2.1 Homo-coupling of organic halides

In 2000, Cassol et al. demonstrated the synthesis of 6,6′-dimethyl-2,2′-bipyridine (2) using homocoupling of 6-bromopicoline (Scheme 1) (Cassol et al., 2000). They introduced two appropriate and high-yielding methods for the 6,6′-dimethyl-2,2′-bipyridine synthesis (74–67 %). The first procedure was electroconductive homocoupling of 6-bromopicoline (1) by Zinc rod anode, nickel foam cathode was used for electrolysis, and another one was palladium(II) acetate catalyzed coupling of 6-bromopicoline. Either method was straightforward and described a significant advance over the recognized syntheses of disubstituted bipyridine.

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Scheme 1

Electroreductive homocoupling of 6-bromopicoline.

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Electrochemical reactions were accomplished in an undivided cell by Zn (anode) and NiBr₂bpy in the role of the catalyst. Various products were obtained in mild status with isolated yields between 58 and 98 %. This report surveyed the effects of methyl groups' position on the ring, temperature, and solvent. The nature of the influence of the methyl and its place on the aromatic ring influenced the dimerization effectiveness. These effects were presumably associated with one or more stage (s) of the catalytic cycle. At ambient temperature, the electric resistance of the media became excessively elevated, and the solubility of the nickel complex was low; also, at 50 °C, no development was reached; therefore, the ambient temperature appeared to be the most suitable.

In 2002, França et al. indicated electrochemical homocoupling of 2-bromomethylpyridines and 2-bromopyridine by nickel complexes (De França et al., 2002). Electrochemical reactions were accomplished with Zinc as (anode) and NiBr₂bpy as the catalyst. With mild conditions, divers of products were provided with isolated yields (58–98 %). This report surveyed the effects of the methyl group's position on the ring, temperature, and solvent. The nature of the methyl (Me) group effect and its position on the ring influenced the dimerization efficiency. These effects were presumably associated with one or more stag (s) of the catalytic cycle. Under room temperature, the electric resistance of the medium became too high, and the solubility of the nickel complex was low, also at 50 °C; no development was reached. Therefore, the ambient temperature appeared to be the most suitable. DMF was also better than acetonitrile because the nickel complex solubility in acetonitrile was low.

In 2003, Barhdadi et al. illustrated the direct or nickel-mediated electroconductive homocoupling of organic halides and coupling of them by activated alkenes at room temperature in ionic liquids (RTILs) as a solvent-electrolyte media (Scheme 2) (Barhdadi et al., 2003). They accomplished the electroreductive homocoupling reaction of several alkyl/benzyl halides 3. The electrolysis was performed by arranging a steady current severity between the Mg or Al bar anode and the nickel grid cathode. Bibenzyl 4 (78–48 %) was achieved from benzyl chloride/bromide in good yields.

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Scheme 2

Direct electroreductive coupling of organic halides in [octyl mim][BF₄].

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Two years later, Navarro et al. studied the supporting electrolyte Zn anode's mixed impact on the homocoupling of 2-bromopyridines (1, Scheme 3) (De França et al., 2005). A spectrum of reactions was conducted with different sorts and concentrations of supporting electrolytes, and it was shown that an essential stage for this procedure was the production of aryl zinc via a nickel-zinc transmetalation. Dimerization with ammonium salts was inefficient. In contrast, using sodium iodide (supporting electrolyte) did not inhibit the dimerization, although the organozinc was prepared, and their yields of product 5 were excellent.

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Scheme 3

The electrochemical 2-bromopyridine homocoupling.

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Then, the group Navarro reported mono and dihalopyridines (6 and 7) electrochemical coupling for the formation of unsymmetrical 2,2′-bipyridines 8 catalyzed using nickel complex (Scheme 4) (Oliveira et al., 2012). Reactions were applied to an undivided cell enclosed by surrounded by a nickel foam (cathode) and a zinc rod (anode). 2,2′-Bipyridine derivatives 8 were attained in good yields (58–98 %).

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Scheme 4

Heterocoupling of 2-bromopyridines with 2-bromomethylpyridines.

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In 2013, the group Tanaka described the electroreductive homocoupling of aryl bromides 9 successfully to prepare the corresponding biaryls 4 (6–73 %) (Scheme 5) (Tanaka et al., 2013). By using a double mediatory system including N-alkyl-4-alkoxycarbonylpyridinium salts and [(C₆H₅)₃P]₂PdCl₂, the electro-reductive coupling was conducted in an undivided cell with a platinum (cathode) and a zinc (anode). The mechanism proposal (Scheme 6) showed that in the cathode, Oct^V2+ reduced to quinoid Oct^V0, which reduces Pd(II) to provide Pd(0) active species together with Oct^V0+. Oxidative addition of aryl bromides on the Pd(0) generated Br—Pd(II)—Ar, then this compound was reduced by Oct^V0 and afforded [Ar—Pd(0)]⁻. In the next step, aryl bromide reacted with [Ar—Pd(0)]⁻ and produced Ar—Pd(II)—Ar, which finally underwent reductive elimination to have biaryls.

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Scheme 5

Electroreductive homocoupling of various aryl halides with Pt (cathode) and Zn (anode).

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Scheme 6

Mechanistic pathway representing the reductive coupling.

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At the same time, this group developed the electrochemical treatment of N-alkylpyridinium salts and electro-reductive coupling of aryl bromides 10 in a double mediatory system, including PdCl₂(PPh₃)₂ (catalytic amounts) and N-alkylpyridinium salts (Scheme 7) (Kuroboshi et al., 2013). Viologen served as a recyclable organic reductant. Compared with viologen compounds, pyridinium derivatives have complex structures, and their redox potential could be adjusted by introducing substituents. Zinc (anode) and platinum (cathode) were used for electrolysis. The variety of the desired biaryls 11 were obtained in good to moderate (65–93 %). They proposed a mechanism like the previous work.

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Scheme 7

Electroreductive homocoupling of aryl halides.

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A new electrochemical procedure for synthesizing symmetrical 2,2′-bipyridines 13 (31–89 %) from the transition metal-catalyzed homocoupling of 2-halopyridine derivatives 12 was reported by Oliveira et al. in 2015 (Scheme 8) (Oliveira et al., 2015). The graphite/nanotube powder cavity cell was employed for the electrocatalytic homocoupling of 2,6-dihydropyridines and 2-bromopyridines. A graphite powder/carbon nanotube (9:1 ratio) was used for the optimum cathode material. The homocoupling product obtained excellent yield. The cathode material could be reusable with three cycles with no tremendous drop.

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Scheme 8

Electrocatalytic homocoupling of 2-halopyridines.

