Chemoselective (Hetero)Arene Electroreduction Enabled by Rapid Alternating Polarity

Mild Photochemical Reduction of Alkenes and Heterocycles via Thiol-Mediated Formate Activation

The reduction of alkenes to their respective alkanes is one of the most important transformations in organic chemistry, given the abundance of natural and commercial olefins. Metal-catalyzed hydrogenation is the most common way to reduce alkenes; however, the use of H2 gas in combination with the precious metals required for these conditions can be impractical, dangerous, and expensive. More complex substrates often require extremely high pressures of H2, further emphasizing the safety concerns associated with these hydrogenation reactions. Here we report a safe, cheap, and practical photochemical alkene reduction using a readily available organophotocatalyst, catalytic thiol, and formate. These conditions reduce a variety of di-, tri-, and tetra-substituted alkenes in good yield as well as dearomatize pharmaceutically relevant heterocycles to generate sp3-rich isosteres of benzofurans and indoles. These formal-hydrogenation conditions tolerate a broad range of functionalities that would otherwise be sensitive to typical hydrogenations and are likely to be important for industry applications.

Photochemical dearomative skeletal modifications of heteroaromatics

Dearomatization has emerged as a powerful tool for rapid construction of 3D molecular architectures from simple, abundant, and planar (hetero)arenes. The field has evolved beyond simple dearomatization driven by new synthetic technology development. With the renaissance of photocatalysis and expansion of the activation mode, the last few years have witnessed impressive developments in innovative photochemical dearomatization methodologies, enabling skeletal modifications of dearomatized structures. They offer truly efficient and useful tools for facile construction of highly complex structures, which are viable for natural product synthesis and drug discovery. In this review, we aim to provide a mechanistically insightful overview on these innovations based on the degree of skeletal alteration, categorized into dearomative functionalization and skeletal editing, and to highlight their synthetic utilities.

Intricacies of Mass Transport during Electrocatalysis: A Journey through Iron Porphyrin-Catalyzed Oxygen Reduction

Electrochemical steps are increasingly attractive for green chemistry. Understanding reactions at the electrode-solution interface, governed by kinetics and mass transport, is crucial. Traditional insights into these mechanisms are limited, but our study bridges this gap through an integrated approach combining voltammetry, electrochemical impedance spectroscopy, and electrospray ionization mass spectrometry. This technique offers real-time monitoring of the chemical processes at the electrode-solution interface, tracking changes in intermediates and products during reactions. Applied to the electrochemical reduction of oxygen catalyzed by the iron(II) tetraphenyl porphyrin complex, it successfully reveals various reaction intermediates and degradation pathways under different kinetic regimes. Our findings illuminate complex electrocatalytic processes and propose new ways for studying reactions in alternating current and voltage-pulse electrosynthesis. This advancement enhances our capacity to optimize electrochemical reactions for more sustainable chemical processes.

Recent Advances in Dearomative Partial Reduction of Benzenoid Arenes

Dearomative partial reduction is an extraordinary approach for transforming benzenoid arenes and has been well-known for many decades, as exemplified by the dehydrogenation of Birch reduction and the hydroarylation of Crich addition. Despite its remarkable importance in synthesis, this field has experienced slow progress over the last half-century. However, a revival has been observed with the recent introduction of electrochemical and photochemical methods. In this Minireview, we summarize the recent advancements in dearomative partial reduction of benzenoid arenes, including dihydrogenation, hydroalkylation, arylation, alkenylation, amination, borylation and others. Further, the intriguing utilization of dearomative partial reduction in the synthesis of natural products is also emphasized. It is anticipated that this Minireview will stimulate further progress in arene dearomative transformations.

Mechanochemical Birch Reduction with Low Reactive Alkaline Earth Metals

Birch reduction and similar dissolved metal-type transformations hold significant importance in the organic synthesis toolbox. Historically, the field has been dominated by alkali metal reductants. In this study, we report that largely neglected, low-reactive alkaline earth metals can become powerful and affordable reductants when used in a ball mill under essentially solvent-free conditions, in the presence of ethylenediamine and THF as liquid additives. Calcium can reduce both electron-deficient and electron-rich arenes, with yields of products similar to those obtained with lithium metal. Magnesium reveals enhanced reducing power, enabling the reduction of benzoic acids while keeping electron-rich aromatic moieties intact and allows for chemoselective transformations. The developed mechanochemical approach uses readily available and safer-to-handle metals, operates under air and ambient temperature conditions, and can be used for gram-scale preparations. Finally, we demonstrate that the developed conditions can be used for other dissolved metal-type reductive transformations, including reductive amination, deoxygenation, dehalogenation, alkene and alkyne reductions.

