Organic Electrosynthesis: From Laboratorial Practice to Industrial

Review pubs.acs.org/OPRD

Organic Electrosynthesis: From Laboratorial Practice to Industrial Applications David S. P. Cardoso, Biljana Šljukić, Diogo M. F. Santos, and César A. C. Sequeira*

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Materials Electrochemistry Group, Center of Physics and Engineering of Advanced Materials (CeFEMA), Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal ABSTRACT: Organic electrosynthesis has received great attention as a powerful green tool for synthesis, affording less waste production, less chemicals spent, and often fewer reaction steps than conventional methods. Functional group interconversion and C−C bond generation by imposition of a proper electrode potential is what lies behind organic electrosynthesis processes. Paired electrochemical reactions, indirect electrosynthesis, electrochemical microreactors, and the use of ionic liquids are some of the highlighted means that contribute to optimization of the overall process. Necessity to use specific organic solvents combined with supporting electrolytes is one of the main limitations to be overcome to make the electrochemical process more economically feasible when compared to nonelectrochemical processes. Numerous examples from the bench scale to industrial routes such as adiponitrile, substituted benzaldehydes, anthraquinone, fluorinated products, and succinic acid production are well described throughout this review.

1. INTRODUCTION Basic chemistry of organic and inorganic processes is, per definition, the foundation for chemical industry. In particular, for the past 120 years, synthetic organic chemistry has been a conventional and largely used tool in chemical synthesis, both at laboratory and technical scale. Currently, significant advances in photoredox chemistry, enzymatic processes, organo-catalysis, and other methodologies are contributing to the increased establishment of chemical synthesis in the chemical industry. Still, modern society demands new synthesis processes, namely in terms of clean alternative processes. Thus, it is advisable to achieve high product yields with considerably lower/no environmental emissions, by avoiding environmentally complicated byproducts as well as extending the lifetime of materials and equipment and focusing attention toward the workup of process streams, simple isolation, simple recycling of solvents, and other important preconditions for the realization of new synthetic processes in industry.1−4 The present availability of electrochemical expertise and equipment on the market, the emergence of new suppliers, and the search by developing countries for new clean technologies make electrosynthesis with its intensive use of electrons (note that electrons on a molar scale are in the same low cost range as hydrogen or oxygen at an integrated chemical plant using these gases for hundreds of processes) a competitive technology with very good potential (but not a general solution). These potential technical and/or economic advantages over conventional and/or competitive processes are patented in many situations such as pollutants treatment,5,6 sustainable energy production,7,8 and synthesis of chemical products.8−12 Organic electrosynthesis, enabling the replacement of dangerous and toxic chemicals by electric current, or “clean” electrons, together with its high versatility, has attracted great attention at the laboratorial3,9,13 and industrial scales in areas of fine chemicals, environmental mitigation, pharmaceuticals, agrochemicals, and others.14,15 © 2017 American Chemical Society

The advantage of imposing a certain cell voltage to handle the selectivity of a process is crucial to avoid unwanted products obtained by side reactions. In this review, key topics within this area will be addressed, from the theoretical fundamentals to the industrial scale.

2. BASIC CONCEPTS IN ORGANIC ELECTROSYNTHESIS In a chemical synthesis, it is frequent that the main reaction does not occur spontaneously under the initial set of conditions. Typically, the high activation energy of a process is one of the critical barriers that has been exhaustively studied. The use of catalysts to decrease the activation energy of a chemical reaction often requires high temperatures. One way to overcome this limitation is by using alternative methods, such as electrochemical ones, where the reaction can occur under mild conditions and key products are obtained using greener approaches.16 Electrochemical reactions of organic compounds can proceed by different paths depending on the experimental conditions, but in all of them, the electron transfer converts the original molecule (RX) into a reactive intermediate and subsequently into the desired product (Scheme 1).1 In the first step at the cathode side, an electron is removed from the electrode surface (Sox) and consequently transferred to the lowest unoccupied molecular orbital (LUMO) of the organic compound, resulting in its reduction (RX•−). The opposite process occurs at the anode, in which an electron from the highest occupied molecular orbital (HOMO) of the molecule is removed resulting in its oxidation (RX•+). The intermediates obtained in this step are so unstable that they can react easily with other compounds depending on the electrolytic conditions: a nucleophilic substitution happens when the leaving group is Received: January 6, 2017 Published: July 17, 2017 1213

DOI: 10.1021/acs.oprd.7b00004 Org. Process Res. Dev. 2017, 21, 1213−1226

Organic Process Research & Development

Review

of the species and the selectivity of the process should be electroanalytically studied by cyclic voltammetry (CV) and coulometry21 and optionally coupled with analytical techniques that are performed for physical organic chemistry applications.1,19,22,23 In order to perform these reactions, electrochemical reactors (known as cells) are needed, having typically one of the two possible configurations: an undivided cell (Figure 2A) or a

Scheme 1. Electrochemical Functional Group Interconversion Pathways

Figure 2. Schematic diagrams of typical (A) undivided and (B) divided electrochemical cells.

replaced by a nucleophilic one via electrode oxidation or by the same logical reasoning an electrophilic substitution can occur in the cathode; in the presence of an excess of radicals, a dimerization via radical addition shall occur. This environmentally friendly technique has several advantages compared to conventional synthesis methods, since the use of electric energy avoids employing hazardous reagents; the suitable choice of electrodes and electrolyte conditions is used to control the selectivity; adjustment of the potential or current density applied help regulate the reaction rate; and the reactions mostly operate under mild conditions, such as room temperature and atmospheric pressure.17−19 This electron-transfer-driven method is more favorable when the reduction is carried out at a more negative potential and oxidation occurs at a more positive potential. However, owing to the instability of the intermediate species, the reaction can take different pathways. When the HOMO levels of the species are adjacent to each other it is difficult to oxidize a substrate without affecting another. Thus, a functional group known as the electroauxiliary (EA) may sometimes be used to accomplish a more controlled and selective electron-transfer route, essentially for an oxidation (Figure 1).20 The use of an EA leads to an increase of the key substrate HOMO level, and consequently this powerful method promotes a desired selective oxidation. Therefore, the position where the electron transfer should occur and the subsequent formation/cleavage of the chemical bonds to obtain the key product need to be considered. To optimize the yield of the process, the reactivity

