Efficient Electrocatalysis for the Preparation of (Hetero)aryl Chlorides and Vinyl Chloride with 1,2-Dichloroethane

Late-Stage Halogenation of Complex Substrates with Readily Available Halogenating Reagents

ConspectusLate-stage halogenation, targeting specific positions in complex substrates, has gained significant attention due to its potential for diversifying and functionalizing complex molecules such as natural products and pharmaceutical intermediates. Utilizing readily available halogenating reagents, such as hydrogen halides (HX), N-halosuccinimides (NXS), and dichloroethane (DCE) reagents for late-stage halogenation shows great promise for expanding the toolbox of synthetic chemists. However, the reactivity of haleniums (X+, X = Cl, Br, I) can be significantly hindered by the presence of various functional groups such as hydroxyl, amine, amide, or carboxylic acid groups. The developed methods of late-stage halogenation often rely on specialized activating reagents and conditions. Recently, our group (among others) has put great efforts into addressing these challenges and unlocking the potential of these readily available HX, NXS, and DCE reagents in complex molecule halogenation. Developing new methodologies, catalyst systems, and reaction conditions further enhanced their utility, enabling the efficient and selective halogenation of intricate substrates.With the long-term goal of achieving selective halogenation of complex molecules, we summarize herein three complementary research topics in our group: (1) Efficient oxidative halogenations: Taking inspiration from naturally occurring enzyme-catalyzed oxidative halogenation reactions, we focused on developing cost-effective oxidative halogenation reactions. We found the combination of dimethyl sulfoxide (DMSO) and HX (X = Cl, Br, I) efficient for the oxidative halogenation of aromatic compounds and alkenes. Additionally, we developed electrochemical oxidative halogenation using DCE as a practical chlorinating reagent for chlorination of (hetero)arenes. (2) Halenium reagent activation: Direct electrophilic halogenation using halenium reagents is a reliable method for obtaining organohalides. However, compared to highly reactive reagents, the common and readily available NXS and dihalodimethylhydantoin (DXDMH) demonstrate relatively lower reactivity. Therefore, we focused on developing oxygen-centered Lewis base catalysts such as DMSO, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and nitromethane to activate NXS or DXDMH, enabling selective halogenation of bioactive substrates. (3) Halogenation of inert substrates: Some substrates, such as electron-poor arenes and pyridines, are inert toward electrophilic functionalization reactions. We devised several strategies to enhance the reactivity of these molecules. These strategies, characterized by mild reaction conditions, the ready availability and stability of catalysts and reagents, and excellent tolerance for various functional groups, have emerged as versatile protocols for the late-stage aromatic halogenation of drugs, natural products, and peptides. By harnessing the versatility and selectivity of these catalysts and methodologies, synthetic chemists can unlock new possibilities in the synthesis of halogenated compounds, paving the way for the development of novel functional materials and biologically active molecules.

Repurposing of halogenated organic pollutants via alkyl bromide-catalysed transfer chlorination

Halogenated organic pollutants (HOPs) are causing a significant environmental and human health crisis due to their high levels of toxicity, persistence and bioaccumulation. Urgent action is required to develop effective approaches for the reduction and reuse of HOPs. Whereas current strategies focus primarily on the degradation of HOPs, repurposing them is an alternative approach, albeit a challenging task. Here we discover that alkyl bromide can act as a catalyst for the transfer of chlorine using alkyl chloride as the chlorine source. We demonstrate that this approach has a wide substrate scope, and we successfully apply it to reuse HOPs that include dichlorodiphenyltrichloroethane, hexabromocyclododecane, chlorinated paraffins, chloromethyl polystyrene and poly(vinyl chloride) (PVC). Moreover, we show that the synthesis of essential non-steroidal anti-inflammatory drugs can be achieved using PVC and hexabromocyclododecane, and we demonstrate that PVC waste can be used directly as a chlorinating agent. Overall, this methodology offers a promising strategy for repurposing HOPs.

HCl/DMSO/HFIP-Mediated Chlorination of Pyrrolo[2,1-a]isoquinolines and Other Electron-Rich Heteroarenes

An efficient oxidative chlorination of pyrrolo[2,1-a]isoquinolines has been established using HCl (aq) as the chlorine source and DMSO as the terminal oxidant in HFIP at ambient temperature. A variety of chlorinated pyrrolo[2,1-a]isoquinoline derivatives have been prepared readily in 23 to 99% yields. This chlorination strategy can be expanded to the functionalization of other electron-rich heteroarenes including substituted pyrroles, indoles, and naphthols.

