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.

Recent advances in the electrochemical functionalization of N-heterocycles

Nitrogen-containing heterocyclic cores are of immense importance due to their high abundance in naturally occurring or synthetic molecules having wide applications in different fields of basic and applied sciences. The functionalities introduced in an N-heterocyclic core play an important role in regulating the physiochemical behavior of the particular N-heterocycles to alter their chemical and biological reactivity. Suitably functionalized N-heterocycles demonstrate their widespread applications in pharmaceuticals, agronomy, materials sciences, synthetic chemistry, pigments, etc. During the last decade, electrochemistry has emerged as a sustainable alternative to conventional synthetic approaches by minimizing reagent uses and chemical waste. Synthetic chemists have extensively utilized the tool to functionalize N-heterocycles. This is evidenced by the appearance of more than a hundred methods on the topic over recent years, signifying the importance of the synthetic area. This review is focused on the accumulation of synthetic methods based on the electrochemical functionalization of N-heterocycles developed over the recent decade. Literature reports on the C-/N-H-functionalization and functional modifications of N-heterocycles that are accessible through the available search engines are included in the review. Relevant mechanistic details in support of the reported reactions are discussed to present a clear picture of the reaction pathways. The review aims to provide a clear picture of the possible pathways of electron transfer, the electrochemical behavior of different N-heterocyclic cores, functionalization reagents, and the chemical processes that occur during the electrochemical functionalization/modification of N-heterocycles.

Synthesis of N-heterocyclic amides from imidazoheterocycles through convergent paired electrolysis

An efficient ring opening of imidazoheterocycles induced by a direct C-H azidation resulting in an unusual formation of N-heterocyclic amides has been successfully developed through convergent paired electrolysis. A broad scope of pyridylbenzamides could be obtained in moderate to excellent yields under exogenous-oxidant, electrolyte- and metal-free electrochemical conditions. The methodology was transferred to continuous flow conditions resulting in notable improvements particularly in terms of cost-efficiency over traditional batch versions.

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.

Organophotoelectrocatalytic C(sp2)-H alkylation of heteroarenes with unactivated C(sp3)-H compounds

An organophotoelectrocatalytic method for the C(sp2)-H alkylation of heteroarenes with unactivated C(sp3)-H compounds through dehydrogenation cross-coupling has been developed. The C(sp2)-H alkylation combines organic catalysis, photochemistry and electrochemistry, avoiding the need for external metal-reagents, HAT-reagents, and oxidants. This protocol exhibits good substrate tolerance and functional group compatibility, providing a straightforward and powerful pathway to access a variety of alkylated heteroarenes under green conditions.

Mechanisms for radical reactions initiating from N-hydroxyphthalimide esters

Due to their ease of preparation, stability, and diverse reactivity, N-hydroxyphthalimide (NHPI) esters have found many applications as radical precursors. Mechanistically, NHPI esters undergo a reductive decarboxylative fragmentation to provide a substrate radical capable of engaging in diverse transformations. Their reduction via single-electron transfer (SET) can occur under thermal, photochemical, or electrochemical conditions and can be influenced by a number of factors, including the nature of the electron donor, the use of Brønsted and Lewis acids, and the possibility of forming charge-transfer complexes. Such versatility creates many opportunities to influence the reaction conditions, providing a number of parameters with which to control reactivity. In this perspective, we provide an overview of the different mechanisms for radical reactions involving NHPI esters, with an emphasis on recent applications in radical additions, cyclizations and decarboxylative cross-coupling reactions. Within these reaction classes, we discuss the utility of the NHPI esters, with an eye towards their continued development in complexity-generating transformations.

Direct alkylation of quinoxalinones with electron-deficient alkenes enabled by a sequential paired electrolysis

The metal-free alkylation of N-heterocycles with alkenes has remained a synthetic challenge. We report here the successful implementation of metal-free alkylation of quinoxalinones with electron-deficient alkenes enabled by a sequential paired electrolysis. This protocol provides a mechanistically distinct approach to prepare a variety of C-3 alkylated quinoxalinones that are otherwise quite difficult to synthesize by other means.