divided cell (Figure 2B).24,25 For the undivided cells, two electrodes (anode and cathode) are immersed in an electrolyte solution that contains the electroactive species without any physical separation. This cell has cost benefits, not only due to its simpler design but also because of its lower internal resistance and longer lifetime. In the case of divided cells, the separator between anolyte and catholyte solutions is usually a membrane or ion-permeable barrier that increases the cell resistance, owing to its resistivity and the fact that it increases the interelectrode spacing. Moreover, the mechanical properties and chemical stability of this physical separator usually limit the cell lifetime.26 When organic electrosynthesis is applied in an undivided cell, both reduction and oxidation occur in the same compartment. Consequently, this setup is not appropriate if processes at the counter electrode affect either intermediates or products of the reaction of interest. A divided cell with two distinct compartments is commonly used to overcome this limitation. For laboratory scale applications, a simple divided H-cell is typically employed, where glass frits and ion-exchange membranes are typically chosen as suitable cell separators. In particular cases where a charge transport resistance is observed, a Nafion membrane can be employed, thus enhancing the ion transport and maintaining the high conductivity of the cell.1,21 The applied electrode potential is given relative to a reference electrode (RE), typically a Ag/AgCl or saturated calomel electrode (SCE).9 When performing constant current experiments, a current is imposed and the electrode potential starts to rise. For anodic oxidation, the electrode potential will rise until the redox potential is attained and the oxidation discharge takes place. The electrode potential will remain fairly constant until the electroactive species are fully oxidized at the electrode surface. Afterward the electrode potential rises to a more positive value, which corresponds to a second electroactive species or solvent to be oxidized. So, the redox potential for the oxidation of the electroactive species must be known, otherwise undesired reactions may occur.9 The selection of electrolyte conditions may carry some restrictions in terms of cell conductivity and mass transfer on the surface of the electrode. As the recovery of the electrolyte may involve extra costs, lower concentrations of the supporting electrolyte shall

Figure 1. Diagram for selective and nonselective electron transfer of the substrate. 1214



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be used or even created in situ.27 The use of micro flow cells with reduced distance between the electrodes can be an alternative option to deal with this limitation.28,29 On the other hand, the solution has to have high conductivity to ensure the charge transport process. Recently, ionic liquids have received much interest for being ecological supporting electrolytes that may be used in reactions run at room temperature. These salts show high conductivity at wide potential range and can be recovered and recycled after the reactions take place.30−32 Another important concept regarding the optimization of the overall process is the paired electrochemical process where the two electrochemical reactions are performed in parallel to obtain one or more different products.3,33 This allows a significant reduction not only in the addition of supporting electrolyte but also in energy consumption, as the electric energy spent is both used for anodic and cathodic reactions.34 As referred to above, the reaction rate can be controlled by means of electrical parameters. Typically, the operation modes in electrolytic cells are potentiostatic and galvanostatic. The first one relies on the use of constant cell voltage and is used for the development of batch applications in which the reaction kinetics can be studied over a wide range of reactant and product concentrations, with these parametric studies being carried out with minimum consumption of chemicals. It is rarely applied in larger-scale cells, which are extremely expensive due to the power required. Galvanostatic operation employs constant cell current, which is simpler, cheaper, and easier for charge balancing when compared with the potentiostatic mode. This case is appropriate for continuous operation in steady state and is typical of industrial operation. Batch applications in galvanostatic mode may only be realized if solely harmless side reactions can occur; otherwise the potentiostatic mode is recommended.35,36 The following sections will summarize contributions to organic electrosynthesis processes from laboratorial practice to industrial applications.

addition reactions. Oxazolidinones belong to a group of antimicrobial agents that are effective against Gram-positive pathogenic bacteria.41 Saravanan et al.42 reported a methoxylation of a 2-oxazolidinone using boron doped diamond (BDD) or graphite as the anode (Scheme 2). Higher yields Scheme 2. Anodic Methoxylation of a 2-Oxazolidinone

were obtained for BDD, with a maximum value of 88% obtained for a current density of 50 mA cm−2. Beyond that current density, yields start to decrease due to the electrochemical degradation of the starting material and the solvent oxidation. Considering alkyl aromatic derivatives in solution, they tend to lose a benzylic proton to form a stable benzyl radical, which is consequently oxidized to the cation and further attacked by the electrolyte compounds. Recent studies43 reported that 1(trifluoromethyl)benzene dissolved in dry acetonitrile/ Bu4NBF4 becomes oxidized to 2-(trifluoromethyl) acetanilide with 86% yield allowing new routes to many synthetic therapeutic agents.44 Several different electrolyte conditions and selective pathways for the oxidative intermolecular coupling of arylethers, arylamines, and phenols were reported by Schäfer.45 Intramolecular cyclizations were also performed by Schäfer, and one example can be found in Scheme 3.46 Scheme 3. Synthesis of (a) Isoquinolines and (b) Benzazepines.46

3. ELECTROCHEMICAL REACTIONS FOR ORGANIC SYNTHESIS Over the past few decades, important progress in the understanding of the underlying mechanisms of electrolysis has been achieved. Electrolysis can be classified as direct or indirect electrolysis. The former is based on the direct electron transfer between the electrode and the compound, whereas the latter is focused on a mediated electron transfer by adding redox mediators dissolved in the electrolyte.37−39 The present section describes a variety of such conversion reactions with yields mostly above 80%. 3.1. Anodic Functionalization of Organic Compounds. Organic substrates can be interconverted at the anode via one of the following reactions: anodic oxidation, addition, substitution, or cleavage. Anodic oxidation of molecules in organic solvents involves, for example, reaction between the electrolyte and the dissolved substrate in which the functional groups from the solvent are inserted in the organic substrate. Additionally, to optimize this process, the oxidation can be coupled with hydrogen production occurring in the cathodic compartment of the electrochemical cell.40 One important industrial reaction focused on this anodic conversion is the oxidation of p-methoxytoluene into p-anisaldehyde produced by BASF (section 4.2). Over the past few years, methoxylation of organic compounds has been one of the most highlighted anodic