Total Synthesis of Dragocins A-C through Electrochemical Cyclization

The first total synthesis of dragocins A-C, remarkable natural products containing an unusual C4' oxidized ribose architecture bridged by a polyhydroxylated pyrrolidine, is presented through a route featuring a number of uncommon maneuvers. Several generations towards the target molecules are presented, including the spectacular failure of a key C-H oxidation on a late-stage intermediate. The final route features rapid, stereocontrolled access to a densely functionalized pyrrolidine and an unprecedented diastereoselective oxidative electrochemical cyclization to forge the hallmark 9-membered ring. Preliminary studies suggest this electrochemical oxidation protocol is generally useful.

Electrochemically Mediated Atom Transfer Radical Polymerization Driven by Alternating Current

Alternating current (AC) and pulsed electrolysis are gaining traction in electro(organic) synthesis due to their advantageous characteristics. We employed AC electrolysis in electrochemically mediated Atom Transfer Radical Polymerization (eATRP) to facilitate the regeneration of the activator CuI complex on Cu0 electrodes. Additionally, Cu0 served as a slow supplemental activator and reducing agent (SARA ATRP), enabling the activation of alkyl halides and the regeneration of the CuI activator through a comproportionation reaction. We harnessed the distinct properties of Cu0 dual regeneration, both chemical and electrochemical, by employing sinusoidal, triangular, and square-wave AC electrolysis alongside some of the most active ATRP catalysts available. Compared to linear waveform (DC electrolysis) or SARA ATRP (without electrolysis), pulsed and AC electrolysis facilitated slightly faster and more controlled polymerizations of acrylates. The same AC electrolysis conditions could successfully polymerize eleven different monomers across different mediums, from water to bulk. Moreover, it proved effective across a spectrum of catalyst activity, from low-activity Cu/2,2-bipyridine to highly active Cu complexes with substituted tripodal amine ligands. Chain extension experiments confirmed the high chain-end fidelity of the produced polymers, yielding functional and high molecular-weight block copolymers. SEM analysis indicated the robustness of the Cu0 electrodes, sustaining at least 15 consecutive polymerizations.

Alternating polarity as a novel strategy for building synthetic microbial communities capable of robust Electro-Methanogenesis

Electro-methanogenic microbial communities can produce biogas with high efficiency and have attracted extensive research interest. In this study an alternating polarity strategy was developed to build electro-methanogenic communities. In two-chamber bioelectrochemical systems amended with activated carbon, the electrode potential was alternated between +0.8 V and -0.4 V vs. standard hydrogen electrode every three days. Cumulative biogas production under alternating polarity increased from 45 L/L/kg-activated carbon after start-up to 125 L/L/kg after the 4th enrichment, significantly higher than that under intermittent cathode (-0.4 V/open circuit), continuous cathode (-0.4 V), and open circuit. The communities assembled under alternating polarity were electroactive and structurally different from those assembled under other conditions. One Methanobacterium population and two Geobacter populations were consistently abundant and active in the communities. Their 16S rRNA was up-regulated by electrode potentials. Bayesian networks inferred close associations between these populations. Overall, electro-methanogenic communities have been successfully assembled with alternating polarity.

Revisiting Alternating Current Electrolysis for Organic Synthesis

This review summarizes the recent advancements in alternating current (AC)-driven electroorganic synthesis since 2021 and discusses the reactivities AC electrolysis provides to achieve new and unique organic transformations.

Pulsed electrolysis: enhancing primary benzylic C(sp3)-H nucleophilic fluorination

Electrosynthesis is an efficient and powerful tool for the generation of elusive reactive intermediates. The application of alternative electrolysis waveforms provides a new level of control for dynamic redox environments. Herein, we demonstrate that pulsed electrolysis provides a favourable environment for the generation and fluorination of highly unstable primary benzylic cations from C(sp3)-H bonds. By introduction of a toff period, we propose this waveform modulates the electrical double layer to improve mass transport and limit over-oxidation.

Comprehensive Comparisons between Directing and Alternating Current Electrolysis in Organic Synthesis

Organic electrosynthesis has consistently aroused significant interest within both academic and industrial spheres. Despite the considerable progress achieved in this field, the majority of electrochemical transformations have been conducted through the utilization of direct-current (DC) electricity. In contrast, the application of alternating current (AC), characterized by its polarity-alternating nature, remains in its infancy within the sphere of organic synthesis, primarily due to the absence of a comprehensive theoretical framework. This minireview offers an overview of recent advancements in AC-driven organic transformations and seeks to elucidate the differences between DC and AC electrolytic methodologies by probing into their underlying physical principles. These differences encompass the ability of AC to preclude the deposition of metal catalysts, the precision in modulating oxidation and reduction intensities, and the mitigation of mass transfer processes.

