation of organics, thus avoiding efficiency losses associated with mass transfer limitations in the degradation of dilute organic solutions. Thus, using the ClO2 gaseous oxidant, a concentration of 0.1 mM of methomyl from a solution containing 500 mL is completely removed with an energy consumption as low as 50 Wh. The application of the same energy to a direct electrolytic process for treating the same wastewater can only reach less than half of this removal. These findings may have a very important application in the use of electrochemical technology to achieve the remediation of persistent pollutants in wastewater, where their low concentrations typically make direct processes very inefficient.

Transition‐metal Substituted Polyoxometalates as Soluble RedOx Mediators in Electrocatalytic Biomass Conversion

AbstractPolyoxometalates (POMs) are known for their unique redox properties, making them interesting candidates as redox mediators in electro‐organic transformations. We investigated derivatives of H3[PMo12O40], a classical Keggin‐type POM, as a potential redox mediator in the indirect electrolysis of biobased glyceraldehyde (GLAD), focusing on the effects of double substitution of the Mo framework metal by redox active transition metals (Mo replaced by V, Ni, Co, Mn). By combining electrochemical techniques with HPLC, UV‐Vis spectroscopy, and 31P‐NMR spectroscopy, a comprehensive overview of the reaction pathways, as well as the electronic and structural changes of the POM during the reaction were revealed. This work not only contributes to the fundamental understanding of POMs as redox mediators, but also paves the way for innovative development of sustainable and environmentally friendly electro‐organic transformations.

The transformations of thiols and their dimers in the redox-mediated thiol-disulfide exchange reaction

A search for new approaches to sulfurous waste utilization is one of the urgent tasks of chemical technology. Thiol-disulfide exchange reaction (TDE) is one of the possible ways to involve technogenic wastes in organic synthesis. Electricity can promote such type of interactions. In this paper, we have studied TDE reactions involving low molecular weight thiols or their dimers under electrochemical conditions. The exchange processes were examined using the model reaction between 1-propanethiol and phenyl disulfide. Electrolysis was performed in the presence of redox mediators such as arylphosphines, substituted amines, o-, p-aminophenols or catechol. These compounds can initiate a TDE process with a formation of unsymmetrical disulfides. 4-Amino-2,6-diphenylphenol was chosen as the most effective redox mediator, which reduces the anodic overvoltage of a thiol oxidation by 1.20 V. The advantage of electrolysis in an undivided cell is the increased yield of target unsymmetrical disulfides due to the possibility of reduction ofhomodimers at the cathode. The involvement of refining waste, such as C3–C4 disulfide oil, in the reaction with substituted thiophenols made it possible to obtain a number of unsymmetrical arylalkyl disulfides with biologically active fragments in a high yield (up to 97%) under indirect electrolysis conditions.

Electrochemical Coupling Reactions Using Non‐Transition Metal Mediators: Recent Advances

AbstractIndirect electrolysis method using appropriate mediators enables numerous chemical reactions. The general principles of mediators were described herein with a particular focus on non‐transition metal mediators. Recent representative examples of bond formation reactions by indirect electrolysis are summarized and discussed here.

Heterocyclic Electrochemistry: Renewable Electricity in the Construction of Heterocycles.

Numerous applications in the realm of biological exploration and drug synthesis can be found in heterocyclic chemistry, which is a vast subject. Many efforts have been developed to further improve the reaction conditions to access this interesting family to prevent employing hazardous ingredients. In this instance, it has been stated that green and environmentally friendly manufacturing methodologies have been introduced to create N-, S-, and O-heterocycles. It appears to be one of the most promising methods to access these types of compounds avoiding use of stoichiometric amounts of oxidizing/reducing species or precious metal catalysts, in which only catalytic amounts are sufficient, and it represent an ideal way of contributing toward the resource economy. Thus, renewable electricity provides clean electrons (oxidant/reductant) that initiate a reaction cascade via producing reactive intermediates that facilitate in building new bonds for valuable chemical transformations. Moreover, electrochemical activation using metals as catalytic mediators has been identified as a more efficient strategy toward selective functionalization. Thus, indirect electrolysis makes the potential range more practical, and less side reactions can occur. The latest developments in using an electrolytic strategy to create N-, S-, and O-heterocycles are the main topic of this mini review, which was documented over the last five years.

