Electrochemical Nozaki-Hiyama-Kishi Coupling: Scope, Applications, and Mechanism

Recent advances in electrochemical utilization of NHPI esters

Electrochemical methods employing N-hydroxyphthalimide (NHPI) esters have emerged as powerful tools for sustainable organic synthesis. Derived from abundant and stable alkyl carboxylic acids, NHPI esters enable the generation of alkyl radicals through single-electron transfer (SET) and decarboxylation, facilitating carbon-carbon (C-C) and carbon-heteroatom (C-X) bond formation. These reactions are vital for synthesizing complex molecules used in pharmaceuticals, agrochemicals, and advanced materials, yet traditional approaches often depend on harsh conditions, toxic reagents, or costly metals. This review explores recent progress in electrochemical applications of NHPI esters, highlighting their role in both metal-catalyzed (e.g., Ni, Cr) and metal-free systems.

Pd-Catalyzed Synthesis of Acyclic 1,2-Dioxygenated Dienes and Their Regioselective Decarboxylative Diels-Alder Cycloaddition/Aromatization Reactions to Access Multisubstituted Phenols

A Pd-catalyzed protocol that provides efficient access to acyclic 1,2-dioxygenated dienes has been established. The installation of the bifunctional carbonate electrofuge to the diene cores enabled such dienes to undergo a regioselective decarboxylative Diels-Alder cycloaddition/aromatization reaction, affording diverse synthetic challenging multisubstituted phenols with ease.

Mechanism Switch Between Radical-Polar Crossover and Radical Buffering

Radical-polar crossover (RPC) is a classic concept that bridges one- and two-electron chemistry. It has been widely used in Cr-catalyzed carbonyl addition reactions to clarify the formation of alkyl chromium(III) intermediate and subsequent carbonyl insertion. Herein, we proposed an orthogonal bonding model, the radical buffering scenario, for Cr-catalyzed carbonyl alkylation. This radical bonding model features the radical dissociation from the alkyl chromium(III) complex followed by the Cr(II)-carbonyl-coupled radical addition to form the C─C bond. The mechanism switch between the radical and polar bonding models is affected by the radical stability, radical nucleophilicity, radical size, and the presence of an α-heteroatom or α-π bond. The collaborative computational and experimental studies have verified the reliability of the radical mechanism. More importantly, we demonstrated that this radical buffering scenario possesses a different stereoselectivity control model from that in the RPC scenario. A general enantioselectivity and diastereoselectivity control model derived from the multiple ligand-radical interactions is thus established for CrCl2/bisoxazoline-catalyzed asymmetric radical addition.

Enantioselective Total Syntheses of Vallesamidine and Schizozygane Alkaloids

A general streamlined strategy for the enantioselective total syntheses of the schizozygane family of natural products and related alkaloid vallesamidine is described. Specifically, a catalytic enantioconvergent cross-coupling sets the quaternary stereogenic center in a pluripotent intermediate adorned with an olefin and three orthogonal carboxylate groups, upon which the modularity of the synthesis relies. A late-stage catalytic oxidative lactamization of an alkyne is instrumental in the first-generation synthesis of the schizozygane alkaloids. In the second-generation approach, a novel application of the van Leusen reaction for the generation of lactams is pivotal to accessing the schizozygane alkaloids from a common intermediate. The strategies outlined here are expected to enable detailed biological investigations as well as facilitate access to and provide solutions for the late-stage modification of bioactive alkaloids.

Recent Advances in Asymmetric Organometallic Electrochemical Synthesis (AOES)