Chemoselective Quinoline and Isoquinoline Reduction by Energy Transfer Catalysis Enabled Hydrogen Atom Transfer

(Hetero)arene reduction is one of the key avenues for synthesizing related cyclic alkenes and alkanes. While catalytic hydrogenation and Birch reduction are the two broadly utilized approaches for (hetero)arene reduction across academia and industry over the last century, both methods have encountered significant chemoselectivity challenges. We hereby introduce a highly chemoselective quinoline and isoquinoline reduction protocol operating through selective energy transfer (EnT) catalysis, which enables subsequent hydrogen atom transfer (HAT). The design of this protocol bypasses the conventional metric of reduction reaction, that is, the reductive potential, and instead relies on the triplet energies of the chemical moieties and the kinetic barriers of energy and hydrogen atom transfer events. Many reducing labile functional groups, which were incompatible with previous (hetero)arene reduction reactions, are retained in this reaction. We anticipate that this protocol will trigger the further advancement of chemoselective arene reduction and enable the current arene-rich drug space to escape from flatland.

Electrochemical hydrogen isotope exchange of amines controlled by alternating current frequency

Here, we report an electrochemical protocol for hydrogen isotope exchange (HIE) at α-C(sp3)-H amine sites. Tetrahydroisoquinoline and pyrrolidine are selected as two model substrates because of their different proton transfer (PT) and hydrogen atom transfer (HAT) kinetics at the α-C(sp3)-H amine sites, which are utilized to control the HIE reaction outcome at different applied alternating current (AC) frequencies. We found the highest deuterium incorporation for tetrahydroisoquinolines at 0 Hz (i.e., under direct current (DC) electrolysis conditions) and pyrrolidines at 0.5 Hz. Analysis of the product distribution and D isotope incorporation at different frequencies reveals that the HIE of tetrahydroisoquinolines is limited by its slow HAT, whereas the HIE of pyrrolidines is limited by the overoxidation of its α-amino radical intermediates. The AC-frequency-dependent HIE of amines can be potentially used to achieve selective labeling of α-amine sites in one drug molecule, which will significantly impact the pharmaceutical industry.

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.

Scalable Electrochemical Decarboxylative Olefination Driven by Alternating Polarity

A mild, scalable (kg) metal-free electrochemical decarboxylation of alkyl carboxylic acids to olefins is disclosed. Numerous applications are presented wherein this transformation can simplify alkene synthesis and provide alternative synthetic access to valuable olefins from simple carboxylic acid feedstocks. This robust method relies on alternating polarity to maintain the quality of the electrode surface and local pH, providing a deeper understanding of the Hofer-Moest process with unprecedented chemoselectivity.

Overcoming the Potential Window-Limited Functional Group Compatibility by Alternating Current Electrolysis

The functional group compatibility of an electrosynthetic method is typically limited by its potential reaction window. Here, we report that alternating current (AC) electrolysis can overcome such potential window-limited functional group compatibility. Using alkene heterodifunctionalization as a model system, we design and demonstrate a series of AC-driven reactions that add two functional groups sequentially and separately under the cathodic and anodic pulses, including chloro- and bromotrilfuoromethylation as well as chlorosulfonylation. We discovered that the oscillating redox environment during AC electrolysis allows the regeneration of the redox-active functional groups after their oxidation or reduction in the preceding step. As a result, even though redox labile functional groups such as pyrrole, quinone, and aryl thioether fall in the reaction potential window, they are tolerated under AC electrolysis conditions, leading to synthetically useful yields. The cyclic voltammetric study has confirmed that the product yield is limited by the extent of starting material regeneration during the redox cycling. Our findings open a new avenue for improving functional group compatibility in electrosynthesis and show the possibility of predicting the product yield under AC electrolysis from voltammogram features.

Elucidating the Underlying Reactivities of Alternating Current Electrosynthesis by Time-Resolved Mapping of Short-Lived Reactive Intermediates

Alternating current (AC) electrolysis is an emerging field in synthetic chemistry, however its mechanistic studies are challenged by the effective characterization of the elusive intermediate processes. Herein, we develop an operando electrochemical mass spectrometry platform that allows time-resolved mapping of stepwise electrosynthetic reactive intermediates in both direct current and alternating current modes. By dissecting the key intermediate processes of electrochemical functionalization of arylamines, the unique reactivities of AC electrosynthesis, including minimizing the over-oxidation/reduction through the inverse process, and enabling effective reaction of short-lived intermediates generated by oxidation and reduction in paired electrolysis, were evidenced and verified. Notably, the controlled kinetics of reactive N-centered radical intermediates in multistep sequential AC electrosynthesis to minimize the competing reactions was discovered. Overall, this work provides direct evidence for the mechanism of AC electrolysis, and clarifies the underlying reasons for its high efficiency, which will benefit the rational design of AC electrosynthetic reactions.