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Electrochemical homocoupling of aryl/hetero halides 9 for synthesizing symmetrical biaryls 4 (54–90 %) was developed by Léonel and colleagues in 2018 (Scheme 9) (Rahil et al., 2018). The electrochemical reaction was done in an undivided electrochemical cell surrounded by a nickel foam (cathode) and a nickel/iron (36/64) (anode). The former reports comprised homo couplings of aryl halides using nickel catalysis and faced obstacles like long reaction times, mandatory thermal activation, and scope limitations (Semmelhack et al., 1971; Hashim and Kappe, 2007).

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Scheme 9

Electrochemical homocoupling of aryl halides.

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The proposal mechanism demonstrated that NiBr₂bp by cathodic dual electron transfer reduced to [Ni]⁰ (Scheme 10). Oxidative addition of Ni(0) to the aryl halide carbon-halide bond makes Ar[Ni]^IIX complex an intermediate. Monoelectronic reduction of the mediator, pursued by a second oxidative addition to the aryl halide, led to the Ar[Ni]^III(X)Ar complex. Finally, a reductive elimination furnished the related coupling compounds.

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Scheme 10

Probable reaction mechanism.

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2.2 Cross-coupling of organic halides

Electrochemical synthesis is a very particular type of organic transformation and has been an ongoing research area in recent years. The improvement of the electrochemical cross-coupling method was significantly considered, and various research groups have entered this field. In 2000, Gosmini et al. reported electrochemical coupling of 2-chloropyrazine/2-chloropyrimidine 14 and functionalized aryl halides 3 catalyzed by 2,2′-bipyridine nickel complex (Scheme 11) (Gosmini et al., 2000a). This reaction was conducted with an iron rod (anode) and a nickel foam (cathode). 2-Aryl pyrimidine/pyrazine derivatives 15 were afforded good yields (31–79 %).

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Scheme 11

Electrochemical coupling 2-chloropyrimidine and aryl halides.

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In the same years, this group explained the electrosynthesis of 2-arylpyridines 17 from the reaction between aryl halides 3 and 2-halo pyridine 16 (Scheme 12) (Gosmini et al., 2000b). Nickel foam cathode and iron anode were used. Under mild condition derivations, 2-arylpyridines achieved good to high yields (40–76 %).

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Scheme 12

Electrosynthesis of functionalized 2-arylpyridines.

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The electrochemical coupling reaction of 4-chloroquinoline 18 and aryl halides 3 by cobalt-catalyzed described by Gosmini's group (Scheme 13) (Le Gall et al., 2001a). By using an iron rod (anode) and a stainless steel grid (cathode), scarcely 4-phenylquinoline derivatives 19 obtained 48–81 % yield.

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Scheme 13

Electrochemical coupling of 4-chloroquinoline and aryl halides.

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Both withdrawing and electron-donating groups coupling products achieved acceptable yields. The selection of iron rods was essential for the effectiveness of the electrochemical reaction.

In 2002, this group showed the electrochemical coupling of diverse aryl halides (chlorides, bromides, and iodides) 3 and 20 for synthesizing unsymmetrical biaryls 4 (Scheme 14) (Gomes et al., 2002a). A stainless steel grid (cathode) and an iron rod (anode) were used. The coupling reaction was suitable with diverse withdrawing or electron-donating substituents and efficient with o-substituted aryl halides.

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Scheme 14

Electrochemical coupling of two various aryl halides.

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Electrochemical coupling of aryl/hetero halides 3 and various allylic substrates such as carbonates or acetates 21 using the cobalt-pyridine complex was reported by the group of Gosmini in 2003 (Scheme 15) (Gomes et al., 2003a). A stainless steel cathode and an iron anode were used for electrolysis. Under mild conditions, the corresponding arylallyls 22 were achieved in high yields (30–74 %). This procedure appeared beneficial for synthesizing aryl-allyl compounds.

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Scheme 15

Coupling substituted aryl halides and allyl acetate.

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In 2003, the group Gosmini represented a novel coupling reaction of aryl halides 3 with vinylic acetates 23 using cobalt as a catalyst in an undivided cell with a stainless steel grid (cathode) and a consumable iron (anode) (Scheme 16) (Gomes et al., 2003b) This simple electrochemical procedure is a mild and helpful method for synthesizing different vinyl-aryl compounds. Styrene derivatives 24 were obtained with good yields (20–92 %). Cobalt associated with bi-pyridine appeared more efficient for this reaction than palladium complexes.

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Scheme 16

Electrochemical coupling vinylic acetates and aryl halides.

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In 2007, Leonel et al. exhibited a proper electrochemical procedure for the syntheses of aryl/hetero aryl pyridazine 26 from the reaction of 3-chloro-6-aryl pyridazine 25 and phenyl halides 3 using Ni-catalyzed (Scheme 17) (Sengmany et al., 2007). An iron rod (anode) covered by a nickel foam (cathode) was applied. Under the standard condition, the differences substituted aryl/hetero arylpyridazines obtained moderate to good yields (22–83 %).

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Scheme 17

Electrochemical synthesis of substituted aryl/hetero arylpyridazines.

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In 2011, Léonel et al.displayed an electrochemical reductive coupling aryl halides 3 and 4-amino-6-chloropyrimidines 27 for the synthesis of scope of 4-amino-6-arylpyrimidines 28 (Scheme 18) (Sengmany et al., 2011). Using an iron rod (anode) and a nickel foam (cathode) several 4-amino-6-arylpyrimidines were provided with moderate to excellent yields (34–99 %). The putative mechanism showed that, firstly, Ni(II) was redacted into Ni(0) (Scheme 19). In this step, the oxidative addition of Ni(0) into the carbon-halide bond of either aryl halide or chloropyrimidine occurred, leading to the generation of HetArNi(II)Cl. The monoelectronic reduction of the complex provided a HetArNiI that was affected in another oxidative addition onto the aryl halide, providing a HetArNi(III)(Ar)X complex. Reductive elimination in the final step produced the desired coupling product.

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Scheme 18

The coupling of aryl halides with 4-amino-6-chloropyrimidines.

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Scheme 19

Proposed reaction mechanism.

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The indirect electroreductive coupling of pyrroles 29 and aryl halides 3 employing perylene bisimide (as an electron-transfer mediator) was offered (Scheme 20) (Sun et al., 2016). For the first time, this group introduced perylene bisimides as an indirect electroreductive mediator. The extraordinary electron transfer capabilities and outstanding stabilities of perylene bisimides, with their flexibility for structural modification, make them encouraging nominees as indirect electroconductive mediators. The electrochemical reaction was performed with no utilization of a metal catalyst and the addition of a base using a sacrificial zinc anode. Under mild status, pyrrole compounds 30 were provided with an average to good yield (50–71 %). A proposal mechanism depicted those radical anions deriving from the cathodic reduction reduced aryl halides to aryl radicals, which underwent other reactions by pyrroles generating coupling products. (Scheme 21).