Radical meta-C-H Halogenation of Azines via N-Benzyl Activation Strategy

Regioselective halogenation of six-membered N-heteroarenes is crucial for precise functional derivatization. We present a meta-selective halogenation method for pyridines, quinolines, and isoquinolines via electrophilic halogen radical addition utilizing an N-benzyl activation strategy. This method achieves C3- and C5-dihalogenation in pyridines, C3- and C6-dihalogenation in quinolines, and C3-monohalogenation in isoquinolines. The feasibility and potential applications of this method were validated through scale-up reactions and the bromination of quinoline derivatives with biomolecular fragments.

Switchable Site-Selective Benzanilide C(sp2)-H Bromination via Promoter Regulation

Regioselective benzanilide bromination that generates either regioisomer from the same starting material is desirable. Herein, we develop switchable site-selective C(sp2)-H bromination by promoter regulation. This protocol leads to regiodivergent brominated benzanilide starting from the single substrate via selection of promoters. The protocol demonstrates excellent regioselectivity and good tolerance of functional groups with high yields. The utility effectiveness of this method has been well exemplified in the late-stage modification of biologically important molecules.

Bioremediation of trichloroethylene-contaminated groundwater using green carbon-releasing substrate with pH control capability

In this research, a sustainable substrate, termed green and long-lasting substrate (GLS), featuring a blend of emulsified substrate (ES) and modified rice husk ash (m-RHA) was devised. The primary objective was to facilitate the bioremediation of groundwater contaminated with trichloroethylene (TCE) using innovative GLS for slow carbon release and pH control. The GLS was concocted by homogenizing a mixture of soybean oil, surfactants (Simple Green™ and soya lecithin), and m-RHA, ensuring a gradual release of carbon sources. The hydrothermal synthesis was applied for the production of m-RHA production. The analyses demonstrate that m-RHA were uniform sphere-shape granules with diameters in micro-scale ranges. Results from the microcosm study show that approximately 83% of TCE could be removed (initial TCE concentration = 7.6 mg/L) with GLS supplement after 60 days of operation. Compared to other substrates without RHA addition, higher TCE removal efficiency was obtained, and higher Dehalococcoides sp. (DHC) population and hydA gene (hydrogen-producing gene) copy number were also detected in microcosms with GLS addition. Higher hydrogen concentrations enhanced the DHC growth, which corresponded to the increased DHC populations. The addition of the GLS could provide alkalinity at the initial stage to neutralize the acidified groundwater caused by the produced organic acids after substrate biodegradation, which was advantageous to DHC growth and TCE dechlorination. The addition of m-RHA reached an increased TCE removal efficiency, which was due to the fact that the m-RHA had the zeolite-like structure with a higher surface area and lower granular diameter, and thus, it resulted in a more effective initial adsorption effect. Therefore, a significant amount of TCE could be adsorbed onto the surface of m-RHA, which caused a rapid TCE removal through adsorption. The carbon substrates released from m-RHA could then enhance the subsequent dechlorination. The developed GLS is an environmentally-friendly and green substrate.

Phase-Transfer Catalysis for Electrochemical Chlorination and Nitration of Arenes

A biphasic anodic oxidation method for aromatic halogenation process was developed, where aqueous metal salts were directly used as halogen source. Ammonium salts serve as both electrolytes and phase transfer catalysis to facilitate anion transport and oxidative transformation. This design allows for chlorination or nitration of multiple types of arenes using NaCl or KNO2.