Steering the Dynamics of Reaction Intermediates and Catalyst Surface during Electrochemical Pulsed CO2 Reduction for Enhanced C2+ Selectivity

Developing alternative electrolysis techniques is crucial for advancing electrocatalysis in addition to tremendous efforts of material developments. Recently, pulse electrochemical CO2 reduction reaction (CO2RR) has demonstrated dramatic selectivity improvement toward multicarbon (C2+) products compared to potentiostatic electrochemical CO2RR, yet the underlying mechanisms remain little understood. Herein, we develop a fast time-resolved in situ Raman spectroscopic method with a time resolution of 0.25 s. We reveal that pulse electrolysis improves the C2+ selectivity of CO2RR through dynamic controls of the surface CuxO/Cu composition that would be unachievable under potentiostatic electrolysis. The population of the surface-adsorbed CO intermediate (COads) is characterized to be the determining factor in controlling reaction selectivity, which depicts the C2+/C1 selectivity of CO2RR under pulse conditions. Meanwhile, the vibrational character of COads, despite transforming dynamically between the low-frequency and high-frequency modes is characterized not to be the key factor in controlling the reaction selectivity. Such an active control of catalyst surface compositions and reaction intermediates enabled by pulse electrolysis offer a general way of regulating the electrocatalysis performance of broad electrochemical reactions beyond CO2RR.

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.

Arylcarboxylation of unactivated alkenes with CO2 via visible-light photoredox catalysis

Photocatalytic carboxylation of alkenes with CO₂ is a promising and sustainable strategy to synthesize high value-added carboxylic acids. However, it is challenging and rarely investigated for unactivated alkenes due to their low reactivities. Herein, we report a visible-light photoredox-catalyzed arylcarboxylation of unactivated alkenes with CO₂, delivering a variety of tetrahydronaphthalen-1-ylacetic acids, indan-1-ylacetic acids, indolin-3-ylacetic acids, chroman-4-ylacetic acids and thiochroman-4-ylacetic acids in moderate-to-good yields. This reaction features high chemo- and regio-selectivities, mild reaction conditions (1 atm, room temperature), broad substrate scope, good functional group compatibility, easy scalability and facile derivatization of products. Mechanistic studies indicate that in situ generation of carbon dioxide radical anion and following radical addition to unactivated alkenes might be involved in the process.

Practical and General Alcohol Deoxygenation Protocol

Herein, we describe a practical protocol for the removal of alcohol functional groups through reductive cleavage of their benzoate ester analogs. This transformation requires a strong single electron transfer (SET) reductant and a means to accelerate slow fragmentation following substrate reduction. To accomplish this, we developed a photocatalytic system that generates a potent reductant from formate salts alongside Brønsted or Lewis acids that promote fragmentation of the reduced intermediate. This deoxygenation procedure is effective across structurally and electronically diverse alcohols and enables a variety of difficult net transformations. This protocol requires no precautions to exclude air or moisture and remains efficient on multigram scale. Finally, the system can be adapted to a one-pot benzoylation-deoxygenation sequence to enable direct alcohol deletion. Mechanistic studies validate that the role of acidic additives is to promote the key C(sp3 )-O bond fragmentation step.

Formation of Isolable Dearomatized [4 + 2] Cycloadducts from Benzenes, Naphthalenes, and N-Heterocycles Using 1,2-Dihydro-1,2,4,5-tetrazine-3,6-diones as Arenophiles under Visible Light Irradiation

We report that the dearomative [4 + 2] cycloaddition between 1,2-dihydro-1,2,4,5-tetrazine-3,6-diones (TETRADs) and benzenes, naphthalenes, or N-heteroaromatic compounds under visible light irradiation affords the corresponding isolable cycloadducts. Several synthetic transformations including transition-metal-catalyzed allylic substitution reactions using the isolated cycloadducts at room temperature or above were demonstrated. Computational studies revealed that the retro-cycloaddition of the benzene-TETRAD adduct proceeds via an asynchronous concerted mechanism, while that of the benzene-MTAD adduct (MTAD = 4-methyl-1,2,4-triazoline-3,5-dione) proceeds via a synchronous mechanism.