ConspectusIn recent years, our research group has dedicated significant effort to the field of asymmetric organometallic electrochemical synthesis (AOES), which integrates electrochemistry with asymmetric transition metal catalysis. On one hand, we have rationalized that organometallic compounds can serve as molecular electrocatalysts (mediators) to reduce overpotentials and enhance both the reactivity and selectivity of reactions. On the other hand, the conditions for asymmetric transition metal catalysis can be substantially improved through electrochemistry, enabling precise modulation of the transition metal's oxidation state by controlling electrochemical potentials and regulating the electron transfer rate via current adjustments. This synergistic approach addresses key challenges inherent in traditional asymmetric transition metal catalysis, particularly those related to the use of redox-active chemical reagents. Furthermore, the redox potentials of molecular electrocatalysts can be conveniently tuned by modifying their ligands, thereby governing the reaction regioselectivity and stereoselectivity. As a result, the AOES has emerged as a powerful and promising tool for the synthesis of chiral compounds.In this Account, we summarize and contextualize our recent efforts in the field of AOES. Our primary strategy involves leveraging the controllability of electrochemical potential and current to regulate the oxidation state of organometallics, thereby facilitating the desired reactions. An efficient asymmetric synthesis platform was established under mild conditions, significantly reducing the reliance on chemical redox reagents. Our research has been systematically categorized into three sections based on distinct electrolysis modes: asymmetric transition metal catalysis combined with anodic oxidation, cathodic reduction, and paired electrolysis. In each section, we highlight our innovative discoveries tailored to the unique characteristics of the respective electrolysis modes.In many transformations, transition metal-catalyzed reactions involving traditional chemical redox reagents and those utilizing electrochemistry exhibit similar reactivities. However, we also observed notable differences in certain cases. These findings include the following: (1) Enhanced efficiency in asymmetric electrochemical synthesis: for instance, the Rh-catalyzed enantioselective electrochemical functionalization of C-H bonds demonstrates superior efficiency. (2) Expanded scope of transformations: certain transformations, previously challenging in traditional transition metal catalysis, can be achieved through electrochemistry due to the tunability of redox potentials. A notable example is the enantioselective reductive coupling of aryl chlorides, which significantly expands the range of accessible transformations. Additionally, our mechanistic studies explore unique techniques intrinsic to electrochemistry, such as controlled potential electrolysis experiments, the impact of electrode materials on catalyst performance, and cyclic voltammetry studies. These investigations provide a more intuitive understanding of the behavior of metal catalysts through the study of electrochemical mechanisms, which can also guide the design of new catalytic systems.The advancements in this field offer a robust platform for environmentally friendly and sustainable selective asymmetric transformations. By integrating electrochemistry with transition metal catalysis, we have developed a versatile approach for organic synthesis that not only enhances the efficiency and selectivity of reactions but also reduces the environmental impact. We anticipate that this Account will stimulate further research and innovation in the realm of AOES, leading to the discovery of new catalytic systems and the development of more sustainable synthetic methodologies.

Cathodic tandem alkylation/dearomatization of heterocycles enabled by Al-facilitated carbonyl deoxygenation

Developing efficient strategies for the deoxygenative functionalization of carbonyl compounds is crucial for enhancing the effective utilization of biomass and the upgrading of chemical feedstocks. In this study, we present an elegant cathodic reduction strategy that enables a tandem alkylation/dearomatization reaction between quinoline derivatives and aryl aldehydes/ketones in a one-pot process. Our approach can be executed via two distinct paths: the aluminum (Al)-facilitated spin-center shift (SCS) path and the Al-facilitated direct deoxygenation path. Both paths are theoretically substantiated by DFT calculations. The crux of this protocol is the in-situ activation of the alcohol intermediates by Al salts, which substantially lowers the activation energy necessary for the formation of key transition states, thereby effectively facilitating the deoxygenation process. Control experiments have not only successfully identified the intermediates but also established that the hydrogen source for the reaction is derived from water and tetrabutylammonium salt. Notably, this method is transition metal-free and compatible with water and oxygen.

A General Three-Component Nozaki-Hiyama-Kishi-Type Reaction Enabled by Delayed Radical-Polar Crossover

Nozaki-Hiyama-Kishi (NHK) reactions offer a mild approach for the formation of alcohol motifs through radical-polar crossover-based pathways from various radical precursors. However, the application of multicomponent NHK-type reactions, which allow the formation of multiple bonds in a single step, has been largely restricted to bulky alkyl radical precursors, thus limiting their expanded utilization. Herein, we disclose a general three-component NHK-type reaction enabled by delayed radical-polar crossover, which efficiently tolerates a plethora of radical precursors that were previously unavailable. This method enables the modular assembly of versatile homoallylic alcohols from feedstock chemicals with excellent chemo-, regio-, diastereo-, and enantioselectivities in a single step. Experimental studies and density functional theory (DFT) calculations reveal that the kinetically favored formation of an allylchromium(III) species is paramount for enforcing the delayed radical-polar crossover over direct radical addition. Finally, straightforward transformations and applications of the homoallylic alcohol products were demonstrated, showcasing the synthetic utility of this method.

Stereoselective Vinylic C-H Addition via Metallaphotoredox Migration

Geometrically defined allylic alcohols with SE, SZ, RE and RZ stereoisomers serve as valuable intermediates in synthetic chemistry, attributed to the stereoselective transformations enabled by the alkenyl and hydroxyl functionalities. When an ideal scenario presents itself with four distinct stereoisomers as potential products, the simultaneous control vicinal stereochemistry in a single step would offer a direct pathway to any desired stereoisomer. Here, we unveil a metallaphotoredox migration strategy to access stereodefined allylic alcohols through vinylic C-H activation with aldehydes. This method harnesses a chiral nickel catalyst in concert with a photocatalyst to enable a 1,4-Ni migration by using readily accessible 2-vinyl iodoarenes as starting materials. The efficacy of this methodology is highlighted by the precise construction of all stereoisomers of allylic alcohols bearing analogous substituents and the efficient synthesis of key intermediates en route to Myristinin family. Experimental and computational studies have shed light on pivotal aspects including the synergy of metal catalysis and photocatalysis, the driving forces behind the migration, and the determination of absolute configuration in the C-H addition process.