Isoquinolines (Scheme 3a) and benzazepines (Scheme 3b) were isolated by the intramolecular cyclization of enaminones in MeOH with NaClO4 as supporting electrolyte. Yields obtained using the glassy carbon anode and Pt cathode were 50% and 55% for isoquinolines and benzazepines, respectively. Typically anodic substitutions can take place in alkanes, aryl compounds, and organics activated by an alkoxy, thio, aryl, and amino group or even by a CC double bond. In the case of aromatic compounds, the C−H bonds can be replaced by acetamide, acetate, or other nucleophiles via anodic oxidation of the aromatic compound to its cation radical.3 CN double bond substrates such as azines and oximes can be synthesized from electrochemical oxidation of hydrazones and alcohols, respectively, using platinum anodes (Scheme 4).47,48 Zhang et al. performed the oxime synthesis by using indium tin oxide (ITO) as the cathode and SnCl2 as the mediator.48 Oximes and azines are used for synthesis of pharmaceuticals and conducting polymers, respectively. 1215



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Scheme 4. Anodic Synthesis of (a) Azine via Hydrazone47 and of (b) Oxime via Alcohol.48

Scheme 6. Electrochemical Hydrogenation of Acetophenone

Other authors studied the electrochemical hydrogenation of coal in the presence of PVP-NiB/SiO2 catalysts under clean and mild reaction conditions as an alternative to the conventional coal hydrogenation.61 It was observed that structural and electronic effects influence the electrocatalytic performance. Thus, partial electron transfer from B to Ni and the weaker bond strength of Ni−H facilitates the coal hydrogenation by activating the adsorbed hydrogen. Furthermore, compounds with higher BET areas and better dispersion of the Ni active nanoparticles exhibited electrocatalytic activity. However, an excessive amount of catalyst contributed to agglomeration, covering the active sites and thus reducing activity. 3.3. Electrogenerated Acids. Some catalytic reactions can be induced due to the in situ generation of protons in an anodic process. These strong acids acting as catalysts for alternative routes in applied electrosynthesis are called electrogenerated acids (EGAs).1 A pioneer of this method was Torii that studied multiple induced reactions including acetalization of carbonyls,62 cyanation and allylation of acetals,63,64 epoxide opening leading to ketones65 and alcohols,66 protection−deprotection of alcohols,67 and cyclization of isoprenoids.68 Carbohydrates were reported as raw materials to produce other organic products via dehydration by electrogenerated acid. The high efficiency of this method is directly related with the acidity promoted by the in situ EGA formation at the anode surface (eq 1). Therefore, DMSO acts as the driving force to further dehydrate fructose (eq 2) and sucrose (eq 3), and consequently synthesize 5-hydroxymethylfurfural (HMF) with at least 90% yield.69

As reported,49 several anodic cleavages in stilbenes were applied to form the corresponding aldehydes with 93% to 98% yield using triphenylamine derivatives as electrocatalysts in acetonitrile solutions. Fernández et al.50 performed a detailed study of the mechanism of glycerol electrooxidation to CO2 at Pt electrodes using isotopically labeled glycerol and gained insight into the glycerol C−C bond cleavage. It should be mentioned that glycerol electrooxidation creates 14 F per mol, i.e., more than twice the methanol energy content, so this study opened new perspectives for efficient electrocatalysis in direct alcohol fuel cells. Kolbe electrolysis is one powerful method to obtain symmetrical dimers. The oxidative decarboxylation of carboxylic salts via radicals can result in formation of a dimer by coupling according to Scheme 5, path (a). Depending on the Scheme 5. Oxidative Decarboxylation via (a) Kolbe Electrolysis and (b) Non-Kolbe Electrolysis

reaction parameters (electrode material, current density, additives), the radical can alternatively be oxidized to a carbocation (Scheme 5, path (b)). In the latter case, the cation is further subjected to solvolysis and rearrangement to obtain products such as esters, ethers, olefins, or amides; the process is known as non-Kolbe electrolysis.3,51 3.2. Cathodic Conversion of Organic Compounds. The electroreductive cleavages are often applied to R−X bonds, where X is an electroactive group, mostly a halogen substituent. The overall reaction results in a dissociative electron transfer to form a radical R• and an anion X−, normally followed by the radical reduction to the corresponding carbanion R− (Scheme 1). These cathodic cleavages have extensive use in analytical, synthetic, and environmental applications.52 Exhaustive studies showed that this mechanism is mainly influenced by the molecular structure, position of the halogen substituent (benzylic, aliphatic, aromatic, i.e., the halogen group type and the steric hindrance effect), the solvent proticity, and electrode surface.53−55 Over the past few years, electrochemical hydrogenation has been studied as an alternative route to catalytic hydrogenation under benign conditions.56 What lies behind this alternative process is a multistep mechanism: first an adsorbed hydrogen is formed on the metal surface, followed by the adsorption of the organic compound and the electrocatalytic hydrogenation of the organic molecule.57,58 Sáez et al.59 performed a study of a two-electron reduction of acetophenone to form 1-phenylethanol that has several applications in pharmaceutical and fine chemical industries.60 The hydrogenation using Pd/C 30 wt % in ethanol solutions showed 90% selectivity by applying 2 F charge and 10 mA cm−2 current density (Scheme 6).