Direct Decarboxylative C-2 Alkylation of Azoles through Minisci-Type Coupling

This note discusses the application of a Minisci-type reaction for the direct alkylation of azoles with carboxylic acids as radical precursors. Different reaction conditions were investigated to achieve high yield of the desired products, focusing on acid strength and solvent screening. Moreover, the reactivity of imidazoles with various carboxylic acids was investigated, showing good yield for most cases. The study reveals the potential of this approach for late-stage functionalization in drug discovery.

Photoredox catalysis harvesting multiple photon or electrochemical energies

Photoredox catalysis (PRC) is a cutting-edge frontier for single electron-transfer (SET) reactions, enabling the generation of reactive intermediates for both oxidative and reductive processes via photon activation of a catalyst. Although this represents a significant step towards chemoselective and, more generally, sustainable chemistry, its efficacy is limited by the energy of visible light photons. Nowadays, excellent alternative conditions are available to overcome these limitations, harvesting two different but correlated concepts: the use of multi-photon processes such as consecutive photoinduced electron transfer (conPET) and the combination of photo- and electrochemistry in synthetic photoelectrochemistry (PEC). Herein, we review the most recent contributions to these fields in both oxidative and reductive activations of organic functional groups. New opportunities for organic chemists are captured, such as selective reactions employing super-oxidants and super-reductants to engage unactivated chemical feedstocks, and scalability up to gram scales in continuous flow. This review provides comparisons between the two techniques (multi-photon photoredox catalysis and PEC) to help the reader to fully understand their similarities, differences and potential applications and to therefore choose which method is the most appropriate for a given reaction, scale and purpose of a project.

Stirring-Free Scalable Electrosynthesis Enabled by Alternating Current

Alternating current (AC) electrolysis is receiving increased interest as a versatile tool for mild and selective electrochemical transformations. This work demonstrates that AC can enable the concept of a stirring-free electrochemical reactor where the periodic switch of electrode polarity, inherent to AC, provides uniform electrolysis across the whole volume of the reactor. Such design implies a straightforward approach for scaling up electrosynthesis. This was demonstrated on the range of electrochemical transformations performed in three different RVC-packed reactors on up to a 50-mmol scale. Redox-neutral, oxidative, and reductive processes were successfully implemented using the suggested design and the applicable frequency ranges were further investigated for different types of reactions. The advantages of the AC-enabled design - such as the absence of stirring and a maximized surface area of the electrodes - provide the possibility for its universal application both for small-scale screening experimentation and large-scale preparative electrosynthesis without significant optimization needed in between.

Desulfinative Alkylation of Heteroarenes via an Electrostatic Electron Donor-Acceptor Complex

Functionalized pyridine and quinoline rings are important components of numerous bioactive molecules and natural products; however, diversification of these rings often requires de novo heterocycle ring synthesis or demanding reaction conditions. We report a method for desulfinative alkylation of pyridine and quinoline N-methoxide salts that operates under both photocatalytic and electrostatic electron donor-acceptor-mediated pathways. Unlike most EDA-mediated processes, this reaction operates in the absence of light and with the desulfination of the donor compound.

Cobalt-catalyzed synthesis of aryl ketones and aldehydes from redox-active esters

A cobalt-catalyzed decarboxylative oxidation of benzylic redox-active esters is described. This protocol efficiently converts secondary or primary aliphatic carboxylic acids into aromatic ketones or aldehydes. A wide range of substrates selectively reacted in good to excellent yields with broad functional group tolerance. Notably, various biologically active molecules could also work well, which indicated the synthetic application of such a methodology.

Visible-Light-Induced Decarboxylative Alkylation/Ring Opening and Esterification of Vinylcyclopropanes

A visible-light-induced four-component reaction of vinylcyclopropanes, N-(acyloxy)phthalimide esters, N,N-dimethylformamide (DMF), and H₂O through an oxidative ring opening of cyclopropane is presented. This procedure provides a new and effective way to construct formate esters. DMF is employed as both a solvent and the source of CHO. This difunctionalization of vinylcyclopropanes shows good functional group tolerance under room temperature. A radical pathway is involved, and carbonyl oxygen of ester originated from water in this transformation.