Enantioselective nickel-catalyzed electrochemical reductive conjugate alkenylation of α,β-unsaturated ketones

Catalytic electrochemical asymmetric catalysis is emerging as a promising strategy for the synthesis of chiral compounds. Herein, we report an asymmetric electrochemical nickel-catalysed reductive conjugate addition of alkenyl bromides/aryl iodides to α,β-unsaturated ketones in an undivided cell, leading to addition products with high yields and excellent enantioselectivities (up to 96% yield and 96% ee).

Electrochemical Enantioselective Nickel-Catalyzed Cross-Coupling of Aldehydes with Aryl Iodides

The preparation of enantioenriched diarylmethanol derivatives is described using nickel-catalyzed electrochemical cross-couplings between various alkyl/aryl aldehydes and aryl iodides. Performed in an electrochemical cell equipped with an iron anode and a nickel cathode, this electrocatalytic variant led to the scalemic targeted products in the presence of 2,2-bis((4 R,5S)-4,5-diphenyl-4,5-dihydrooxazol-2-yl)acetonitrile (L2), as enantiopure cyano-bis(oxazoline) ligand. X-ray structure analysis of a pre-catalyst, for instance the [Ni(II)(L2)2] complex, with L2 as an anionic bisoxazolinate ligand, confirms the chemical formulation of one nickel surrounded by two ligands. The redox behavior of the new Ni complexes generated in situ was first assessed by cyclic voltammetry showing a redox wave at ca. -1.5 V that can be assigned to the two-electron reduction of the Ni(II) center to the Ni(0) state. Oxidative addition between the electrogenerated Ni(0) complex and aryl iodide was evidenced. An intense current was observed in presence of a mixture of the two substrates pertaining an electrocatalytic process. Interestingly, we found that the sacrificial iron anode plays a crucial role in the catalytic mechanism.

Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Coupling To Access Chiral Secondary Benzylic Alcohols

Transition-metal-catalyzed asymmetric cross-coupling represents a powerful strategy for C-C bond formation and the synthesis of enantiomerically pure molecules. Here, we report a dual nickel/photoredox-catalyzed enantioselective reductive cross-coupling of aryl halides with α-bromobenzoates, readily generated from aliphatic aldehydes, to provide diverse chiral secondary benzylic alcohols that are important motifs in bioactive natural products and pharmaceuticals. This dual catalytic system features mild conditions, good functional group tolerance, broad substrate scope, excellent enantiocontrol, and avoidance of stoichiometric metal reductants, presenting great potential for late-stage functionalization of complex molecules.

Electrochemically Driven Nickel-Catalyzed Enantioselective Hydro-Arylation/Alkenylation of Enones

Herein, the study reports the first electrochemical nickel-catalyzed enantioselective hydro-arylation/alkenylation of enones in an undivided cell with low-cost electrodes in the absence of external reductants and supporting electrolytes. Aryl bromides/iodides/triflates or alkenyl bromides are employed as electrophiles for the efficient preparation of more than 56 valuable β-arylated/alkenylated ketones in a simple manner (up to 97% yield, 97% ee). With the advantages of electrochemistry, excellent functional group tolerance and late-stage modification of complex natural products and pharmaceuticals made the established protocol greener and more economic. Mechanism investigation suggests that a NiI/NiIII cycle may be involved in this electro-reductive reaction rather than metal reductant driven Ni0/NiII cycle. Overall, the efficient electrochemical activation and turnover of the nickel catalyst avoid the drawbacks posed by the employment of stoichiometric amount of sensitive metal powder reductants.

Cobalt-Catalyzed Asymmetric Migratory Nozaki-Hiyama-Kishi Coupling

Selective functionalization of ubiquitous C-H bonds based on 1,n-metal migration provides an attractive and sustainable route to access complex molecules from readily available precursors. Herein, we report a Co-catalyzed asymmetric reductive migratory Nozaki-Hiyama-Kishi (NHK) coupling between two readily available electrophiles, aryl (pseudo)halides and aldehydes, via an unprecedented through-space aryl-to-alkenyl 1,4-cobalt/hydride shift. The judicious choice of ligands is crucial for selectivity, leading to either ipso- or migratory NHK products with exquisite control of regio-, E/Z-, and enantioselectivity. Enabled by a ligand relay catalytic strategy, this platform has been further extended to aryl-to-aryl asymmetric migratory NHK coupling. These high-value NHK adducts, including α-chiral allylic alcohols and benzyl alcohols, are readily convertible to a variety of useful synthons.