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Scheme 20

The electroreductive coupling of pyrroles and aryl halides.

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Scheme 21

Proposal catalytic cycle.

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During the same time, Khrizanforov et al. presented a novel approach to single-step preparation of perfluoro alkylated arenes 33 through cross-coupling of bromo/chloro arenes or hetero arenes (aryl, furan, and pyridine) 31 and organic perfluoro alkyl halides 32 (Scheme 22) (Khrizanforov et al., 2016). Perfluoroalkylated compounds were achieved in good yields. They chose several complexes, including nitrogen ligands that stabilize the low-oxidation-state metals, for instance, α-di-imine compound (dtbpy, tpy, bpy, and dmphen). These reactions used a complex of cobalt and nickel in various ligand environments. The more negative the reduction potential is, the more efficient the catalyst is.

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Scheme 22

The electrocatalytic cross-coupling of organic halides by nickel and cobalt complexes catalyst.

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Also in 2016, Leonel’s group demonstrated the synthesis of the 4-aryl/hetero-6-pyrrolyl pyrimidines 36 (34–70 %) by electroreductive nickel catalyzed from cross-coupling chloro pyrimidines 34 and aryl halides 35 (Scheme 23) (Sengmany et al., 2016). This reaction was a catalyst in an undivided cell wrapped by nickel foam (cathode), iron (anode), and NiBr₂bpy. They revealed that the scarcely described aryl/hetero aryl pyrrolyl pyrimidines, potentially attractive novel structures for the transition-metal ligands or pharmaceutical industry, could be easily provided by reductive and electrochemical cross-coupling.

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Scheme 23

Electrochemical cross-couplings aryl/hetero halides and 4-chloro-6-pyrrolylpyrimidine.

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In 2017, Atobe et al. designed a new cathodic coupling reaction of arenes 37 with aryl halides 3 (Scheme 24) (Qu et al., 2017). Using the cathodic SET mechanism for the activation of aryl halides allows the coupling reaction (31–76 %) to continue with no the requirement for any transition metal catalysts and single electron donors in a moderate status. Mechanism studies reveal that the SET from a cathode starts a radical chain by producing an anion radical of the aryl halide (Scheme 25). Platinum plates were utilized both for the cathode and anode.

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Scheme 24

The cathodic coupling reaction of arenes with aryl halides.

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Scheme 25

The suitable mechanism for the cathodic reaction.

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The Huang group created a novel pathway for directly coupling allylic/alkyl halides utilizing an electrochemical approach (Scheme 26) (Lai and Huang, 2017). This reaction was performed in an undivided cell with zinc foil (cathode) and a platinum wire (anode) in an aqueous medium by catalyzed palladium(II) acetate. This method was helpful for the coupling reaction (76–91 %) of alkyl halides 39 with allylic halides 40, such as tertiary, secondary, and inactivated primary halides, as well as activated halides.

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Scheme 26

Electrochemical allylic alkylation between allylic and alkyl halides.

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This year, Hansen et al. declared the electrochemical procedure for nickel-catalyzed reductive sp³-sp² couplings alkyl 42 and aryl bromides 3 (Scheme 27) (Perkins et al., 2017). A Reticulated Vitreous Carbon (RVC) foam (cathode) and zinc rod (anode) were used for electrolysis. RVC was selected as the cathode substance for its chemical inertness and high surface area. The yields of product 43 were attained at 51–86 %. The mechanism offered by Weix et al. for metal powder reductive coupling (Biswas and Weix, 2013), where Ni(0) enters the catalytic cycle through the reduction of Ni(II) at the cathode (Scheme 28).

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Scheme 27

Electrochemical for sp³–sp² cross-electrophile coupling of unactivated alkyl halides.

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Scheme 28

The mechanism that explains the electrochemical cross-electrophile coupling.

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In 2018, Li et al. indicated nickel-catalyzed reductive coupling of N-hydroxy phthalimide esters 44 and aryl halide derivatives 45 (Scheme 29) (Li et al., 2018).This report provides a new and practical approach to using carboxylic acids as precursors to carbon–carbon bond-making. The reaction proceeded with mild status in a divided cell and employed triethylamine (as the reductant). This decarboxylative C(sp²)-C(sp³) bond- making showed great substrate functional group adaptability 46 (39–76 %). They proposed to utilize electron-rich amines as sacrificial reductants, which readily underwent anodic oxidation and donated electrons to the system.

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Scheme 29

Electrochemical decarboxylative arylations.

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Mechanism determination revealed that cathodic reduction of the redox-active NHP ester 44 appeared in a decarboxylative fragmentation, resulting in the production of the C(sp³) radical (a) (Scheme 30). Afterward, alkyl radical (a) is intercepted by a homogeneous nickel catalyst, which could be either Ni(II) or Ni(0) created from the oxidative addition of aryl halides. Upon production of Ni(III) species (b), the aryl/alkyl groups were expected to undergo reductive elimination to afford the C(sp²)-C(sp³) coupling product.

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Scheme 30

Proposed mechanism of electrochemical-driven decarboxylative arylation by nickel-catalyzed.

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In 2021, Jiao et al. reported a simple synthesis of 1,1-dialkyl ketones 49 via a nickel catalyst electrochemical reductive cross-coupling of alkyl bromides 48 with alkyl acids 47 in an undivided cell. Alkyl bromides having various functional groups like ether, fluoro, chloro trifluoromethoxy, trifluoromethyl, and OTIPS were used and obtained products in good yields in contrary alkyl bromide with polysubstituted aryl rings did not went well and provided low results (Scheme 31) (Jiao et al., 2021).

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Scheme 31

Electrochemical nickel-catalyzed coupling of alkyl halides with alkyl carboxylic acids .

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The chemical method for reductive acylation between alkyl bromides and alkyl carboxylic acids by nickel-catalyzed has been reported Wang et al. This approach suffers from a long reaction time (24h) (He et al., 2019). A common limitation is employing stoichiometric amounts of manganese or zinc as reductants. The electrochemical method can be an excellent choice for solving this matter. The mechanism is offered in Scheme 32.

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Scheme 32

Plausible catalytic process.

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In 2023, Cantillo et al. described the electrochemical coupling of alkyl bromides 42 and alkyl tosylates 50 by nickel catalyzed (Scheme 33) (Ibrahim et al., 2023). The results illustrated that the reaction proceeds high selectivity under a constant current (4 mA) using NaBr as the electrolyte in DMSO. The alkyl–alkyl bond-making reactions (42–78 %) are essential in medicinal chemistry, so electroorganic synthesis of this compound is growing. The mechanistic pathway representing electrochemical cross-coupling was depicted in Scheme 34.