Regio- and Stereoselective Electrochemical Alkylation of Morita-Baylis-Hillman Adducts

Electrosynthesis is effectively employed in a general regio- and stereoselective alkylation of Morita-Baylis-Hillman compounds. The exposition of N-acyloxyphthalimides (redox-active esters) to galvanostatic electroreductive conditions, following the sacrificial-anode strategy, is proved an efficient and practical method to access densely functionalized cinnamate and oxindole derivatives. High yields (up to 80%) and wide functional group tolerance characterized the methodology. A tentative mechanistic sketch is proposed based on dedicated control experiments.

General electrochemical Minisci alkylation of N-heteroarenes with alkyl halides

Herein, we report, a general, facile and environmentally friendly Minisci-type alkylation of N-heteroarenes under simple and straightforward electrochemical conditions using widely available alkyl halides as radical precursors. Primary, secondary and tertiary alkyl radicals have been shown to be efficiently generated and coupled with a large variety of N-heteroarenes. The method presents a very high functional group tolerance, including various heterocyclic-based natural products, which highlights the robustness of the methodology. This applicability has been further proved in the synthesis of various interesting biologically valuable building blocks. In addition, we have proposed a mechanism based on different proofs and pieces of electrochemical evidence.

Diastereoselective addition of redox active esters to azomethine imines by electrosynthesis

Thanks to metal- and catalyst-free electrochemical conditions in an undivided cell, a series of readily available redox-active N-(acyloxy)phthalimide esters led to an efficient and highly stereoselective addition (85 : 15 to 95 : 5 dr) of putative radical species to chiral (racemic and enantioenriched) C5-substituted azomethine imines to provide an array of 31 polyaminated hydrazine derivatives as a single diastereoisomer.

In Situ Alkyl Radical Recycling-Driven Decoupled Electrophotochemical Deamination

Molecular electrophotocatalysis has emerged as a powerful strategy for the development of sustainable synthetic protocols. With the proof-of-concept, we exploited a versatile electrophotocatalytic deaminative alkylation approach. Mechanistic investigation indicated that in situ recycling of the alkyl radicals was the key point. Notably, ligand modification and late-stage functionalization of pharmaceuticals were also established, highlighting its feasibility in practical utilization.

Recent Advances in C(sp3)-C(sp3) and C(sp3)-C(sp2) Bond Formation through Cathodic Reactions: Reductive and Convergent Paired Electrolyses

The formation of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds is one of the major research goals of synthetic chemists. Electrochemistry is commonly considered to be an appealing means to drive redox reactions in a safe and sustainable fashion and has been utilized for C-C bond-forming reactions. Compared to anodic oxidative methods, which have been extensively explored, cathodic processes are much less investigated, whereas it can pave the way to alternative retrosynthetic disconnections of target molecules and to the discovery of new transformations. This review provides an overview on the recent achievements in the construction of C(sp³)-C(sp³) and C(sp³)-C(sp²) bonds via cathodic reactions since 2017. It includes electrochemical reductions and convergent paired electrolyses.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Electroreductive Cross-Coupling of Trifluoromethyl Alkenes and Redox Active Esters for the Synthesis of Gem-Difluoroalkenes

An electroreductive access to gem-difluoroalkenes has been developed through the decarboxylative/defluorinative coupling of N-hydroxyphtalimides esters and α-trifluoromethyl alkenes. The electrolysis is performed under very simple reaction conditions in an undivided cell using cheap carbon graphite electrodes. This metal-free transformation features broad scope with good to excellent yields. Tertiary, secondary as well as primary alkyl radicals could be easily introduced. α-aminoacids L-aspartic and L-glutamic acid-derived redox active esters were good reactive partners furnishing potentially relevant gem-difluoroalkenes. In addition, it has been demonstrated that our electrosynthetic approach toward the synthesis of gem-difluoroalkenes could use an easily prepared Kratitsky salt as alkyl radical precursor via a deaminative/defluorinative carbofunctionalization of trifluoromethylstyrene.

Electrochemical Deoxygenative Thiolation of Preactivated Alcohols and Ketones

This work describes an electrochemically promoted nickel-catalyzed deoxygenative thiolation of alcohols and ketones under mild conditions. Excellent substrate tolerance and good chemical yields can be achieved by graphene/nickel foam electrodes in an undivided cell. Further study to gain mechanistic insight into this electrochemical cross-coupling has been carried out.