Chromium- and Metal-Reductant-Free Asymmetric Nozaki-Hiyama-Kishi (NHK) Reaction Enabled by Metallaphotoredox Catalysis

Chiral allylic alcohols are highly prized in synthetic chemistry due to their versatile reactivity stemming from both alkenyl and hydroxyl functionalities. While the Nozaki-Hiyama-Kishi (NHK) reaction is a widely used method for the synthesis of allylic alcohols, it suffers from drawbacks such as the use of toxic chromium salts, high amounts of metal reductants, and poor enantiocontrol. To address these limitations, we present a novel approach involving a metallaphotoredox-catalyzed asymmetric NHK reaction for the production of chiral allylic alcohols. This method marries alkenyl (pseudo)halides with aldehydes, leveraging a synergistic blend of a chiral nickel catalyst and a photocatalyst. This innovative technique enables both oxidative addition and insertion just using nickel, diverging significantly from the conventional NHK reaction pathway mediated by nickel and chromium salts. The adoption of this methodology holds immense promise for crafting a spectrum of intricate compounds, particularly those of significance in pharmaceuticals. Detailed experimental investigations have shed light on the metallaphotoredox process, further enhancing our understanding and enabling further advancements.

Electrochemical C-H Alkylation of Azauracils Using N-(Acyloxy)phthalimides

We present an electrochemical alkylation of azauracils using N-(acyloxy)phthalimides (NHPI esters) as readily available alkyl radical progenitors under metal- and additive-free conditions. Several azauracils are shown to undergo alkylation with an array of NHPI esters (1°, 2°, 3°, and sterically congested), providing the desired products in good to excellent yields. This operationally simple method is robust, scalable, and suitable for both batch and flow setups.

Chromium Catalyzed Asymmetric Reformatsky Reaction

This study describes an unprecedented chromium-catalyzed asymmetric Reformatsky reaction, enabling the synthesis of chiral β-hydroxy carbonyl compounds from α-chlorinated or α-brominated esters and amides. By employing a chiral chromium/diarylamine bis(oxazoline) catalyst, we achieved relatively broad functional group tolerance. Distinct from known reports, the protocol operates under both classical and photoredox conditions, facilitated by the in situ formation of a nucleophilic chiral chromium intermediate through a radical-polar crossover mechanism. Preliminary mechanistic insights, supported by DFT calculations, identify the nucleophilic aldehyde addition as the key stereo-determining step. This approach not only overcomes the limitations of existing Reformatsky reactions but also provides a versatile strategy for accessing complex chiral molecules.

Nickel-Catalyzed Enantioselective Reductive Arylation of Common Ketones

A nickel complex of chiral bisoxazolines catalyzed the stereoselective reductive arylation of ketones in high enantioselectivity. A range of common acyclic and cyclic ketones reacted without the aid of directing groups. Mechanistic studies using isolated complex of a chiral bis(oxazoline) (L)Ni(Ar)Br revealed that Mn reduction was not needed, while Lewis acidic titanium alkoxides were critical to ketone insertion.

Electrochemically Driven para-Selective C(sp2)-H Alkylation Enabled by Activation of Alkyl Halides without Sacrificial Anodes

With alkyl halides (I, Br, Cl) as a coupling partner, an electrochemically driven strategy for para-selective C(sp2)-H alkylation of electron-deficient arenes (aryl esters, aldehydes, nitriles, and ketones) has been achieved to access diverse alkylated arenes in one step. The reaction enables the activation of alkyl halides in the absence of sacrificial anodes, achieving the formation of C(sp2)-C(sp3) bonds under mild electrolytic conditions. The utility of this protocol is reflected in high site selectivity, broad substrate scope, and scalable.

Carbon quaternization of redox active esters and olefins by decarboxylative coupling

The synthesis of quaternary carbons often requires numerous steps and complex conditions or harsh reagents that act on heavily engineered substrates. This is largely a consequence of conventional polar-bond retrosynthetic disconnections that in turn require multiple functional group interconversions, redox manipulations, and protecting group chemistry. Here, we report a simple catalyst and reductant combination that converts two types of feedstock chemicals, carboxylic acids and olefins, into tetrasubstituted carbons through quaternization of radical intermediates. An iron porphyrin catalyst activates each substrate by electron transfer or hydrogen atom transfer, and then combines the fragments using a bimolecular homolytic substitution (SH2) reaction. This cross-coupling reduces the synthetic burden to procure numerous quaternary carbon---containing products from simple chemical feedstocks.