DMSO + H 2O → DMSO2 + 2H+ + 2e−

(1)

DMSO/H+

C6H12O6 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ C6H6O3 + (CH3)2 SO2 + 2H 2O + 2H+ + 2e− DMSO/H

(2)

+

C12H 22O11 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ C6H12O6 + C6H6O3 + (CH3)2 SO2 + H 2O + 2H+ + 2e−

(3)

Formation of well-defined aryl-tethered films is another example of a process proceeding in the presence of the EGA. First, the EGA is formed via oxidation of N−N-diphenylhydrazine close to the electrode surface. Therefore, the protons obtained were used to convert an aryltriazene to form the corresponding aryldiazonium salt. Upon negative polarization of the working electrode, the aryldiazonium ions were reduced to form the aryl radicals that consequently facilitate the grafting process and create well-defined aryl-tethered films, with the arene unit anchored to the electrode surface (Scheme 7).70 3.4. Electrogenerated Bases. Anion radicals or anions can be generated by electroreduction and act as bases to deprotonate or initiate base-catalyzed reactions. These compounds, known as electrogenerated bases (EGBs), can be obtained in situ in aprotic solvents to overcome problems related with handling strong bases.18,71 Li et al. reported an 1216



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excellent ligands known as N-heterocyclic carbenes were shown to have numerous applications as electrocatalysts in the chemical industry and to enable a huge variety of organic reactions.79 3.5. Ionic Liquids (ILs). Room-temperature molten salts, known as ionic liquids, have received great attention in electrochemistry due to their specific properties, mainly good ionic conductivity, nonvolatility, thermal stability, nonflammability, and reusability.30,80,81 Still, problems related to difficult isolation of the desired product, ILs’ high viscosity, some environmental instability, and the price of ILs might pose some restrictions to their use. In organic reactions, most reagents are not soluble in aqueous solutions which brings a considerable limitation for organic electrosynthesis applications. The use of ILs overcomes this drawback, as they typically have in their structure an organic cation providing the solubility of the organic substrate during the reaction (Figure 3) and thus operate without the

Scheme 7. Electrografting of an Aryltriazene in the Presence of Electrogenerated Acid69

electrochemical synthesis of N-bromoamino acid using benzylideneaniline as raw material (Scheme 8).71

Figure 3. Structure of some ionic liquid cations used in electrochemistry. Typical counterions include AlCl4−, BF4−, HF−, N(CF3SO2)2−[NTf2], PF6−, and SCN−.

Scheme 8. Electrochemical Synthesis of N-Bromoamino Acid from Benzylideneaniline71

addition of supporting electrolytes.30 Furthermore, they allow the replacement of hazardous organic solvents that have volatility and flammability restrictions. Figure 3 summarizes the typical IL ions employed in electrochemical reactions, with bulky organic cations generally being coupled with weakly coordinating anions. Their wide electrochemical potential window, wide liquid range, and tunable solvents properties are peculiar advantages that caught the attention of researchers to employ these promising solvents for organic electrosynthesis applications.82 Halogenated organic compounds gained much attention for having an important role in the development of several pharmaceuticals and agrochemicals.83,84 Anodic partial fluorination became very attractive for being carried out under mild conditions in the absence of hazardous reagents.80,85 Still, when organic solvents are employed, anodic passivation can occur, leading to a decrease of the process efficiency.30 Several studies reported the enhancement of the fluorides reactivity when employing the ILs in the preparation of fluorinated molecules.86−89 Other authors90−92 showed good yields (>80%) when performing the reactions under aprotic solvents containing HF salt ionic liquids in the presence and absence of imidazolium ionic liquids (Et3N·nHF, n = 1−5) (Scheme 9). The electrochemical synthesis of conductive polymers has been developed by several researchers, since these materials present interesting features for numerous electronic devices.93,94 Alkyl-imidazolium and alkyl-pyrrolidinium derivatives

At the cathode, a carboxylate anion with carbon dioxide is formed by reduction of the R4+ cation and that anion deprotonates the imino group of the radical anion formed by reduction of benzylideneaniline proceeding in parallel with R4+ cation reduction. At the same time, at the anode, a bromide ion is oxidized to hypobromous acid which, being a strong oxidant, easily oxidizes the deprotonated anion, leading to the synthesis of N-bromoamino acid. Several organic compounds were synthesized by Feroci et al.72−75 using EGBs such as oxazolidinones, carbamates, and lactams. Imidazolium-based ionic liquids were found to have an important role as good precursors in the generation of heterocyclic species containing a carbene carbon.76−78 These 1217



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Scheme 9. Anodic Fluorination of Phthalide (a),90 Tetrahydrofuran (b),91 and 1,4-Difluorobenzene (c)92

to the selectivity of the process where the secondary product with difficult separation can be avoided.38 Some processes of indirect oxidation of organic substrates include the following: (1) generation of hydrogen peroxide (H2O2) by cathodic reduction of dioxygen; (2) homolytic cleavage of electrogenerated H2O2 under the action of a mediator; (3) substrate oxidation with the hydroxyl radicals formed (could be either a hydrogen atom abstraction from substrate or even in the case of aromatic compounds an addition to the substrate).38 Indirect anodic electrooxidation reactions were already performed using different types of mediators, such as 2,2,6,6tetramethylpiperidinyl-1-oxyl (TEMPO) compounds,38,106−108 triarylamines,38,106 arylimidazoles,109−111 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ),112 hypervalent iodine,113 or ferrocene.114 Mediators such as halide salts are more suitable for industry approaches due to their simple and easy separation from the mixture.115,116 In some cases, anodic oxidations of alcohols are unsuccessful due to the high potentials needed to complete their conversion. Thus, oxidation of primary and secondary alcohols at low potentials was developed with high conversion rates by using TEMPO derivative molecules.117−119 In alkaline solutions the benzyl alcohol is oxidized to benzaldehyde with 90% yield, where the TEMPO mediator can be simultaneously recycled (Scheme 11).118

have proved to be suitable solvents for the electrosynthesis of polypyrrole,95,96 polythiophene,97−99 and polyaniline.100 Ionic liquids have also been tested as electrolytes for the synthesis of other compounds, mainly via electrooxidation of alcohols,101,102 carboxylic acids,103,104 and aromatic derivatives.47,105 3.6. Indirect Electrosynthesis. Indirect electrolysis describes a particular case of an organic electrosynthesis in which the electron transfer occurs in another redox intermediate (homogeneous), instead of in the electrode/ electrolyte interface (heterogeneous) as shown in Scheme 10.