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Scheme 33

Electrochemical cross-coupling of alkyl tosylates with alkyl bromides.

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Scheme 34

Mechanistic pathway for electrochemical cross-coupling.

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In the same year, Qiu et al. reported NiH-catalyzed electrochemical coupling of alkyl alkenes/halides in an undivided cell with 4.0 mA (constant current) for 12h (Scheme 35) (Li et al., 2023). Alkyl halides perform double roles as coupling substrates and hydrogen sources to cause NiH species to be at electrochemical status. The approach displays a broad precursor range and generates good to high yields with up to seventy examples.

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Scheme 35

Electrochemical coupling of unactivated alkenes and alkyl bromide.

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2.3 Addition of organic halides to unsaturated groups

Conjugate additions onto activated alkenes are valuable for carbon–carbon bond creation in organic synthesis (Perlmutter, 1992). In 2000, Gosmini and colleagues demonstrated that CoBr₂ was an impressive catalyst for the electrochemical addition of aryl bromides 3 onto activated olefins 54 (Scheme 36) (Gomes et al., 2000). No additional product was acquired for other metals, e.g., Mg, Zn, or Al; nickel foam (cathode) and iron (anode) were used. Electrolysis was conducted at 0.2 A (constant current) in an undivided cell with good yield products 55 (20–70 %). The solvent acetonitrile /pyridine offered advantages over pyridine/DMF, affording slightly better results and avoiding using 2,2′-bipyridine as a ligand.

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Scheme 36

Electrochemical addition of aryl halides onto activated alkenes.

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In the same year, Condon et al. demonstrated a direct procedure of activation of alkenyl halides, enabling the fabrication of substantial target compounds, including ketones, γ,δ- or β,γ- unsaturated esters, nitriles, or conjugated dienes as well as alkenylated aryl products (Scheme 37) (Cannes et al., 2000). Homo and cross-coupling, including alkenyl halides 56, were carried out efficiently using electroassisted nickel complex catalysis with (50–77 %) yield. They also exhibited that activating alkenyl chlorides, bromides, or iodides was possible. However, the reaction was born at 50 °C or higher. In cross-coupling reactions, using alkenyl iodides is not beneficial because of the dimerization; this is also observed with bromo styrene.

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Scheme 37

Electrochemical couplings of vinyl halides.

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In 2001, Durandetti et al. indicated the electrochemical coupling of aldehydes 58 with organic halide 3 to synthesize arenecarboxaldehydes 59 (the Nozaki-Hiyama-Kishi reaction) (Scheme 38) (Durandetti et al., 2001). The electrochemical procedure includes a one-compartment cell fitted with a nickel sponge (cathode) and a stainless steel rod (anode) using chromium and nickel catalyst. Secondary alcohols were achieved in the mild to good yields (50–70 %). Different benzhydrol derivatives could be obtained to satisfy outcomes in one stage by a highly facile electrochemical procedure. The reaction was catalytic in chromium salt (7 %/C₆H₅CHO). Also, the process was used to add α-chloroester, allyl acetate, or vinyl halide to aryl aldehydes.

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Scheme 38

Electroreductive cross-coupling between aryl halides and benzaldehydes.

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One year later, Condon et al. described electrochemical heteroarylation of activated olefins 61 by nickel catalyst (Scheme 39) (Condon et al., 2002). The conjugate addition reaction of hetero/aryl halides 60 to functionalized olefins had been favorably attained with nickel catalysis combined with the consumable anode method and prepared available to diverse functionalized hetero aryl products 62 (5–76 %). The structures of hetero aromatic are critical building blocks for synthesizing biologically active fragments as agrochemicals or pharmaceuticals. The reactions were conducted in an undivided cell flushed with a concentric nickel foam (cathode) and a stainless steel rod or iron (anode) in an argon atmosphere.

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Scheme 39

Reaction of activated olefins with 3-bromoquinoline.

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The same year, Gosmini et al. demonstrated electrochemical vinylation of vinyl/aryl halides 3 with acrylate esters 63 using CoBr₂ (Scheme 40) (Gomes et al., 2002b). A stainless steel grid (cathode) and an iron (anode) were performed. Pyridine/triethylamine/acetonitrile was used as a solvent, and bipyridine was used as a ligand. The CoBr₂, 2,2′-bipyridine (specifically substituted by an electron-donating group), and acrylate esters with vinyl chlorides were used. This stereoselective reaction generated only E-olefins 64 (45–61 %) in all the investigated cases. Aryl bromides (substituted by an electron-donating group) in the para status gave the best results.

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Scheme 40

Coupling of acrylate esters with vinyl/aryl halides using CoBr₂.

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In 2002, Nédélec et al. revealed two pathways to obtain medium-sized lactones based on the electrochemical arylation of activated olefins 63 catalyzed by a nickel complex (Scheme 41) (De Mendonça Cavalcanti et al., 2002). Preparation of 6-, 7-, and 8- membered lactones 68 was applied through electrochemical arylation of electron-deficient olefins. The electrochemical reaction was done in the undivided cell with an iron rod (anode) and a nickel foam (cathode). This group found an exciting alternative based on the coupling between an α-β-unsaturated ester 63 and an o-aryl halide 66 (60–85 %), followed by lactonization.

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Scheme 41

Bimolecular coupling between esters of acrylic acid and o-aryl halides.

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Direct electrochemical cross-coupling between a mixture of hetero/aryl chlorides or bromides 3 and carbonates or allylic acetates 22 was offered by Gomes et al. (Scheme 42) (Gomes et al., 2003a). The electrochemical cell has a stainless steel grid (cathode) and a sacrificial iron (anode). This catalytic reaction is a suitable path for the SN₂ with aryl halides. Various aryl allyl compounds 22 were achieved in good yields (30–81 %).

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Scheme 42

Coupling reaction between allyl acetate and aryl halides.

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In 2003, Condon et al. illustrated the arylation of organic halides 3 with acrolein diethyl acetal 69 by nickel(II) bromide as a catalyst precursor (Scheme 43) (Condon et al., 2003). (E)- and (Z)-enol ethers 70 (28–58 %) were obtained by allylic displacement of an alkoxy group. The reactions were carried out using an iron or stainless steel (Fe/Cr/Ni, 72/18/10) anode in steady current intensity at 70 °C, and SN₂ substitution derivatives were not attained. Furthermore, the Z-isomer is barely preferred; the isomer ratio was constant in the reaction.

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Scheme 43

Arylation of acrolein diethyl acetal using nickel catalyzed.