Electrocatalytic Asymmetric Nozaki-Hiyama-Kishi Decarboxylative Coupling: Scope, Applications, and Mechanism

The first general enantioselective alkyl-Nozaki-Hiyama-Kishi (NHK) coupling reactions are disclosed herein by employing a Cr-electrocatalytic decarboxylative approach. Using easily accessible aliphatic carboxylic acids (via redox-active esters) as alkyl nucleophile synthons, in combination with aldehydes and enabling additives, chiral secondary alcohols are produced in a good yield with high enantioselectivity under mild reductive electrolysis. This reaction, which cannot be mimicked using stoichiometric metal or organic reductants, tolerates a broad range of functional groups and is successfully applied to dramatically simplify the synthesis of multiple medicinally relevant structures and natural products. Mechanistic studies revealed that this asymmetric alkyl e-NHK reaction was enabled by using catalytic tetrakis(dimethylamino)ethylene, which acts as a key reductive mediator to mediate the electroreduction of the CrIII/chiral ligand complex.

Metal-free photocatalytic cross-electrophile coupling enables C1 homologation and alkylation of carboxylic acids with aldehydes

In contemporary drug discovery, enhancing the sp3-hybridized character of molecular structures is paramount, necessitating innovative synthetic methods. Herein, we introduce a deoxygenative cross-electrophile coupling technique that pairs easily accessible carboxylic acid-derived redox-active esters with aldehyde sulfonyl hydrazones, employing Eosin Y as an organophotocatalyst under visible light irradiation. This approach serves as a versatile, metal-free C(sp3)-C(sp3) cross-coupling platform. We demonstrate its synthetic value as a safer, broadly applicable C1 homologation of carboxylic acids, offering an alternative to the traditional Arndt-Eistert reaction. Additionally, our method provides direct access to cyclic and acyclic β-arylethylamines using diverse aldehyde-derived sulfonyl hydrazones. Notably, the methodology proves to be compatible with the late-stage functionalization of peptides on solid-phase, streamlining the modification of intricate peptides without the need for exhaustive de-novo synthesis.

Cobalt-Catalyzed Asymmetric Aza-Nozaki-Hiyama-Kishi (NHK) Reaction of α-Imino Esters with Alkenyl Halides

Chromium-catalyzed enantioselective Nozaki-Hiyama-Kishi (NHK) reaction represents one of the most powerful approaches for the formation of chiral carbon-heteroatom bond. However, the construction of sterically encumbered tetrasubstituted stereocenter through NHK reaction still posts a significant challenge. Herein, we disclose a cobalt-catalyzed aza-NHK reaction of ketimine with alkenyl halide to provide a convenient synthetic approach for the manufacture of enantioenriched tetrasubstituted α-vinylic amino acid. This protocol exhibits excellent functional group tolerance with excellent 99 % ee in most cases. Additionally, this asymmetric reductive method is also applicable to the aldimine to access the trisubstituted stereogenic centers.

Synthesis of the C13-C27 Fragment of Madeirolide A Using Visible-Light-Promoted Radical Cyclization

The convergent synthesis of a fully elaborated C13-C27 fragment of madeirolide A has been achieved. The key features of the synthesis include the stereocontrolled construction of both the THF and THP rings via visible-light-induced iridium-catalyzed radical cyclization and the late-stage union of the two oxacyclic subunits through nickel-catalyzed decarboxylative cross-coupling.

Interfacial Tuning of Electrocatalytic Ag Surfaces for Fragment-Based Electrophile Coupling

Construction of C‒C bonds in medicinal chemistry frequently draws on the reductive coupling of organic halides with ketones or aldehydes. Catalytic C(sp3)‒C(sp3) bond formation, however, is constrained by the competitive side reactivity of radical intermediates following sp3 organic halide activation. Here, an alternative paradigm deploys catalytic Ag surfaces for reductive fragment-based electrophile coupling compatible with sp3 organic halides. We use in-situ spectroscopy, electrochemical analyses, and simulation to uncover the catalytic interfacial structure and guide reaction development. Specifically, Mg(OAc)2 outcompetes the interaction between Ag and the aldehyde, thereby tuning the Ag surface for selective product formation. Data are consistent with an increased population of Mg-bound aldehyde facilitating the addition of a carbon-centered radical (product of Ag-electrocatalyzed organic halide reduction) to the carbonyl. Electron transfer from Ag to the resultant alkoxy radical yields the desired alcohol. Molecular interfacial tuning at reusable catalytic electrodes will accelerate development of sustainable organic synthetic methods.

Exploring an Electrochemical Route for Water-Enhanced Oxygenation Reactions Utilizing Nickel Molecular Structures: A Case Study

Recently, Ni molecular catalysis has been extensively applied in oxygenation reactions. This work is underpinned by the characterization techniques and the discovered instability of the Ni-bipyridine/phenanthroline system, which results in Ni (hydr)oxide production under oxidative conditions. The practical applications of this mechanism by employing a prepared Ni (hydr)oxide-based electrode specifically in the oxygenation of sulfides, achieving noteworthy yields in contrast to noncatalyst control experiments, are explored. Thus, a Ni (hydr)oxide-based material is proposed as a candidate for the true catalyst for sulfide oxidation in the presence of the Ni-bipyridine/phenanthroline system. The findings of this study are expected to stimulate discussion and encourage new viewpoints within the chemical community regarding the potential applications and mechanisms of molecular catalysts in oxidation reactions.