Scheme 11. Proposed Mechanism for Benzyl Alcohol Oxidation Using TEMPO Molecule As Mediator

Scheme 10. Anodic Oxidation Diagram of Organic Compounds by the Use of Mediators

In turn, this intermediate, called mediator (M), allows the transformation of the organic compounds (RH) at lower oxidation potentials compared to the substrate molecule.38 During this indirect electrooxidation process, mediators are involved either in an electron transfer derived from the RH without the formation of an intermediate with compounds of the solution (outer-sphere oxidation, i.e., oxidation process in which electron transfer occurs between chemical species that remain separate and intact during the transfer, redox catalysis) or converted into oxidants during the electron transfer (innersphere oxidation, i.e., oxidation process in which electron transfer occurs between redox sites connected by a chemical bridge, chemical catalysis).38,106 The summarized multistep process is represented in Scheme 10. The mediator is initially oxidized (Mox), and then it can be either regenerated to its original outer-sphere form by oxidizing the RH or it can oxidize the substrate species to form a radical (R•) by adsorbing an inner-sphere proton by the mediator and originate an intermediate (MH+). Then, a base leads to a deprotonation of the outer-sphere oxidation product and contributes to the regeneration of the mediator. Finally, the key products are obtained by the oxidation of R•. The mediators also contribute

Triarylamines can also be used as organic mediators for alcohols electrooxidation, although to perform a reversible oxidation process they need to be substituted in the paraposition.120 Steckhan, one of the pioneers of this approach, reported several reactions using triarylamines mediators, such as deprotonation of thioacetals, oxidative cleavage of benzyl ethers, allylic and benzylic alcohols, and oxidation of aliphatic ethers, among others.121,122 Lu et al.123 showed a pathway for 4-methoxybenzyl alcohol oxidation by using tris(4bromophenyl)amine (TBPA) as the mediator. Triarylamines are also used for anodic cleavages to form aldehydes starting from stilbene compounds.49 By opposition, the progress in indirect electroreduction processes has slowed down during recent decades. Recently, the studies have been focused on the use of fullerenes, transition metal salen complexes, and carboranes as redox mediators for cathodic processes.106 Cathodic reduction of 1218



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Table 1. List of Typical Industrial Processes Based on Organic Electrosynthesis127 Product

Raw Material

Product

Company

Succinic acid 3,4,5-Trimethoxy benzaldehyde 3,4,5-Trimethoxytolyl alcohol

Commercial processes Acetoin Acetylenedicarboxylic acid Adipoin dimethyl acetal Adiponitrile 4-Aminomethylpyridine Anthraquinone

Butanone 1,4-Butynediol Cyclohexanone Acrylonitrile 4-Cyanopyridine Anthracene

BASF BASF BASF BASF, Monsanto Reilly Tar L. B. Holliday, ECRC Johnson Matthey Co. Clariant Sandoz, India Sandoz, India Spain

Azobenzene

Nitrobenzene

Bleached montan wax Calcium gluconate Calcium lactobionate S-Carbomethoxy methylcysteine L-Cysteine

Raw montan wax Glucose Lactose Cysteine + chloroacetic acid L-Cystine

Diacetone-2-ketogulonic acid

Diacetone-L-sorbose

Dialdehyde starch 1,4-Dihydronaphthalene 2,5-Dimethoxy2,5-dihydrofuran 2,5-Dimethoxy2,5-dihydrofuryl-1-ethanol Dimethylsebacate Gluconic acid Hexafluoropropylene oxide m-Hydroxybenzyl alcohol p-Anisaldehyde Perfluorinated hydrocarbons

Starch Naphthalene Furan

Wacker Chemie AG HoffmanLa Roche CECRI Clariant BASF

Furfuryl-1-ethanol

Otsuka

Monomethyladipate Glucose Hexafluoropropylene m-Hydroxybenzoic acid p-Methoxytoluene Alkyl substrates

Polysilanes Salicylic aldehyde

Chlorosilanes o-Hydroxybenzoic acid

Asahi Chemical Sandoz, India Clariant Otsuka BASF 3M, Bayer, Clariant Osaka Gas India

Raw Material

Company

Maleic acid 3,4,5-Trimethoxy toluene 3,4,5-Trimethoxy toluene

CERCI, India Otsuka Chemical

1-Acetoxynaphthalene 2-Aminobenzyl alcohol Anthraquinone Arabinose

Naphthalene Anthranilic acid Naphthalene, butadiene Gluconate

1,2,3,4-Butanetetracarboxylic acid Ceftibuten

Dimethyl maleate

BASF BASF Hydro Quebec Electrosynthesis Co. Monsanto

3,6-Dichloropicolinic acid

3,4,5,6-Tetrachloro picolinic acid p-Iodotoluene, toluene Formaldehyde

Otsuka Chemical

Piloted processes

Ditolyliodonium salts Ethylene glycol Glyoxylic acid Hydroxymethylbenzoic acid Monochloroacetic acid Nitrobenzene 5-Nitronaphthoquinone Partially fluorinated hydrocarbons Pinacol Propiolic acid Propylene oxide Substituted benzaldehydes

Cephalosporin C

Oxalic acid Dimethyl terephthalate Tri- and dichloroacetic acid p-Aminophenol 1-Nitronaphthalene Alkanes and alkenes Acetone Propargyl alcohol Propylene Substituted toluenes

Electrosynthesis Co. Dow Eastman Chemical Electrosynthesis Co. Rhone Poulenc Clariant Clariant India, Monsanto Hydro Quebec Philips Petroleum Diamond Shamrock BASF Kellog, Shell Hydro Quebec

Figure 4. Scheme of industrial adiponitrile electrosynthesis process (ACN = acrylonitrile; ADN = adiponitrile; QS = quaternary ammonium salt).120

dihalides,124 catalytic reduction of CFCs,125 and intramolecular cyclization of α-bromo β-propargyloxy esters126 are some of the reactions where redox mediators are used for cathodic reduction.