Practical and Regioselective Synthesis of C-4-Alkylated Pyridines

The direct position-selective C-4 alkylation of pyridines has been a long-standing challenge in heterocyclic chemistry, particularly from pyridine itself. Historically this has been addressed using prefunctionalized materials to avoid overalkylation and mixtures of regioisomers. This study reports the invention of a simple maleate-derived blocking group for pyridines that enables exquisite control for Minisci-type decarboxylative alkylation at C-4 that allows for inexpensive access to these valuable building blocks. The method is employed on a variety of different pyridines and carboxylic acid alkyl donors, is operationally simple and scalable, and is applied to access known structures in a rapid and inexpensive fashion. Finally, this work points to an interesting strategic departure for the use of Minisci chemistry at the earliest possible stage (native pyridine) rather than current dogma that almost exclusively employs Minisci chemistry as a late-stage functionalization technique.

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

One of the most oft-employed methods for C-C bond formation involving the coupling of vinyl-halides with aldehydes catalyzed by Ni and Cr (Nozaki-Hiyama-Kishi, NHK) has been rendered more practical using an electroreductive manifold. Although early studies pointed to the feasibility of such a process, those precedents were never applied by others due to cumbersome setups and limited scope. Here we show that a carefully optimized electroreductive procedure can enable a more sustainable approach to NHK, even in an asymmetric fashion on highly complex medicinally relevant systems. The e-NHK can even enable non-canonical substrate classes, such as redox-active esters, to participate with low loadings of Cr when conventional chemical techniques fail. A combination of detailed kinetics, cyclic voltammetry, and in situ UV-vis spectroelectrochemistry of these processes illuminates the subtle features of this mechanistically intricate process.

Electrochemical Generation and Use in Organic Synthesis of C-, O-, and N-Centered Radicals

During the last decade several research groups have been developing electrochemical procedures to access highly functionalized organic molecules. Among the most exciting advances, the possibility of using free radical chemistry has attracted the attention of the most important synthetic groups. Nowadays, electrochemical strategies based on these species with a synthetic purpose are published continuously in scientific journals, increasing the alternatives for the synthetic organic chemistry laboratories. Free radicals can be obtained in organic electrochemical reactions; thus, this review reassembles the last decade's (2010-2020) efforts of the electrosynthetic community to generate and take advantage of the C-, O-, and N-centered radicals' reactivity. The electrochemical reactions that occur, as well as the proposed mechanism, are discussed, trying to give clear information about the used conditions and reactivity of these reactive intermediate species.

Reaching the Full Potential of Electroorganic Synthesis by Paired Electrolysis

Electroorganic synthesis has recently become a rapidly blossoming research area within the organic synthesis community. It should be noted that electrochemical reaction is always a balanced reaction system with two half-cell reactions-oxidation and reduction. Most electrochemical strategies, however, typically focus on one of the two sides for the desired transformations. Paired electrolysis has two desirable half reactions running simultaneously, thus maximizing the overall margin of atom and energy economy. Meanwhile, the spatial separation between oxidation and reduction is the essential feature of electrochemistry, offering unique opportunities for the development of redox-neutral reactions that would otherwise be challenging to accomplish in a conventional reaction flask setting. This review discusses the most recent illustrative examples of paired electrolysis with special emphasis on sequential and convergent processes.

Electro-Oxidative Coupling Reactions Leading to π-Conjugated Compounds

Electrochemical reactions are rapidly gaining attention today as a powerful and environmentally benign reaction processes for organic synthesis. We found that the electro-oxidation of palladium acetate afforded cationic palladium species and thus-generated cationic Pd species were efficient mediators for electro-oxidative coupling reactions. Homo-coupling of arylboronic acids and terminal alkynes proceeded efficiently to afford biaryls and butadiyne, respectively. Cross-coupling reactions between terminal alkynes and arylboronic acids were also achieved with the use of a Ag anode. As an advantage of electrochemical reactions, we developed a sequential reaction system switched between oxidative and neutral conditions by the on/off application of electricity, and several π-extended butadiynes were obtained in one-sequence by the system. Electrochemical intramolecular C-S coupling for the synthesis of thienoacene was also developed. The use of Bu₄ NBr as a halogen mediator was essential for the reaction.