One-Pot Transformation of Aldehydes to Ketones via Minisci-Type Reaction of Imines

A method for the conversion of aldehydes to ketones via the preliminary formation of aldiminines is described. The imines are involved in acid promoted Minisci-type reaction with alkyl radicals generated from esters of N-hydroxylphthalimide under photoredox conditions. Aminyl radical cations formed after the addition of the iminium ions are believed to be key intermediates, which determine the reaction outcome.

Enabling Al sacrificial anodes in tetrahydrofuran electrolytes for reductive electrosynthesis

Al0 is widely used as a sacrificial anode in organic electrosynthesis. However, there remains a notable knowledge gap in the understanding of Al anode interface chemistry under electrolysis conditions. We hypothesize that Al interfacial chemistry plays a pivotal role in the discernible bias observed in solvent selections for reductive electrosynthesis. The majority of existing methodologies that employ an Al sacrificial anode use N,N-dimethylformamide (DMF) as the preferred solvent, with only isolated examples of ethereal solvents such as tetrahydrofuran (THF). Given the crucial role of the solvent in determining the efficiency and selectivity of an organic reaction, limitations on solvent choice could significantly hinder substrate reactivity and impede the desired transformations. In this study, we aim to understand the Al metal interfaces and manipulate them to improve the performance of an Al sacrificial anode in THF-based electrolytes. We have discovered that the presence of halide ions (Cl-, Br-, I-) in the electrolyte is crucial for efficient Al stripping. By incorporating halide additive, we achieve bulk Al stripping in THF-based electrolytes and successfully improve the cell potentials of electrochemically driven reductive methodologies. This study will encourage the use of ethereal solvents in systems using Al sacrificial anodes and guide future endeavors in optimizing electrolytes for reductive electrosynthesis.

Electrochemical synthesis of the protected cyclic (1,3;1,6)-β-glucan dodecasaccharide

Automated electrochemical assembly is an electrochemical method to synthesise middle-sized molecules, including linear oligosaccharides, and some linear oligosaccharides can be electrochemically converted into the corresponding cyclic oligosaccharides effectively. In this study, the target cyclic oligosaccharide is a protected cyclic (1,3;1,6)-β-glucan dodecasaccharide, which consists of two types of glucose trisaccharides with β-(1,3)- and β-(1,6)-glycosidic linkages. The formation of the protected cyclic dodecasaccharide was confirmed by the electrochemical one-pot dimerisation-cyclisation of the semi-circular hexasaccharide. The yield of the protected cyclic dodecasaccharide was improved by using a stepwise synthesis via the linear dodecasaccharide.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

First Reusable Catalyst for the Reductive Coupling Reaction of Organohalides with Aldehydes

In this study, we simulate the reductive coupling (Barbier-Grignard-type) reaction of organohalides with aldehydes using a new reusable catalyst. In this regard, bimetallic alloys of NiCo encapsulated in melamine-based dendrimers (MBD) immobilized on magnetic nanoparticles symbolized as γ-Fe2O3-MBD/NiCo were designed and synthesized. The structure and properties of the catalyst were studied by a variety of techniques such as Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), vibrating sample magnetometry (VSM), energy-dispersive spectrometry (EDS) mapping, and inductively coupled plasma (ICP). The presence of NiCo nanoalloys was confirmed by XRD and XPS analysis, TEM images, and EDS mapping. Various secondary alcohols were produced in good to high yields by reductive coupling of different types of aldehydes and organohalides in the presence of HCO2K as a nonmetallic reducing agent in aqueous media catalyzed by γ-Fe2O3-MBD/NiCo. In these reactions, the high catalytic performance of γ-Fe2O3-MBD/NiCo was achieved in comparison to monometallic counterparts due to the synergistic cooperative effect of Co and Ni in the NiCo nanoalloys. Magnetic and hydrophilic properties of the catalyst facilitate the catalyst recyclability for seven runs. The reusability of γ-Fe2O3-MBD/NiCo, use of water as an environmentally friendly solvent, ease of processing, and absence of metal additives make this process an excellent choice for the reductive coupling reaction to produce secondary alcohols from aldehydes. This is the first report on these kinds of reactions using a reusable catalyst.