Indirect Electrochemical‐Induced Trifluoromethylation of Tryptophan Containing Oligopeptides

AbstractElectrochemical synthesis in chemical biology is a milder, biocompatible, and environmentally friendly process for constructing novel types of bioconjugated systems. Herein, we report on the development of an indirect electrochemical strategy that allows a trifluoromethyl group to be incorporated into a tryptophan residue. This protocol allowed fluorinated peptides to be produced, thus allowing their characterization by 19F‐NMR. Interestingly, the trifluoromethylation of tryptophan did not appear under conditions of direct electrolysis; instead, as we introduced ferrocene as a mediator to allow indirect electrosynthesis, the desired product could be obtained in 67% isolated yield. Furthermore, various oligopeptides are successfully coupled to the Trp residue selectively. Mechanistic studies involving CV and DFT calculations indicated that the transformation proceeds through the homo‐cleavage of CF3SO2Na via ferrocene‐mediated indirect electrolysis. Subsequently, the addition of a CF3 radical allowed the labelled Trp to be incorporated into the corresponding products. This electrochemical protocol provides a new toolkit for the trifluoromethylation of peptides that contain tryptophan.

Sustainable electrochemical C(sp3) − H oxygenation using water as the oxygen source

Electrochemical C(sp3) − H oxygenation of phenols derivatives were accomplished using H2O as the sole oxygen source. Thus, selectively oxidative 4-cresols to 4-acylphenols derivatives proved viable with the aid of indirect electrolysis, through metal- and external oxidant free environmental-friendly and safe electrocatalytic conditions. Advances of this strategy were proved by its ability to transform various sulfonamides with excellent site and regioselectivity. Detailed mechanistic studies allowed to delineate the exact profile of the generation of the oxygenation events.

Electrochemical Enantioselective Nucleophilic α-C(sp3)-H Alkenylation of 2-Acyl Imidazoles.

Merging electrochemistry with asymmetric catalysis promises to provide an environmentally friendly and efficient strategy for the construction of nonracemic chiral molecules. However, in practice, significant challenges arise from the instability or incompatibility of the chiral catalysts under the electrochemical conditions at the interface of electrode and solution. Herein, we report a catalytic asymmetric indirect electrolysis employing the combination of a redox mediator and a chiral-at-rhodium Lewis acid, which achieves a previously elusive enantioselective nucleophilic α-C(sp3)-H alkenylation of ketones. Specifically, 2-acyl imidazoles react with potassium alkenyl trifluoroborates in high yields (up to 94%) and with exceptional enantioselectivities (27 examples with ≥99% ee) without the need for any additional stoichiometric oxidants (overall 40 examples). The new indirect electrosynthesis can be scaled to gram quantities and was applied to the straightforward synthesis of intermediates of the natural product cryptophycin A and a cathepsin K inhibitor.

PVDF HFP_RuO2 Nanocomposite Aerogels Produced by Supercritical Drying for Electrochemical Oxidation of Model Tannery Wastewaters.

A supercritical CO2 drying process was used to prepare an innovative nanocomposite, formed by a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF HFP) aerogel loaded with RuO2 nanoparticles. The produced nanocomposites, at 10% and 60% w/w of RuO2, were tested for the electrochemical oxidation of model tannery wastewaters. The effect of the electrochemical oxidation parameters, like pH, temperature, and current density, on tannic acid, intermediates, and chemical oxygen demand (COD) removal, was investigated. In particular, the electrolysis of a simulated real tannery wastewater, using PVDF HFP_RuO2 60, was optimized working at pH 10, 40 °C, and setting the current density at 600 A/m2. Operating in this way, surfactants, sulfides, and tannins oxidation was achieved in about 2.5 h, ammonium nitrogen oxidation in 3 h, and COD removal in 5 h. When chloride-containing solutions were tested, the purification was due to indirect electrolysis, related to surface redox reactions generating active chlorine. Moreover, sulfide ions were converted into sulfates and ammonium nitrogen in gaseous N2.