Advances in Electrochemical Decarboxylative Transformation Reactions

Owing to their non-toxic, stable, inexpensive properties, carboxylic acids are considered as environmentally benign alternatives as coupling partners in various organic transformations. Electrochemical mediated decarboxylation of carboxylic acid has emerged as a new and efficient methodology for the construction of carbon-carbon or carbon-heteroatom bonds. Compared with transition-metal catalysis and photoredox catalysis, electro-organic decarboxylative transformations are considered as a green and sustainable protocol due to the absence of chemical oxidants and strong bases. Further, it exhibits good tolerance with various functional groups. In this Minireview, we summarize the recent advances and discoveries on the electrochemical decarboxylative transformations on C-C and C-heteroatoms bond formations.

Construction of Peptide Macrocycles via Radical-Mediated Intramolecular C-H Alkylations

Enzyme-catalyzed radical-mediated C-H functionalization reactions allow nature to create natural products of unusual three-dimensional structures from simple linear peptide precursors. In comparison, chemist's ability to harness radical C-H functionalization reactions for synthesis of complex peptides remains limited. In this work, new methods have been developed to construct peptide macrocycles via radical-mediated intramolecular C-H alkylation reactions under photoredox catalysis. Linear peptide precursors equipped with a C-terminal N-(acyloxy)phthalimide ester can cyclize with the α C-H bond of N-terminal glycine or aryl C-H bond of N-heteroarene capping units in high yield and selectivity under mild conditions. The strategy uses the C-H cyclization step to incorporate lysine, homolysine, and various heteroarene-derived amino acid linchpins into peptide macrocycles, enabling convergent and flexible synthesis of complex peptide macrocycles from simple building blocks.

Metal-free electrochemical synthesis of α-ketoamides via decarboxylative coupling of α-keto acids with isocyanides and water

A mild and convenient electrochemical protocol has been developed for the preparation of α-ketoamides via a decarboxylative coupling reaction of α-keto acids with isocyanides and water.

Recent progress on electrochemical synthesis involving carboxylic acids

Carboxylic acids are not only essential sections of medicinal molecules, natural products and agrochemicals but also basic building blocks for organic synthesis. However, high temperature, expensive catalysts and excess oxidants are normally required for carboxylic acid group transformations. Therefore, more eco-friendly and efficient methods are urgently needed. Organic electrochemistry, as an environmentally friendly and sustainable synthetic method, can potentially avoid the above problems and is favored by more and more organic chemists. This review summarized the recent progress on the electrochemical synthesis of carboxylic acids to construct more complex compounds, emphasizing the development of electrosynthesis methodologies and mechanisms in order to attract more chemists to recognize the importance and applications of electrochemical synthesis.

Functionalized Polyesters via Stereoselective Electrochemical Ring-Opening Polymerization of O-Carboxyanhydrides

Ring-opening polymerization is used to prepare polyesters with precisely controlled molecular weights, molecular weight distributions, and tacticities. Herein, we report a Co/Zn catalytic system that can be activated by an electrical current to mediate efficient ring-opening polymerization of enantiopure O-carboxyanhydrides, allowing for the synthesis of isotactic functionalized polyesters with high molecular weights (>140 kDa) and narrow molecular weight distributions (M_w/M_n < 1.1). We also demonstrate that these catalysts can be used for stereoselective ring-opening polymerization of racemic O-carboxyanhydrides to synthesize syndiotactic or stereoblock copolymers with different glass transition temperatures compared with their atactic counterparts.

Photocatalytic dual decarboxylative alkenylation mediated by triphenylphosphine and sodium iodide

An efficient photocatalytic dual decarboxylative alkenylation of α,β-unsaturated carboxylic acids and alkyl N-hydroxyphthalimide (NHP) esters mediated by triphenylphosphine and sodium iodide has been developed. This protocol proceeds under 456-nanometer irradiation by visible blue light in the absence of transition metals or organic dye based photoredox catalysts. The reaction is successfully applied to a wide range of redox-active esters derived from aliphatic carboxylic acids (1°, 2° and 3°) and α-amino acids, enabling transformations of diverse α,β-unsaturated carboxylic acids to α,β-alkylated styrenes with high efficiency and excellent selectivity under mild conditions.

Electrochemical decarboxylative C3 alkylation of quinoxalin-2(1H)-ones with N-hydroxyphthalimide esters

Electrochemical decarboxylative C3 alkylation of a wide range of quinoxalin-2(1H)-ones under metal- and additive-free conditions was reported.