Parallel paired electrolysis-enabled asymmetric catalysis: simultaneous synthesis of aldehydes/aryl bromides and chiral alcohols

Metal-catalyzed asymmetric electro-reductive couplings have emerged as a powerful tool for organic synthesis, wherein a sacrificial anode is typically required. Herein, a parallel paired electrolysis (PPE)-enabled asymmetric catalysis has been developed, and the alcohols and ketones could be simultaneously converted to the corresponding aldehydes and chiral tertiary alcohols with high yields and enantioselectivity in an undivided cell. Additionally, this Ni-catalyzed asymmetric reductive coupling can well match the anodic oxidative C-H bond bromination of (hetero)arenes. This protocol opens an alternative avenue for organic synthesis.

Electronic Structures of Nickel(II)-Bis(indanyloxazoline)-dihalide Catalysts: Understanding Ligand Field Contributions That Promote C(sp2)-C(sp3) Cross-Coupling

NiII(IB) dihalide [IB = (3aR,3a'R,8aS,8a'S)-2,2'-(cyclopropane-1,1-diyl)bis(3a,8a-dihydro-8H-indeno[1,2-d]-oxazole)] complexes are representative of a growing class of first-row transition-metal catalysts for the enantioselective reductive cross-coupling of C(sp2) and C(sp3) electrophiles. Recent mechanistic studies highlight the complexity of these ground-state cross-couplings but also illuminate new reactivity pathways stemming from one-electron redox and their significant sensitivities to reaction conditions. For the first time, a diverse array of spectroscopic methods coupled to electrochemistry have been applied to NiII-based precatalysts to evaluate specific ligand field effects governing key Ni-based redox potentials. We also experimentally demonstrate DMA solvent coordination to catalytically relevant Ni complexes. Coordination is shown to favorably influence key redox-based reaction steps and prevent other deleterious Ni-based equilibria. Combined with electronic structure calculations, we further provide a direct correlation between reaction intermediate frontier molecular orbital energies and cross-coupling yields. Considerations developed herein demonstrate the use of synergic spectroscopic and electrochemical methods to provide concepts for catalyst ligand design and rationalization of reaction condition optimization.

Deaminative Addition of Alkylpyridinium Salt to Aldehyde

Here we show that a primary amine can engage in the nucleophilic addition to an aldehyde to synthesize an alcohol following preactivation of the amine. The enabling reagent for this radical-polar crossover process is CrCl₂. This reaction is selective for aldehydes and compatible with numerous functional groups, which are not tolerated under classical Grignard-type conditions. Complementary to the well-established imine synthesis, this deaminative alcohol synthesis can broadly expand the chemical space constructed by aldehydes and amines.

Deep Electroreductive Chemistry: Harnessing Carbon- and Silicon-based Reactive Intermediates in Organic Synthesis

This Viewpoint outlines our recent contribution in electroreductive synthesis. Specifically, we leveraged deeply reducing potentials provided by electrochemistry to generate radical and anionic intermediates from readily available alkyl halides and chlorosilanes. Harnessing the distinct reactivities of radicals and anions, we have achieved several challenging transformations to construct C-C, C-Si, and Si-Si bonds. We highlight the mechanistic design principle that underpinned the development of each transformation and provide a view forward on future opportunities in growing area of reductive electrosynthesis.

Ni-Electrocatalytic Enantioselective Doubly Decarboxylative C(sp3)-C(sp3) Cross Coupling

The first examples of enantioselective doubly decarboxylative cross coupling are disclosed. Malonate half amides are smoothly coupled to a variety of primary carboxylic acids after formation of the corresponding redox-active esters under Ni-electrocatalytic conditions using a new chiral ligand based on PyBox, resulting in amides with α-alkylated stereocenters. The scope of the reaction is broad, tolerating numerous functional groups, and uniformly proceeds with high ee. Finally, the potential utility of this enantioselective radical-radical reductive cross coupling to simplify synthesis is demonstrated with numerous case studies.

Nickel/biimidazole-catalyzed electrochemical enantioselective reductive cross-coupling of aryl aziridines with aryl iodides

Here, we report an asymmetric electrochemical organonickel-catalyzed reductive cross-coupling of aryl aziridines with aryl iodides in an undivided cell, affording β-phenethylamines in good to excellent enantioselectivity with broad functional group tolerance. The combination of cyclic voltammetry analysis of the catalyst reduction potential as well as an electrode potential study provides a convenient route for reaction optimization. Overall, the high efficiency of this method is credited to the electroreduction-mediated turnover of the nickel catalyst instead of a metal reductant-mediated turnover. Mechanistic studies suggest a radical pathway is involved in the ring opening of aziridines. The statistical analysis serves to compare the different design requirements for photochemically and electrochemically mediated reactions under this type of mechanistic manifold.