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In 2005, Moeller and Tian published the status for electrochemically assisted Heck reactions (Scheme 44) (Tian and Moeller, 2005). This reaction not only notably accelerates the room-temperature Heck reactions without adding ligands but also leads to high product yields 71 (62–89 %). Coupling iodobenzene 3 to electron-rich, electron-poor, and conjugated olefins series 61 was performed. The reactions were accomplished using a straightforward design utilizing a battery as the energy supply.

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Scheme 44

Coupling bromo/iodobenzene to different olefin series.

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2.4 Carboxylation and carbonylation of organic halides

Electrochemical fixation of CO₂ to organic chemicals is a beneficial procedure for synthesizing different carboxylic acids under neutral and moderate status. These electrochemical carboxylations occur efficiently even in the atmospheric pressure of CO_2; meanwhile, an active metal, including Al or Mg, is utilized as a sacrificial anode in the electrolysis (Wang et al., 2022; Zhao et al., 2023). CO₂ is a plentiful, cheap, non-toxic, and renewable carbon source; the kinetic and thermodynamic stability of the molecule limits its utilization in the chemical industry for the transformation to helpful chemicals (Costentin et al., 2012; Benson et al., 2009; Kumar et al., 2012).

In 2001, Troupel et al. reported the fabrication of unsymmetrical carbonyls 73 from aryl iodide 3 and benzylic chlorides 72 employing iron pentacarbonyl like the origin of CO (carbon monoxide) (Scheme 45) (Dolhem et al., 2001). The electrochemical reaction was done in an undivided cell supplied by a sacrificial stainless steel rod (anode) encircled by a cylindrical nickel grid (cathode). Applying carbon monoxide gas in mild status, this procedure provided symmetrical ketones derived with good yields (76–88 %).

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Scheme 45

Electrosynthesis of various ketones from different aryl halides.

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Reddy et al. declared electrochemical carboxylation of benzyl chlorides 74 catalyzed using Pd(II) complex for synthesizing 2-aryl propionic acids (Scheme 46) (Damodar et al., 2001). This reaction was performed in a cell with an Mg (anode) and Pt (cathode) under atmospheric pressure of carbon dioxide at constant current, producing 2-aryl propionic acids 75 in good yields (74–84 %). Organometallic catalysts did not play any chemical role in the stage of carboxylation but aided in activating the C—X bond, allowing the two-electron reduction of organic halide. Mechanistic research illustrated the reduced Palladium species' cooperative function for carbon dioxide activation.

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Scheme 46

Electrochemical preparation of 2-aryl propionic acids.

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In 2002, Gennaro and Isse explained that electrocarboxylation of benzyl chlorides was performed with no use to homogeneous catalysts (Isse and Gennaro, 2002). This work's most significant advantage is that the silver cathode process occurred at a more optimistic potential than other published catalysts. This method gave carboxylic acids excellent yields. Silver has displayed satisfactory catalytic attributes in the electroreduction of various organic halides.

One year later, Raju and colleagues described the preparation of 6-amino nicotinic acid 78 from 2-amino-5-chloropyridine 76 using H₂SO₄/CO₂ in a cathode surface (Scheme 47) (Raju et al., 2003). Electrochemical carboxylation was executed at an undivided cell by platinum (cathode) and magnesium (anode).

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Scheme 47

The carboxylation of 2-amino-5-chloropyridine by electrochemical reaction.

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This new procedure involved the combination of electrochemical carboxylation and hydrogenation for the synthesis of 6-amino nicotinic acid. Scheme 48 offers a proposed mechanistic rationalization for reductive carboxylation. The two-electron reduction of 2-amino-5-chloropyridine generates an intermediate carbanion, which attacks CO₂ to generate CO₂⁻. The catch of the anion by metal ions afforded by the dissolution of the anodic metal provided a metal carboxylate. Lastly, treating the metal carboxylate with acid furnished the desired product (60–84 %).

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Scheme 48

Proposed mechanism of reductive-carboxylation.

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In 2004, Gennaro et al. demonstrated a novel method for preparing 6-amino nicotinic acid with the electrocatalytic reduction of 2-amino-5-bromopyridine at silver cathodes (Gennaro et al., 2004). They discovered that the procedure required just the stoichiometric charge at Ag cathodes. They gave good yields of 6-amino nicotinic acid, while in DMF/CH₃CN, reduction of the halide compounds in the typically employed electrodes, including Pt, Hg, and GC, required negative potentials, including Pt, Hg, and GC, required very negative possibilities, which were only slightly more positive than that of carbon dioxide.

In 2005, Isse et al. surveyed the electrochemical reduction of some aryl ethyl chlorides by silver and glassy carbon electrodes in dimethylformamide /acetonitrile (Isse et al., 2005). In these two solvents, silver showed a significant catalytic influence. Better results were obtained at lower temperatures. The acid product yields were 70–81 % and 61–73 % at 273.15 K and 298.15, respectively.

In 2007, Aishah et al. explained the electrosynthesis of benzoic acid 80 from chlorobenzene 79 by CO₂ fixation procedure (Scheme 49) (Aishah et al., 2007). The electrolysis was conducted at a one-compartment cell provided by palladium (cathode) and aluminium (anode) under the optimum status of electrocarboxylation, which showed a 72 % yield of C₆H₅CO₂H from C₆H₅Cl. In addition, these statuses were employed in 1,2- and 1,3- dichlorobenzene to transform them into their desired benzoic acids.

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Scheme 49

Electrosynthesis of C₆H₅CO₂H from C₆H₅Cl.

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At the same time, Yamauchi et al. indicated the electrochemical carboxylation of α,α-difluorotoluene derivatives 81, and 84 by the electrochemical reduction using carbon dioxide in an undivided cell with a magnesium rod (anode) and platinum plate (cathode). (Scheme 50) (Yamauchi et al., 2008). They illustrated that this procedure is highly effective for synthesizing α-fluorinated nonsteroidal and 2-fluoro-2-arylpropanoic acids 82, 83, and 85. In an efficient carbon dioxide fixation, the related α-fluorophenylacetic acids were achieved in high yields (40–74 %).

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Scheme 50

The carboxylation of α,α-difluorotoluene compound by electrochemical method.

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In 2008, Niu et al. demonstrated an efficient and easy electrocarboxylation reaction of aliphatic halides 86 (Scheme 51) (Niu et al., 2008). They showed that diverse status, including the supporting electrolyte, temperature, and the nature of the electrode, could affect C₆H₅CO₂H yield. Under the standard reaction status, medium to high yields (22–89 %) were achieved to form corresponding carboxylic acids 87.

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Scheme 51

Electrochemical carboxylation of 3-chloro-2-methylpropene.