desired product, rate of production, and energy consumption behind the synthesis are some of the most important parameters for successful compound production. Our previous report127 shows a table (Table 1) containing examples of compounds synthesized industrially with the respective company. Some of them have not reached the official commercialization mainly due to (a) high energy costs; (b) pollution constrains; and (c) low product selectivity. A wide variety of industrial processes supported by recent extensive

4. INDUSTRIAL ELECTROSYNTHESIS The overall know-how described in sections 2 and 3 revealed new perspectives on the synthesis of organic compounds by electrochemical processes. For industrial scale, the yield of the 1219



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studies in the field of organic electrosynthesis will be approached along this section. 4.1. Adiponitrile. The electrohydrodimerization of acrylonitrile (ACN) to adiponitrile (ADN) is one of the most commercialized processes of organic electrosynthesis. This method developed by Monsanto in the 1960s became important to obtain the precursor hexamethylenediamine, which will form the well-known fiber Nylon-6,6 after reacting with adipic acid.25,128 While the main product in the anode is oxygen, the cathodic reaction can be described by eq 4. 2CH 2CHCN + 2H+ + 2e− → NC(CH 2)4 CN

derivatives can suffer an anodic acetalization in methanol and form dimethylacetal intermediates, which further hydrolyze in acid solutions to form the aldehyde product. A well-known example, 4-methoxybenzaldehyde, also known as p-anisaldehyde, can be synthesized by the oxidation of p-methoxytoluene, also known as 4-methylanisole (Scheme 12). BASF Company Scheme 12. Reaction Scheme for p-Anisaldehyde Production

(4)

Since the process has low conversion rate per mass, 24 electrochemical divided cells are operating in a batch reactor and the catholyte is continuously recycled using an external compartment (Figure 4). Each cell contains 16 electrode pairs using lead−silver as the anode, lead as the cathode, and a cation-exchange membrane separator.129 The electrolyte contains a two-phase solution of an aqueous phase and a dispersed organic phase of ACN, ADN, and propionitrile byproducts.130,131 The system involves a stripper where the ADN product is removed and isolated whereas the unconverted reactant ACN is recycled back. This design was replaced by an undivided cell system, where the acrylonitrile is mixed with a phosphate-borate buffer solution and where a cadmium anode and a stainless steel cathode are employed.129 Quaternary ammonium salts (QS) used in this process are adsorbed on the cathode in preference to protons or water.127 The QS addition increases the solubility of ACN and decreases the propionitrile byproduct production, contributing to an enhancement of the selectivity of the desired product.128 Moreover, the use of salts such as ethyltributylammonium will protect the cathode from corrosion. Annually, more than 300,000 t of adiponitrile are produced via acrylonitrile hydrocoupling.132 Compared with the conventional chemical procedure, this alternative route became more promising because it requires not only one less reactor but also the use of water instead of HCN as the hydrogen source making the process more ecological. Furthermore, it is believed that the dimerization reaction occurs within the cathode’s hydrophobic double layer, which facilitates dimerization and hinders the formation of propionitrile. Therefore, in order to improve the selectivity, a mathematical model was recently developed to provide estimation of the energy required for the voltage optimization.133 The authors studied the effect of the cathode overpotential on the selectivity of products by applying the input potential for cathode activation in the 0.27−0.67 V range. It was proven that the selectivity for ADN was around 0.25 for lower overpotentials, whereas byproducts such as propionitrile and tricyanohexane were obtained for excessively high overpotential values. Therefore, a maximum selectivity of 0.85 was obtained for a cathode overpotential of 0.57 V. 4.2. Substituted Benzaldehydes. Aromatic aldehydes are mainly used as additives in perfumes, flavoring additives, or intermediates for synthesis of pharmaceuticals and were first prepared by chemical oxidation of the substrates.134 However, the low selectivity and low yields of the desired product under harsh conditions led researchers to search for synthesis routes under milder conditions. One way can be via electrochemical oxidation of alkyl aromatic compounds directly at the electrode or reacting in a substrate that is further regenerated.135 Toluene

has been manufacturing the compound in a capillary gap cell since the 1960s and is currently producing 3500 t per year. Typically, the electrodes used are bipolar graphite rings employing a voltage range per gap of 4−6 V and a current density of 3−5 A dm−2. This continuous bipolar electrode stack operates in a temperature range between 40 and 50 °C using methanol as both solvent and reagent to obtain an overall conversion between 90% and 99% with yields higher than 80% and selectivity of 85%.39 Malloy et al. developed a cell that could operate in a voltage range between 2 and 30 V or a current density up to 1000 mA cm−2, with the temperature ranging from ambient to 50 °C.134 Another advantage compared with the conventional methods is that electrochemical methods do not require the use of concentrated acids that would form undesirable byproducts resulting in a decrease of the overall selectivity. Yield optimizations of substituted benzaldehyde products can be achieved by the use of RTILs,136 performing the synthesis in electrochemical microstructured reactors,137,138 or via indirect electrosynthesis,139 allowing a decrease in the supporting electrolyte quantity. A recent work was published testing different substrates in different imidazolium ionic liquid solutions.140 The synthesis of p-anisaldehyde in 3.1 M EMIMBF4 under a cell voltage of 1.45 V showed the highest selectivity and yield, with values of ca. 92% and 90%, respectively. Furthermore, an industrial paired electrolysis is performed by BASF where cathodic hydrogenation of dimethyl phthalate is occurring in parallel with the anodic substitution of 4-tertbutyltoluene to form the dimethyl acetal and further the corresponding benzaldehyde (Scheme 13).3 Both products are obtained with high yield, with a yearly amount of 4000 t, and are used as intermediates for fragrances and agents for crop protection.138 Scheme 13. Reaction Scheme of a Paired Electrolysis Process for Substituted Benzaldehyde Products

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4.3. Anthraquinone. Anthraquinones (AQs) are natural pigments found in plants that find industrial applications in paper pulping and as natural dyes for foods, cosmetics, and pharmaceuticals.141,142 Scheme 14a shows the synthesis of AQ