Organic transformations using electro-generated polyoxometalate redox mediators

The use of electrochemistry in organic synthesis (sometimes termed electro-organic synthesis, or just “electrosynthesis”) is a field that is attracting ever-increasing attention on account of its ability to deliver reaction outcomes that cannot be achieved in any other way and on account of its potential to replace toxic and/or sacrificial reagents with cleanly-generated electricity. In the simplest incarnation of electro-organic synthesis, an organic substrate undergoes direct electron transfer with the electrode in a type of heterogeneous catalysis process. However, such direct electrochemical processes may suffer from slow kinetics, poor product selectivities and/or lack of control over the extent of electron transfer to the substrate, all of which can lead to sub-optimal reaction outcomes. A distinct, but extremely important, sub-section of electro-organic synthesis (so-called “indirect electrolysis”) circumvents these limitations through the use of soluble redox mediator molecules, which act as electron shuttles between the heterogeneous electrode and the organic substrate in bulk solution. The advantages of indirect electrolysis for a number of organic transformations is now becoming increasingly clear, typically using either very simple metal salts or small organic molecules as the mediators. However, one particular class of mediators, the polyoxometalates (well-defined, soluble metal oxide clusters), offers an especially wide diversity of structures and properties that could revolutionise indirect electrolysis for electro-organic synthesis. In this review, we provide a collation of reports that describe use of electro-generated polyoxometalate redox mediators for organic transformations, with a particular focus on the most recent and most exciting results.

Electrochemical Ring Expansion to Synthesize Medium-Sized Lactams Through C–C Bond Cleavage

Medium-sized nitrogen heterocycles are prevalent motifs in many kinds of bioactive molecules and natural products. Owing to the unfavorable enthalpic and entropic barriers during the transition sta...

Statistical optimization of electrochemical conversion of heavy fuel oil to valuable products using response surface methodology

Mediated electro-membrane oxidation was employed to convert a refinery heavy fuel oil (HFO) to valuable liquid and solid products (heavy residuals) at mild conditions. Response surface methodology (RSM) was engaged to obtain maximum conversion of HFO by implementing the central composite design (CCD). Based on the prediction and optimization functions, the optimum conditions for HFO conversion were then established. When Ag (I) concentration, HFO content, electrolysis temperature and time were adjusted to 0.56 M, 1.2 wt. %, 32 °C and 11 min, respectively, the HFO converted to a liquid product comprising 10 % gasoline and kerosene, 20 % atmospheric gasoil, 40 % vacuum gasoil and 30 % vacuum residuals. These were very close to the model predictions representing reliability of the RSM technique.FTIR results after electrochemical conversion confirmed the production of branched aliphatic groups and regeneration of interacting hydroxyl and carbonyl groups for heavy residuals and liquid products. Sim-Dist results revealed that during the indirect electrolysis, cracking of the initial HFO to lighter products occurred. Besides, surface morphology of the heavy residuals showed formation of a calcined coke. Some pores and agglomerated asphaltenes were observed which are likely linked to the former presence of resinous components removed by electrolysis.

Catalytic Electrosynthesis of N,O‐Heterocycles – Recent Advances

Recognized as gentler and more environmentally friendly compared to conventional methods, catalytic electrochemical reactions are a powerful tool for constructing heterocycles. Of special interest is indirect electrolysis, where the use of a redox mediator provides an additional means of reaction control, thus avoiding over‐oxidation and facilitating improved selectivity. This minireview describes recent advances in catalytic electrochemical reactions for synthesizing N,O‐heterocycles that appeared from 2018 until September 2019.

Site-Selective C-H Functionalization via Synergistic Use of Electrochemistry and Transition Metal Catalysis.