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For the first time, the electrocarboxylation of benzyl chloride was surveyed at the silver cathode in CO₂-saturated ambient temperature ionic liquid (BMIMBF₄) solution by Jiaxing et al. in 2009 (Niu et al., 2009). The electrochemical treatment was investigated at various electrodes by cyclic voltammetry, which exhibited the remarkable electrocatalytic impact of the silver electrode in the reduction of benzyl chloride. The most elevated yield of phenylacetic acid (45 %) was acquired under optimization status. The ionic liquid was recovered after four cycles of minimal efficiency loss.

In 2010, Yamauchi et al. illustrated the electrochemical reduction of 1-bromo-2,2,2-trifluoroethyl benzene (88) to synthesize 2-aryl-3,3,3-trifluoropropanoic acid (89) using CO₂ in an undivided cell fitted by a zinc plate (anode) and a Pt plate (cathode) (Scheme 52) (Yamauchi et al., 2010). The desired products with good yields (23–70 %). Also, this current reaction was effectively employed to synthesize β,β,β-trifluorinated non-steroidal anti-inflammatory medicines (NSAIDs).

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Scheme 52

Electrochemical carboxylation of 1-bromo-2,2,2-trifluoroethyl benzene.

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In 2010, Hiejima et al. studied the electrochemical carboxylation of α-chloroethyl benzene 90 with CO₂ in ionic liquids compressed (Scheme 53) (Hiejima et al., 2010). The current efficiency increased with pressure and temperature. The influence of carbon dioxide pressure on the present efficiency of the carboxylation reaction was surveyed. The sacrificial magnesium is satisfactorily utilized in TFSA-DEME compressed with carbon dioxide, allowing it to handle the auxiliary oxidation in the counter electrode and enhance the existing carboxylation efficiency (32–60 %).

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Scheme 53

Electrochemical carboxylation of α-chloroethylbenzene.

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A novel electrochemical method for the electrocatalytic carboxylation of 2-amino-5-bromopyridine and CO₂ in BMIMBF₄ (ionic liquid) to 6-amino nicotinic acid was surveyed in 2010 by Feng et al (Feng et al., 2010). The reactions were completed in three electrodes undivided cells without catalysts or toxic and volatile solvents under mild status. Cyclic voltammetry shows that BMIMBF4 could serve as a medium for 2-amino-5-bromopyridine reduction and the effect of working potential, temperature, passed charge, and concentrations. Under optimized conditions, 6-amino nicotinic acid can be obtained in 75 % yield with excellent selectivity. In addition, the ionic liquid was recycled.

In 2010, Yuan et al. indicated that the electrochemical pathway with nickel cathode was efficient for the electrocarboxylation of polycyclic aryl hydrocarbons 92 with carbon dioxide without additional catalysts under mild status (Scheme 54) (Yuan et al., 2010). Nickel cathode catalyzed in reducing polycyclic aryl hydrocarbons or carbon dioxide. The work provided an efficient and convenient method for synthesizing dicarboxylic acids from CO₂ and polycyclic aryl hydrocarbons. The formation of trans-dicarboxylic acids during electrolyzes is due to electrostatic repel and the steric hindrance effect between two CO2 groups in the products. The related trans-dicarboxylic acids 93 were found in 62–90 % yields.

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Scheme 54

Electrochemical dicarboxylation of naphthalene with CO₂.

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An efficient and straightforward electrocatalytic carboxylation of arylic bromides 3 was extended by Lu et al. in 2011. Magnesium as an anode and silver as a cathode were used (Scheme 55) (Zhang et al., 2011). The influences of fundamental factors (including temperature, the current density, and the nature of the cathode material) on this reaction were examined. The desired carboxylic acids 94 were achieved in 30–78 % yields. The electrochemical treatment was checked in various electrodes (Ni, Ti, Cu, and Ag) by cyclic voltammetry, which exhibited the notable electrocatalytic effect of the silver electrode in the reductive carboxylation of aryl bromides.

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Scheme 55

Electrocatalytic carboxylation of arylic bromides.

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The same group presented the electrochemical reduction of di-bromobenzenes 95 using carbon dioxide (Scheme 56) (Lan et al., 2012). Diverse selectivity of individual isomers determined by cyclo voltammetry. Several cathode materials were surveyed, including titanium, copper, silver, and nickel. The most satisfactory results were achieved with silver as a cathode material owing to the less harmful reduction potential. By using o-di-bromobenzene, a transformation of just 49 % could be observed.

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Scheme 56

The electrochemical carboxylation of di-bromobenzenes compounds.

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Conversely, an average conversion of 71 and 68 % was found in p- and m- di-bromobenzene, respectively. These conversions were not selective. O-di-bromobenzene was mainly monocarboxylate, which could be explained by a significantly more significant reduction potential of both reduction events in 1.61 V.

In 2013, Senboku et al. surveyed the electrochemical carboxylation of hexafluorobenzene 99 (Scheme 57) (Senboku et al., 2013). The electrolysis was stereo- and regio- selective, and polyfluorobenzoic acids 100 were attained in medium to superb yields (44–84 %).

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Scheme 57

Electrochemical carboxylation of polyfluroarenes.

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In 2014, Chen et al. reported the asymmetric electrocarboxylation of 1-phenylethylchloride 90 with carbon oxide using a chiral [Co^I(salen)]-complex catalyst (Scheme 58) (Chen et al., 2014). This process was accomplished in an undivided glass cell by GC (cathode) and magnesium (anode). Although the yield (13–66 %) and the enantiomeric excess values are moderate, this elegant study rendered an alternative method for synthesizing optically active carboxylic acids 101.

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Scheme 58

The asymmetric carboxylation of achiral 1-phenylethyl chloride.

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The electrochemical virtues of ionic liquid and CO₂ systems were surveyed as new reaction media by Tateno et al. in 2015 (Scheme 59) (Tateno et al., 2015). The Electrochemical reaction in ionic liquid and supercritical CO₂ to diverse organohalide compounds 90 illustrated appropriate yields (22–59 %). The obtained consequences exhibit that this system is a valuable medium for electrochemical carboxylation reactions. The chemical fixation of carbon dioxide as a raw material is usually challenging owing to its high stability. Therefore, Grignard and organolithium reagents generally react directly with carbon dioxide (Cai and Xie, 2013). The drawbacks of these approaches are the problem of handling and using costly and toxic reagents and the production of enormous amounts of reagent waste. The electrochemical fixation of carbon dioxide to organic compounds is an attractive alternative to traditional chemical procedures (Senboku, 2021).

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Scheme 59

Electrochemical carboxylation of diverse organohalide compounds using [DEME][TFSI].