4.4. Fluorinated Products. Fluorinated compounds have been useful for several applications such as heat exchange agents, refrigerants, cleaning solvents, and particularly the heterocyclic fluorochemicals for pharmaceuticals and agrochemicals owing to their specific physiological activity. First electrochemical fluorination was developed in the 1940s by John Simons as a perfluorinated process where nickel (Ni) anodes were fluorinated under anhydrous hydrogen fluoride conditions.149 In recent years, Zhejiang Jusheng Fluorochemical Co., Ltd. developed a method to produce perfluorocyclo formyl fluoride derivatives in hydrofluoric acid (HF). The electrolytic solution is prepared by reaction of anhydrous HF with an electrolyte (benzoyl chloride, benzoic acid, phthalic acid chloride, or cyclohexyl chloride) and a solubilizing agent (methyl benzoate, dimethyl phthalate, or phthalic acid ammonium) at temperatures ranging from 10 to 25 °C and pressure ranging from 0.05 to 0.15 MPa for 1 to 5 h. The electrolyte solution is added to a tank and operated under mild conditions (temperature of 15− 35 °C and normal pressure) using Ni and SS as anode and cathode materials, respectively. A continuous 90-day operation allowed obtaining perfluoro cyclohexyl-formyl fluoride products with a yield between 80% and 89% (Scheme 15).150 Recently, an electrosynthesis process for fluorination of ethylene carbonate derivatives using polyhydrofluoride complexes of trialkylamines or tetraalkylammoniumfluorides as fluorinating agents was patented.151 The monofluoration performed during the electrochemical step had a current density range of 10−250 mA cm2 with a Pt, Pb, SS, or Ni cathode and a gas diffusion layer (GDL) anode.151 Alternatively, electrochemical fluorination was proven to contribute to uranium recovery from used nuclear fuel by means of NF3 or XeF2 as the fluorinating agent in a molten salt electrolyte solution.152 Recent approaches focused on the use of ionic liquids as solvents,153 and aromatic compounds as substrates154−158 were found to be promising for further scale up of fluorination electrosynthesis. 4.5. Succinic Acid. Butanedioic acid also known as amber acid or succinic acid was purified for the first time in 1546 by Georgius Agricola.159 The reactivity of this dicarboxylic acid allowed its application in antitumor agents, foods, cosmetics, or even as intermediate for the manufacturing of macromolecular materials.159−161 Due to the limited sources of fossil fuels, alternative sources such as bacterial and other microbial derivatives have become a great focus for succinic acid production.162,163 However, the large amount of wastewater required (as bacteria source), low extraction efficiency, and high production cost are some of the main limitations. In order to overcome these drawbacks, succinic acid production via electrosynthesis was proposed, in which a maleic anhydride hydrolysis typically occurs with further electroreduction under acid conditions (Scheme 16). In past few years several authors have patented electrosynthesis methods that led to higher yields of succinic acid using

Scheme 14. Synthesis of (a) Anthraquinone and (b) Tetrahydroanthraquinone

by anodic oxidation of anthracene (A), which was first reported by Laurent in the 1830s.143 This process is restricted by availability of raw material coming from coal tar.144 Tetrahydroanthraquinone (THAQ) is an AQ derivative that has also important applications in the enhancement of paper pulping process. When naphthalene (C) reacts with oxygen using vanadium(V) oxide as a catalyst, it oxidizes directly to naphthoquinone (D) and further reacts with butadiene via Diels−Alder reaction to form the THAQ (Scheme 14b). Still, the low selectivity due to the formation of byproducts, such as phthalic anhydride (E), led researchers to study alternative mechanisms.144 To overcome this drawback, commercial routes based on electrochemical methods have been developed. For example, since the 1920s, Holliday’s Chemical Company has produced AQ via indirect electrochemical oxidation of anthracene using a cerium(III) methanesulfonate mediator.144,145 Also, British Columbia Research established an indirect electrooxidation mechanism shown in Scheme 14b using the redox couple Ce4+/ Ce3+ in acid solutions. THAQ can be further oxidized to obtain AQ and the Ce couple regenerated.127 Concerning the global threat of persistent organic pollutants, the production of AQ through oxidation of naphthalene and anthracene have increased the attention of the researchers in recent years to push forward the process in ecological terms. Recent studies are being carried out regarding the use of substituted polycyclic aromatic hydrocarbons.146−148 Natarajan et al. developed methods for the AQ production reacting 9anthraldehyde using N-bromosuccinimide in aqueous N,Ndimethylformamide147 or with HOBr generated in situ.148

Scheme 15. Schematic Example of Patented Production Process of Perfluoro Cyclohexyl-Formyl Fluoride

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Scheme 16. Maleic Anhydride Electroreduction for Succinic Acid Production

different types of reactors (Table 2). Gao164 has disclosed a method to couple the maleic anhydride electroreduction with simultaneous oxidation of the iodide ion resulting in a paired electrosynthesis process and therefore in a reduction of the overall cell voltage and cost production. By employing Ti supported RuO2−TiO2 as the anode and Pb alloy as the cathode, it was possible to obtain a current efficiency value of 95%. Alternatively, an electrocarboxylation of ethylene was proposed to obtain succinic acid using Pt and Ni catalysts.170 Although the main products were obtained, side products were also formed and the kinetics were sluggish. Therefore, improvements are necessary before larger scale application of this synthesis method. 4.6. Microreactors. Currently, various industrial synthesis processes using macrobatch reactors are facing barriers that affect the global process owing to the interelectrode ohmic drop, mass transfer phenomena, selectivity, or even environmental factors. 138,171,172 Electrochemical microreactors (ECMRs) are suitable equipment for solving these limitations mainly in electroorganic systems where the electric conductivity is reduced. This ECMR concept is being used for the development of multiple organic synthetic routes with benefits including energy, time, and space savings; increased current efficiencies; resource preservation; waste/emissions minimization; and fewer required separation and synthetic steps.34 At industrial scale, the most employed ECMRs have a plate-toplate configuration containing a flow-through electrolyte in which the main reaction is concluded after electrolyte circulation several times through the interelectrode space. Conductivity and reaction efficiencies are increased compared with the conventional electrolyzers due to the combination of short distance interelectrode spacing with high electrodesurface to reactor-volume.138 The electrodes typically used are graphite, Pt, Ni, and SS, where the working electrode is in contact with the heat-transfer agent. The interelectrode spacer is a polymer material, for example a polyimide foil with a 80− 320 μm thickness (Figure 5). As discussed in section 4.2, BASF Company have produced the p-anisaldehyde dimethylacetal intermediate via methoxylation in a capillary-gap cell for decades. Löwe et al.173,174 developed a microflow electrochemical reactor with a plate-toplate design containing a 75 μm-thick polyimide foil separator, and the selectivity obtained was higher (98%) compared with the traditional macrobatch (85%).171 Alternatively, Bouzek and co-workers175 tested the synthesis of p-anisaldehyde dimethylacetal using microreactors not only