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

A controlled non-radical chlorine activation pathway on hematite photoanodes for efficient oxidative chlorination reactions

Photo(electro)catalytic chlorine oxidation has emerged as a useful method for chemical transformation and environmental remediation. However, the reaction selectivity usually remains low due to the high activity and non-selectivity characteristics of free chlorine radicals. In this study, we report a photoelectrochemical (PEC) strategy for achieving controlled non-radical chlorine activation on hematite (α-Fe2O3) photoanodes. High selectivity (up to 99%) and faradaic efficiency (up to 90%) are achieved for the chlorination of a wide range of aromatic compounds and alkenes by using NaCl as the chlorine source, which is distinct from conventional TiO2 photoanodes. A comprehensive PEC study verifies a non-radical "Cl+" formation pathway, which is facilitated by the accumulation of surface-trapped holes on α-Fe2O3 surfaces. The new understanding of the non-radical Cl- activation by semiconductor photoelectrochemistry is expected to provide guidance for conducting selective chlorine atom transfer reactions.

Recent advances in oxidative chlorination

Considering the wide occurrence and extensive application of organic chlorides in many research fields, the development of easy, practical and green chlorination methodologies is much needed. In the oxidative chlorination strategy, active chlorinating species can be in situ formed by the interaction of easily accessible chlorides such as NaCl, HCl, KCl, CHCl3, etc. and suitable oxidants. Among the established chlorination approaches, this strategy is an attractive one as it features the use of readily available, cheap and safe inorganic or organic chlorides, good atom economy of chlorine, and multiple choices of oxidants. This review summarizes the representative methodologies in the field of oxidative chlorination, covering 2013 to 2023.

DMSO/SOCl2-Enabled Synthesis of 3-Chloroindoles via Desulfonylative Chlorocyclization of N,N-Disubstituted 2-Alkynylanilines

The application of the DMSO/SOCl2 system enabled the intramolecular cyclization/chlorination of N,N-disubstituted 2-alkynylanilines, leading to the synthesis of a series of 3-chloroindoles with moderate to good yields. Differing from the previously reported interrupted Pummerer reaction featuring the introduction of SMe moiety, the current approach adopted an alternative pathway that realized the incorporation of chlorine atom to the indole skeleton via a desulfonylative chlorocyclization process.

Rapid and Practical Synthesis of gem-Dibromoalkanes from Aldehydes by Tribromide Reagent

gem-Dibromoalkanes are important synthetic building block in organic chemistry, but their preparation is still troublesome. Herein, we have developed a simple and practical protocol for the synthesis of gem-dibromoalkanes from aldehydes using tetrabutylammonium tribromide and triphenyl phosphite. A variety of alkyl and aromatic aldehydes can be transformed into the corresponding products within 10 minutes. This protocol is also applicable to alcohols, and the configuration of chiral alcohol is inverted during the process with excellent enantiopurity.

Aromatic C(sp2 )-H Functionalization by Consecutive Paired Electrolysis: Dibromination of Aryl Amines with Dibromoethane at Room Temperature

Herein, we disclose a facile and efficient electrochemical method for the dibromination of aryl amines by double functionalization of aromatic C(sp2 )-H (both para and ortho) under metal- and external oxidant-free conditions at room temperature for the first time. The reaction is demonstrated using 1,2-dibromoethane to dibrominate a wide range of N-substituted aryl amines in a simple setup with C(+)/Pt(-) electrodes under mild reaction conditions. This transformation proceeds smoothly with a broad substrate scope affording the valuable and versatile N-substituted 2,4-dibromoanilines in moderate to excellent yields with high regioselectivity. In this paired electrolysis, cathodic reduction of 1,2-DBE followed by anodic oxidation generates bromonium intermediates, which then couple with anilines to furnish the dibrominated products. It represents a distinctive approach to challenging redox-neutral reactions. The versatility of the electrochemical ortho-, para-dibromination was reflected by unique regioselectivities for challenging aryl amines and gram-scale electrosynthesis without the use of a stoichiometric oxidant or an activating agent.

Transition-Metal-Free Electrochemical Selenylative Cyclization of Alkynyl Phosphonates

Unprecedented regioselective electrochemical tandem selenation/cyclization of alkynyl phosphonates with diselenide is described here. These obtained selenoether products can be chemo-selectively converted into halogen-functionalized cyclic enol phosphonates under our electrochemical conditions. These protocols provide straightforward access to valuable cyclic enol phosphonate or phosphaisocoumarins under the electrochemical and transition-metal-free conditions. The robustness of these transformations was illustrated by their compatibility with various complex natural products and bioactive molecules. The selenoether and halogen functional groups allow the further diversification of the phosphorus heterocycles thus obtained.