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3.1 C—N bond formation reaction

Carbon-nitrogen cross-coupling is one of the most widespread and valuable transforms in organic synthesis fields. C—N cross-coupling reactions are vital for providing N-containing compounds with numerous applications in synthetic, biological, pharmaceutical, and materials science (Kafi-Ahmadi et al., 2021; Parsa Habashi and Poursattar Marjani, 2022; Payamifar et al., 2024a; Payamifar and Poursattar Marjani, 2023a, 2023b, 2024b, Poursattar Marjani et al., 2017, 2019b, 2019a; Schlummer and Scholz, 2004).

In 2017, Baran et al. introduced an electrochemical procedure to attain the cross-coupling between aliphatic amines 103 and aryl halides 3 and 102 (Scheme 60) (Li et al., 2017). The range of the electrolytic protocol comprises aryl chlorides, bromides, iodides, and triflates. Additionally, amides and alcohols could be used as nucleophiles. Electrochemical reactions were performed in an undivided cell by RVC (anode) and nickel (cathode) without an external base. Under milder conditions, various aryl donors (Ar—OTf, Ar—I, Ar—Br, and Ar—Cl) and secondary and primary amines with functional group tolerance are viable electrophiles in this reaction. This work displayed using an inexpensive nickel catalyst to enable carbon–nitrogen bond formation and exhibited a rare instance of anodic and cathodic processes adapted to synergistic produce reactive catalyst species in various oxidation states; therefore, a sacrificial electrode was not included. There are many previous reports on the formation of aryl carbon–nitrogen bonds between aryl halides and amines (Lavoie et al., 2016; Park et al., 2014; Ge et al., 2014; Fine Nathel et al., 2014; Manolikakes et al., 2008). The harsh reaction conditions prevented them from being extended. In this electrochemical method, cross-coupling between aryl halides and alkyl amines was performed without an external base at room temperature.

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Scheme 60

Electrochemical amination of aryl halides by nickel-catalyzed.

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3.2 C—S bond formation reaction

The C—S bond generation reactions are used as powerful means in synthetic organic chemistry for the synthesis of different pharmaceutical, biological, and molecule materials (Feng et al., 2016; Boyd, 2016; Evano et al., 2013).

Recently, Wang et al. revealed electrochemical thiolation of aryl iodides 106 for carbon–sulfur bond formation by nickel-catalyzed (Scheme 61) (Wang et al., 2019). The reaction was attained in an undivided cell by graphene/nickel foam electrodes, which were chosen to enhance the charge exchange. The electrochemical reaction was performed under moderate electrochemical status and provided alkyl and aryl sulfides 108 in superb yields (54–99 %). In an undivided cell unit, radical-mediated nickel species of various oxidation states, the cathodic and anodic processes synergistically harness. Manuel reported that the palladium-catalyzed coupling of aryl halides with thiols was used using the chemical method (Fernández-Rodríguez et al., 2006). This reaction was performed in DME as the solvent and DME as the base at 110 °C. This reaction needs a high mole percent palladium catalyst at high temperatures to afford sulfide derivatives.

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Scheme 61

Electrochemical cross-coupling thiols and aryl halides.

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The proposal mechanism (Scheme 62) showed that the anodic oxidation of thiol 107 caused cation radical F. Hydrogen abstraction from cation radical by pyridine generated the thiol radical G with aryl sulfide H. Meanwhile, Ni⁽⁰⁾—X B was produced from a cathodic reduction of NiCl₂·dtbbpy A. Oxidative addition B with aryl halide 106 generated X—Ni^(II)—Ar species C, which trapped the thiol radical G to deliver a Ni^(III)—complex D. Lastly, reductive elimination of D furnished the coupling product 108 and Ni^(I)—X complex E observed with the cathodic reduction to regenerated Ni⁽⁰⁾—X B.

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Scheme 62

The proposed mechanism reaction.

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The cyclic voltammetry studies were performed, as shown in Fig. 3. According to this data, aryl iodide and nickel (II) catalysts exhibited relatively high oxidative potentials (2.17 and 1.03 V vs SCE, respectively). Thiol indicated multiple irreversible oxidative waves from 0.88 V vs SCE. By adding pyridine to thiol, only one oxidative wave showed at 1.04 V, demonstrating that pyridine could stabilize the oxidation process of thiols. These anodic events substantiated that the initiation of the catalytic cycle was more likely to be the oxidation of thiol to its radical R—S radical.

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Fig. 3

The cyclic voltammetry study Wang et al., 2019.

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3.3 C—P bond formation reaction

Organophosphorus is a beneficial reagent in catalysis, organic synthesis, coordination chemistry, and medicinal chemistry (Montchamp, 2014; Tang and Zhang, 2003). An appropriate procedure for their preparation commonly comprises carbon-phosphorus coupling between nucleophilic phosphorus-having compounds and aryl halides (Tappe et al., 2010; Demmer et al., 2011). The electrochemical synthesis of aryl/hetero phosphonates from di-methyl phosphite and aryl/hetero halides for the first time was published by the group's Léonel in 2018 (Scheme 63) (Sengmany et al., 2018). These reactions were accomplished in the galvanostatic mode in an undivided cell with an iron/nickel rod anode enclosed by a Ni foam cathode. NiBr₂bpy was applied as the comfortably available pre-catalyst, TBAB (Supporting electrolyte), and trimethylamine as a base. Both aryl iodides and bromides could be utilized 109, providing the related hetero/aryl phosphonates 111 with good yields. Electron-rich and electron-deficient aryl bromides coupling with di-methyl phosphite 110 produce products in excellent yields (40–86 %). The other di-alkyl phosphites were used in this process. In the chemical method, Yamaguchi reported the synthesis of arylphosphinates by zinc-mediated nickel-catalyzed coupling of aryl halides and H-phosphinate (Kinbara et al., 2015). This reaction needs long reaction times at high temperatures to afford phosphorus derivatives. The electrochemical method has some advantages, such as a short-time reaction and high yield at room temperature.

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Scheme 63

Electrochemical cross-coupling of aryl halides and di-methyl phosphite.

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The admissible mechanism showed that an initial cathodic reduction of the nickel^(II) pre-catalyst led to the Ni⁽⁰⁾ species, pursued by oxidative addition of the halide and reductive electron transfer to produce Ni^(I) species (Scheme 64). Di-methyl phosphite was then added to create a Ni-phosphorus ylide. Proton abstraction on the hydroxyl of by Et₃N led to the critical complex, which would be oxidized to generate Ar—Ni(II)P(O)(OMe)₂. Reductive elimination afforded the target compound 111 and the catalyst of the process Ni⁽⁰⁾.

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Scheme 64

The suggested mechanism of electrochemical carbon-phosphorus bond fabrication.