Overcoming the limitations of Kolbe coupling with waveform-controlled electrosynthesis

The Kolbe reaction forms carbon-carbon bonds through electrochemical decarboxylative coupling. Despite more than a century of study, the reaction has seen limited applications owing to extremely poor chemoselectivity and reliance on precious metal electrodes. In this work, we present a simple solution to this long-standing challenge: Switching the potential waveform from classical direct current to rapid alternating polarity renders various functional groups compatible and enables the reaction on sustainable carbon-based electrodes (amorphous carbon). This breakthrough enabled access to valuable molecules that range from useful unnatural amino acids to promising polymer building blocks from readily available carboxylic acids, including biomass-derived acids. Preliminary mechanistic studies implicate the role of waveform in modulating the local pH around the electrodes and the crucial role of acetone as an unconventional reaction solvent for Kolbe reaction.

Deuterodehalogenation Under Net Reductive or Redox-Neutral Conditions Enabled by Paired Electrolysis

Interest in deuterated active pharmaceutical ingredients (APIs) is increasing as deuteration holds promise for kinetic isotope effect (KIE) regulated fine-tuning of API performance. Moreover, deuterium isotope labeling is frequently carried out to study organic and bioorganic reaction mechanisms and to facilitate complex target synthesis. As such, methods for highly selective deuteration of organic molecules are highly desirable. Herein, we present an electrochemical method for the selective deuterodehalogenation of benzylic halides via a radical-polar crossover mechanism, using inexpensive deuterium oxide (D2 O) as the deuterium source. We demonstrate broad functional group compatibility across a range of aryl and heteroaryl benzylic halides. Furthermore, we uncover a sequential paired electrolysis regime, which permits switching between net reductive and overall redox-neutral reactions of sulfur-containing substrates simply by changing the identity of the sacrificial reductant employed.

Uncanonical Semireduction of Quinolines and Isoquinolines via Regioselective HAT-Promoted Hydrosilylation

Heterocycles are the backbone of modern medical chemistry and drug development. The derivatization of "an olefin" inside aromatic rings represents an ideal approach to access functionalized saturated heterocycles from abundant aromatic building blocks. Here, we report an operationally simple, efficient, and practical method to selectively access hydrosilylated and reduced N-heterocycles from bicyclic aromatics via a key diradical intermediate. This approach is expected to facilitate complex heterocycle functionalizations that enable access to novel medicinally relevant scaffolds.

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.

Recent advances in the dearomative functionalisation of heteroarenes

Dearomatisation reactions of (hetero)arenes have been widely employed as efficient methods to obtain highly substituted saturated cyclic compounds for over a century. In recent years, research in this area has shifted towards effecting additional C-C bond formation during the overall dearomative process. Moving away from classical hydrogenation-based strategies a wide range of reagents were found to be capable of initiating dearomatisation through nucleophilic addition (typically a reduction) or photochemically induced radical addition. The dearomatisation process gives rise to reactive intermediates which can be intercepted in an intra- or intermolecular fashion to deliver products with significantly increased molecular complexity when compared to simple dearomatisation. In this Perspective recent examples and strategies for the dearomative functionalisation of heteroaromatic systems will be discussed.

Mechanistic and Synthetic Studies of Biaryl Birch Reductions

The Birch reduction of biaryls generally converts one of the two arenes into a cyclohexa-1,4-diene. Biaryls are more reactive than monocyclic arenes under the Birch conditions. Unlike the reduction of monocyclic arenes, biaryl reduction proceeds through two consecutive electron transfer steps before the protonation of the dianion intermediate. The biaryl reductions and subsequent alkylations in one pot rapidly increase the molecular complexity and thus have been used in the synthesis of natural products and drug-like molecules.

1 Introduction

2 The Physical Organic Chemistry of the Birch Reduction of Biaryls

3 Biaryls as the Mediators of Electron Transfer

4 Methods for the Dissolving-Metal Reduction of Biaryls

5 Intercepting the Biaryl Reduction Intermediates with Electrophiles

6 Synthetic Applications of the Dissolving-Metal-Mediated Reductions of Biaryls

7 Outlook

Electrochemical-Promoted Nickel-Catalyzed Reductive Allylation of Aryl Halides

Compared with conventional reductive coupling, reductive coupling under electrochemical conditions without external reductants is greener, milder, and more efficient and is of increasing interest to organic chemists. In this work, we report the sacrificial anode, nickel-catalyzed electrochemical allylation reaction of aryl and alkyl halides. The reaction can be applied to a range of allylation reagents such as trifluoroalkenes, oxalates, and acetates.