A guide to organic electroreduction using sacrificial anodes

Organic electrosynthesis is a green strategy for the synthesis of valuable molecules. Electrochemical reactions using sacrificial metal anodes enable new reactivity to be uncovered that could not be achieved with traditional non-electrochemical methods. Compared with reactions using metal powder as the reducing reagent, the mild electroreduction protocols usually exhibit diverse reactivity and excellent selectivity. The inexpensive metal anodes possess low oxidation potential, which could prevent undesired overoxidation of substrates, active intermediates and products. The in situ generated metal ions from sacrificial anodes could not only serve as Lewis acids to activate the reactants but also as a promoter or mediator. This tutorial review highlights the recent achievements in this rapidly growing area within the past five years. The sacrificial anode-enabled electroreductions are discussed according to the reaction type.

Recent advances in electrochemical deoxygenation reactions of organic compounds

As naturally abundant and recyclable industrial feedstock, alcohols and carboxylic acids have drawn tremendous attention in medicinal chemistry and polymer chemistry. The selective C-O cleavage of the hydroxyl group represents an appealing strategy to deliver alkyl and carbonyl moieties into organic molecules. Classical examples of hydroxyl activation include the Appel reaction, Mitsunobu reaction, and Barton-McCombie deoxygenation. However, these early approaches still require large amounts of oxidants or reductants, and suffer from harsh conditions and low atom economy. Electrosynthesis has proven to be an effective and mild way of the modern chemical industry, avoiding the use of chemical oxidants/reductants through the action of an electric current. In this review, we have summarized the recent advances in electrochemical deoxygenation reactions and categorized the deoxygenation methods by different functionalities.

Cathodic Carbonyl Alkylation of Aryl Ketones or Aldehydes with Unactivated Alkyl Halides

An efficient cathodic carbonyl alkylation of aryl ketones or aldehydes with unactivated alkyl halides has been realized through the electrochemical activation of iron. The protocol is believed to include a radical-radical coupling or nucleophilic addition process, and the formation of ketyl radicals and alkyl radicals has been demonstrated. The protocol provides various tertiary or secondary alcohols by the formation of intermolecular C-C bonds under safe and mild conditions, is scalable, consumes little energy, and exhibits a broad substrate scope.

On-DNA Alkyne Iodination and Acetylenic Coupling as a Useful Tool for DEL Synthesis

1-Iodoalkynes and 1,3-diynes are versatile chemical intermediates and pharmaceutically valuable ingredients. In this study, copper mediated on-DNA alkyne iodination and Cadiot-Chodkiewicz coupling are developed for the first time. This generates diverse, systematic, and unprecedented topographic structural features, which could be invaluable as molecular recognition agents for drug discovery in DEL screening.

Recent advances in electrooxidative radical transformations of alkynes

During the past few years, electrochemical oxidative reactions through radical intermediates have emerged as an environmentally-benign, powerful platform for the facile formation of C–E (E = C, N, S, Se, O and Hal) bonds through single-electron-transfer (SET) processes at the electrodes. Functionalized unsaturated molecules and unusual structural motifs can, for instance, be directly constructed under exceedingly mild reaction conditions through initial radical attack onto alkynes. This minireview highlights the recent advances in electrooxidation in radical reactions until June 2022, with a particular focus on radical additions onto alkynes.

Asymmetric 1,4-functionalization of 1,3-enynes via dual photoredox and chromium catalysis

The merger of photoredox and transition-metal catalysis has evolved as a robust platform in organic synthesis over the past decade. The stereoselective 1,4-functionalization of 1,3-enynes, a prevalent synthon in synthetic chemistry, could afford valuable chiral allene derivatives. However, tremendous efforts have been focused on the ionic reaction pathway. The radical-involved asymmetric 1,4-functionalization of 1,3-enynes remains a prominent challenge. Herein, we describe the asymmetric three-component 1,4-dialkylation of 1,3-enynes via dual photoredox and chromium catalysis to provide chiral allenols. This method features readily available starting materials, broad substrate scope, good functional group compatibility, high regioselectivity, and simultaneous control of axial and central chiralities. Mechanistic studies suggest that this reaction proceeds through a radical-involved redox-neutral pathway.

Facile and general electrochemical deuteration of unactivated alkyl halides

Herein, a facile and general electroreductive deuteration of unactivated alkyl halides (X = Cl, Br, I) or pseudo-halides (X = OMs) using D2O as the economical deuterium source was reported. In addition to primary and secondary alkyl halides, sterically hindered tertiary chlorides also work very well, affording the target deuterodehalogenated products with excellent efficiency and deuterium incorporation. More than 60 examples are provided, including late-stage dehalogenative deuteration of natural products, pharmaceuticals, and their derivatives, all with excellent deuterium incorporation (up to 99% D), demonstrating the potential utility of the developed method in organic synthesis. Furthermore, the method does not require external catalysts and tolerates high current, showing possible use in industrial applications.