Electrochemical synthesis of organic compounds has emerged as an attractive and environmentally benign alternative to conventional approaches for oxidation and reduction of organic compounds that utilizes electric current instead of chemical oxidants and reductants. As such, many useful transformations have been developed, including the Kolbe reaction, the Simons fluorination process, the Monsanto adiponitrile process, and the Shono oxidation, to name a few. Electrochemical C-H functionalization represents one of the most promising reaction types among many electrochemical transformations, since this process avoids prefunctionalization of substrates and provides novel retrosynthetic disconnections. However, site-selective anodic oxidation of C-H bonds is still a fundamental challenge due to the high oxidation potentials of C-H bonds compared to organic solvents and common functional groups. To overcome this issue, indirect electrolysis via the action of a mediator (a redox catalyst) is regularly employed, by which the selectivity can be controlled following reaction of said mediator with the substrate. Since the redox potentials of transition metal complexes can be easily tuned by modification of the ligand, the synergistic use of electrochemistry and transition metal catalysis to achieve site-selective C-H functionalization is an attractive strategy. In this Account, we summarize and contextualize our recent efforts toward transition metal-catalyzed electrochemical C-H functionalization proximal to a suitable directing group. We have developed C-H oxygenation, acylation, alkylation, and halogenation reactions in which a Pd(II) species is oxidized to a Pd(III) or Pd(IV) intermediate by anodic oxidation, followed by reductive elimination to form the corresponding C-O, C-C, and C-X bonds. Importantly, improved monofunctionalization selectivity is achieved in the Pd-catalyzed C(sp3)-H oxygenation compared to conventional approaches using PhI(OAc)2 as the chemical oxidant. Physical separators are sometimes used to prevent the electrochemical deposition of Pd black on the cathode resulting from reduction of high valent Pd species. We skirted this issue through the development a Cu-catalyzed electrochemical C(sp2)-H amination using n-Bu4NI as a redox cocatalyst in an undivided cell. In addition, we developed Ir-catalyzed electrochemical vinylic C-H functionalization of acrylic acids with alkynes in an undivided cell, affording various substituted α-pyrones in good to excellent yield. More importantly, chemical oxidants, including Ag2CO3, Cu(OAc)2, and PhI(OAc)2, resulted in much lower yields in the absence of electrical current under otherwise identical conditions. As elaborated below, progress in the area of electrochemical transition metal-catalyzed synthesis provides an effective platform for environmentally friendly and sustainable selective chemical transformations.

Metalla-electrocatalyzed C-H Activation by Earth-Abundant 3d Metals and Beyond.

To improve the efficacy of molecular syntheses, researchers wish to capitalize upon the selective modification of otherwise inert C-H bonds. The past two decades have witnessed considerable advances in coordination chemistry that have set the stage for transformative tools for C-H functionalizations. Particularly, oxidative C-H/C-H and C-H/Het-H transformations have gained major attention because they avoid all elements of substrate prefunctionalization. Despite considerable advances, oxidative C-H activations have been dominated by precious transition metal catalysts based on palladium, ruthenium, iridium, and rhodium, thus compromising the sustainable nature of the overall C-H activation approach. The same holds true for the predominant use of stoichiometric chemical oxidants for the regeneration of the active catalyst, prominently featuring hypervalent iodine(III), copper(II), and silver(I) oxidants. Thereby, stoichiometric quantities of undesired byproducts are generated, which are preventive for applications of C-H activation on scale. In contrast, the elegant merger of homogeneous metal-catalyzed C-H activation with molecular electrosynthesis bears the unique power to achieve outstanding levels of oxidant and resource economy. Thus, in contrast to classical electrosyntheses by substrate control, metalla-electrocatalysis holds huge and largely untapped potential for oxidative C-H activations with unmet site selectivities by means of catalyst control. While indirect electrolysis using precious palladium complexes has been realized, less toxic and less expensive base metal catalysts feature distinct beneficial assets toward sustainable resource economy. In this Account, I summarize the emergence of electrocatalyzed C-H activation by earth-abundant 3d base metals and beyond, with a topical focus on contributions from our laboratories through November 2019. Thus, cobalt electrocatalysis was identified as a particularly powerful platform for a wealth of C-H transformations, including C-H oxygenations and C-H nitrogenations as well as C-H activations with alkynes, alkenes, allenes, isocyanides, and carbon monoxide, among others. As complementary tools, catalysts based on nickel, copper, and very recently iron have been devised for metalla-electrocatalyzed C-H activations. Key to success were detailed mechanistic insights, prominently featuring oxidation-induced reductive elimination scenarios. Likewise, the development of methods that make use of weak O-coordination benefited from crucial insights into the catalyst's modes of action by experiment, in operando spectroscopy, and computation. Overall, metalla-electrocatalyzed C-H activations have thereby set the stage for molecular syntheses with unique levels of resource economy. These electrooxidative C-H transformations overall avoid the use of chemical oxidants and are frequently characterized by improved chemoselectivities. Hence, the ability to dial in the redox potential at the minimum level required for the desired transformation renders electrocatalysis an ideal platform for the functionalization of structurally complex molecules with sensitive functional groups. This strategy was, inter alia, successfully applied to scale-up by continuous flow and the step-economical assembly of polycyclic aromatic hydrocarbons.