Figure 5. Scheme of an electrochemical microreactor in a plate-toplate electrode design.

for the main reaction but also for the final product isolation. The separation of methanol from the electrolyte solution was performed in the first distillation column, and the unreacted 4methyl anisole was isolated from p-anisaldehyde dimethylacetal in the second column. By comparison with the conventional equipment, the authors reported that it was possible to save energy during the product separation step depending on the organic content in methanol recycled and on the yield of panisaldehyde dimethylacetal. Recently, the same mechanism was tested in bipolar electrochemical microreactors, and the authors verified that it has great potential for industrial applications.176 Other processes were performed using different designs of ECMRs, and some with higher yields are shown in Table 3. No requirement of high-conducting electrolytes, effective heat transfer without localized overheating, and good throughput at high current density with low specific energy consumption are some of the main advantages of ECMRs compared with conventional batch reactors.138 Flow microreactor synthesis enables not only saving energy (since an increase in temperature and residence time generally allows increasing the conversion of starting material and product yield) but also enhancing the selectivity due to easier control of the mass and heat transfer.177,178 Considering that the ECMR size can be a limitation for largescale production, scale up can be more easily accomplished and optimized when numbering-up the ECMRs.177,179,180 Yoshida et al.179 suggested a design for radical polymerization where, within the 8 microtubes, the ones with a small diameter were only applied for the polymerization step. In the final stage, microtubes with a larger channel size are always required in order to minimize the pressure drop with high production volume being obtained simultaneously. The same author tested the Grignard exchange reaction in a pilot plant as well, and during 24 h of continuous operation, it was possible to obtain a 94% yield of the main product.180 Other different chemical syntheses were performed in microflow reactors, including the fluorination of sugars,188 the reduction of substituted benzaldehydes,189 polymerization,190

Table 2. Results for Succinic Acid Scale-up Production Using Different Types of Reactors No.

Reactor type

T/°C

j/A m−2

Anode

Cathode

Yield/%

Reference

1 2 3 4 5

Fixed bed tank Not available Bipolar membrane Pipe reactor H-type

30−80 40−60 20−65 40−80 20−70

200−1000 500−1000 100−1000 100−900 100−2000

Pb Ti Graphite Pb Pb

Ti Ti/TiO2 Pb Pb Pb

92 89−91 >95 70−80 >90

165 166 167 168 169

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Table 3. List of Reactions Using Different ECMRs with Their Main Results

namely in the areas of microreactor engineering, fluid-flow electrochemistry, nanoelectrochemistry, and bipolar electrochemistry, it is believed that soon electroorganic synthesis would occupy a prestigious place in science and engineering, contributing to the specific, efficient, and widespread commercialization of organic electrode processes.

and/or regarding phenomena such as electrophoresis,191 electrogenerated chemiluminescence,192 and bipolar electrochemistry,193 opening new routes for organic electrosynthesis for different scale up purposes.

5. CONCLUDING REMARKS AND FUTURE PROSPECTS Herein, we discussed electrochemical processes as instruments for generating novel and valuable products by using simple reactions and reactors, with minimum environmental problems coupled with reasonable prices. Chemists often encounter difficult situations when dealing with the synthesis of organic compounds. Requirements such as reduced activation energy, high electrocatalytic activity, and low operation temperatures are not easy to achieve. Electrochemical synthesis can generally be carried out under benign-environmental conditions, at moderate or low temperature and pressures, with simple cheap facilities, just requiring proper control of the working electrode potential, or the cell voltage (particularly in the cases of paired electrolysis), or the cell current that is applied to the terminals of the electrolytic cell reactor. In general, the galvanostatic mode is the most used in industrial synthesis, and then, the potential adjusts automatically to the compound with the least positive oxidation potential (anode) or the least negative reduction potential (cathode). This allows one to study oxidation (or reduction) chemistry of a variety of different compounds, essentially under the same conditions and without the need for testing different redox reagents. The main focus of this review is to describe practical applications of electrochemical pathways for organic synthesis, from bench to industrial scale. After a brief description of suitable operating conditions and appropriate electrochemical equipment, it is shown that the electron-transfer process within a chemical reaction is able to convert functional groups through mechanisms and reactive intermediates that are kinetically and environmentally more favorable than those of conventional methods. By being able to avoid hazardous and pollutant reagents, reduce waste production, work at or near room temperature, and use recent advances in electrochemistry,

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

David S. P. Cardoso: 0000-0002-8270-4276 Diogo M. F. Santos: 0000-0002-7920-2638 César A. C. Sequeira: 0000-0001-7556-2858 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank Fundaçaõ para a Ciência e Tecnologia (FCT, Portugal) for financial support under the Contract No. IF/01084/2014/CP1214/CT0003 under the IF2014 Programme (D.M.F.S.), for Postdoctoral Grant SFRH/BPD/77768/2011 (B.Š.), and for a research grant within Project UID/CTM/04540/2013 (D.S.P.C.).

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