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3.4 C—B bond fabrication reactions

The electrochemical methodology suggests an appealing alternative to the classical preparation of organoboronic derivatives. In unique boronic acids and esters, boronic derivatives constitute highly valuable intermediates for their broad applications in the agrochemical and pharmaceutical areas and polymer and material chemistry (Silva et al., 2020). However, the chief interest of esters and boronic acids is their usage as intermediates in coupling reactions. Coupling reactions permit the creation of new carbon–carbon bonds to form novel and complex molecules exhibiting particular features.

In 2002, Dunach et al. reported electrochemical coupling of aromatic or heteroaromatic halides 3 and a trialkyl borate including B(OⁱPr)₃ or B(OMe)₃ 112 for the obtaining of aryl/hetero boronic acids 113 (Scheme 65) (Laza et al., 2002). The selectivities of aryl boronic acids were in the scope of 32–73 %. The reactions were employed in a single-compartment cell with a magnesium or aluminum (anode) and stainless steel or nickel foam (cathode).

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Scheme 65

Electrosynthesis of aryl boronic acids with B(Oⁱpr)₃ or B(OMe)₃.

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One year later, the same group represented a novel electrosynthesis of aryl boronic esters 115 from pinacolborane 114 and aryl halides 3 (Scheme 66) (Laza and Dunach, 2003). They used pinacolborane (HBpin) as a suitable substance for the functionalization of aryl halide. In this case, aryl boronic esters yielded up to 95 %. The electrochemical reaction was conducted in a single-compartment cell fitted with a nickel foam (cathode) and a consumable magnesium (anode). The nature of the electrodes ultimately affected the results. A magnesium anode was shown to be more efficient than aluminium or zinc. This electrochemical method exhibited several advantages compared to the classical boration of aryl halides. The boration occurred in one-pot without the requirement to prepare the Grignard reagent or its Li equivalent.

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Scheme 66

Electrosynthesis of aryl boronic pinacol esters from pinacol borane and aryl halides.

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In 2004, the group Dunach demonstrated the electrochemical procedure, including the straightforward coupling of pinacolborane 114 and benzylic halides 116 for synthesizing benzyl boronic pinacol esters 117 (Scheme 67) (Pintaric et al., 2004). Under mild status, the reaction was conducted in a single-compartment cell with a magnesium (anode) and a nickel (cathode). Benzyl boronic pinacol esters were attained in 61–82 % yields.

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Scheme 67

Electrosynthesis of benzylboronic pinacol esters derivatives.

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The electrochemical reduction of aryl halide compounds 118 using pinacolborane (119) as Dunach's group had reported the electrophile to obtain aryl boronic acids selectively that still bear halogen substituents for further functionalization (Scheme 68) (Laza et al., 2005). Reactions were acted in a single cell compartment using a magnesium (anode) and a nickel foam (cathode) in tetrahydrofuran. The relative nature and place of the halogen substituents affect the boration outcomes. Some halogenated aryl boronic esters 120–122 achieved satisfactory yields at unfinished transformations.

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Scheme 68

Electrosynthesis of aryl boronic esters.

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By control of the electrolysis conditions, p-F, p-Cl, p-Br, and m-Br phenyl pinacol boronic esters were obtained in 45, 75, 67, and 60 % yields, respectively. Also, m-phenyl-di-boronic ester yields 57 % after 4F/mol electrolysis of m-di-bromobenzene.

Electrosynthesis of allyl boronic pinacol esters from the electrochemical boration of pinacolborane 124 with various allyl halides 123 illustrated by the group of Dunach in 2009 (Table 2) (Godeau et al., 2009). The electrosynthesis was done in a single-compartment cell with a nickel foam (cathode) and a consumable Al (anode) in a tetrahydrofuran solution comprising (CF₃SO₂)₂NLi. The allyl boronic 125 and 126 derivatives were acquired in 60–86 % yields, generally as mixtures of linear and branched isomers, with the linear isomers usually being the significant compounds. The process is simple and does not require organometallic catalysis, and the reaction takes one step. The terminal borated substance having the less hindered achieved preferential with 68–91 % selectivities. The E-derivative was the principal or particular stereoisomer.

Table 2 Electrosynthesis of allyl boronic pinacol esters from pinacolborane and allyl halides.

Entry	Substrate	Products ratio	E/Z ratio of 125	Boration yield
1		90:10	70/30	81 %
2		91:9	74/26	86 %
3		84:16	96/4	70 %
4		7:90	–	7 %
5		79:21	–	76 %
6			–	65 %
7		83:17	100/-	74 %
8		68:32	100/-	64

In 2023, for the first time, Wang et al. declared electrochemical borylation of alkyl boronic esters to unactivated alkyl halides 127 at high current (Scheme 69) (Wang et al., 2021). The tertiary, secondary, primary, and alkyl boronic esters provide good to superb yields 129 (70 examples). Mechanistic investigations revealed that B₂cat₂ 128 acts as both a cathodic mediator and a reagent, which could electro reduction at a low potential of about alkyl bromides or chlorides difficult-to-reduce. Recently, transition-metal catalyzed borylation of alkyl halides has developed as a valuable strategy for synthesizing alkylboron compounds. Regardless, these methods depend mainly on transition-metal catalysts, long reaction times, and stoichiometric quantities of activators (Yang et al., 2012; Yi et al., 2012; Wang et al., 2021; Ito and Kubota, 2012). The Electrochemical approach is milder, safer, simpler, and environmentally efficient for synthesizing alkyl boron compounds. The cyclic voltammetry (CV) experiments are shown in Fig. 4. The mechanism for the reaction can be explained using cyclic voltammetry data.

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Scheme 69

Electrochemical borylation of alkyl halides for synthesizing alkyl boronic esters.

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Fig. 4

The cyclic voltammetry data Wang et al., 2021.

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3.5 C—Si bond formation reaction

In 2001, Moreau et al. illustrated the electrochemical synthesis of functional aryl/hetero arylchlorosilanes 132 (Scheme 70) (Moreau et al., 2001). An efficient and selective procedure for synthesizing functionalized di- and mono-chlorosilanes as an essential intermediate for polarized organosilicons that are fascinating for NLO applications was described. This reaction was done in an undivided cell with an aluminium or magnesium (anode), a concentric stainless steel grid, or carbon (cathode). Due to its versatility and selectivity, the electrosynthesis pathway is especially beneficial for constructing new organosilicon models. Electrochemical reduction of aryl halides, halopyridines, halofurans, and halothiophenes 130 for synthesizing various functional aryl/hetero arylchlorosilanes (43–88 %).

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Scheme 70

Ni-catalyzed electrochemical synthesis of p-methoxyphenyldimethylchlorosilane.

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