Visible-light-induced halocyclization of 2-alkynylthioanisoles with simple alkyl halides towards 3-halobenzo[b]thiophenes without an external photocatalyst

A new strategy for the preparation of 3-halobenzo[b]thiophenes via a photo-driven halocyclization/demethylation of 2-alkynylthioanisoles with simple alkyl halides was developed. The reaction can proceed smoothly at room temperature under visible-light irradiation without any external photocatalyst, and the protocol has a range of advantages, including simplicity and mildness of the reaction conditions, good functional-group tolerance, and excellent yields of the products.

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.

Preferential Adsorption of Ethylene Oxide on Fe and Chlorine on Ni Enabled Scalable Electrosynthesis of Ethylene Chlorohydrin

Industrial manufacturing of ethylene chlorohydrin (ECH) critically requires excess corrosive hydrochloric acid or hypochlorous acid with dealing with massive by-products and wastes. Here we report a green and efficient electrosynthesis of ECH from ethylene oxide (EO) with NaCl over a NiFe₂ O₄ nanosheet anode. Theoretical results suggest that EO and Cl preferentially adsorb on Fe and Ni sites, respectively, collaboratively promoting the ECH synthesis. A Cl radical-mediated ring-opening process is proposed and confirmed, and the key Cl and carbon radical species are identified by high-resolution mass spectrometry. This strategy can enable scalable electrosynthesis of 185.1 mmol of ECH in 1 h with 92.5 % yield at a 55 mA cm^-2 current density. Furthermore, a series of other chloro- and bromoethanols with good to high yields and paired synthesis of ECH and 4-amino-3,6-dichloropyridine-2-carboxylicacid via respectively loading and unloading Cl are achieved, showing the promising potential of this strategy.

Copper-catalyzed selective C5-H bromination and difluoromethylation of 8-aminoquinoline amides using ethyl bromodifluoroacetate as the bifunctional reagent

A simple and effective method for the copper-catalyzed selective C5-H bromination and difluoromethylation of 8-aminoquinoline amides with ethyl bromodifluoroacetate as the bifunctional reagent was developed. The combination of cupric catalyst and alkaline additive results in a C5-bromination reaction, whereas cuprous catalyst combined with silver additive results in the C5-difluoromethylation reaction. This method has a broad substrate scope and allows for easy and convenient access to desired C5-functionalized quinolones with good to excellent yields.

Cubic CuFe2O4 Spinel with Octahedral Fe Active Sites for Electrochemical Dechlorination of 1,2-Dichloroethane

CuFe₂O₄ spinel has been considered as a promising catalyst for the electrochemical reaction, while the nature of the crystal phase on its intrinsic activity and the kind of active site need to be further explored. Herein, the crystal phase-dependent catalytic behavior and the main active sites of CuFe₂O₄ spinel for electrochemical dechlorination of 1,2-dichloroethane are carefully studied based on the combination of experiments and theoretical calculations. Cubic and tetragonal CuFe₂O₄ are successfully prepared by a facile sol-gel method combined with high temperature calcination. Impressively, CuFe₂O₄ with the cubic phase shows a higher activity and ethylene selectivity compared to CuFe₂O₄ with the tetragonal phase, suggesting a significant facilitation of electrocatalytic performance by the cubic crystal structure. Moreover, the octahedral Fe atom on the surface of cubic CuFe₂O₄(311) is the active site responsible to produce ethylene with the energy barrier of 0.40 eV. This work demonstrates the significance of crystal phase engineering for the optimization of electrocatalytic performance and offers an efficient strategy for the development of advanced electrocatalysts.

Using waste poly(vinyl chloride) to synthesize chloroarenes by plasticizer-mediated electro(de)chlorination

New approaches are needed to both reduce and reuse plastic waste. In this context, poly(vinyl chloride) (PVC) is an appealing target as it is the least recycled high-production-volume polymer due to its facile release of plasticizers and corrosive HCl gas. Herein, these limitations become advantageous in a paired-electrolysis reaction in which HCl is intentionally generated from PVC to chlorinate arenes in an air- and moisture-tolerant process that is mediated by the plasticizer. The reaction proceeds efficiently with other plastic waste present and a commercial plasticized PVC product (laboratory tubing) can be used directly. A simplified life-cycle assessment reveals that using PVC waste as the chlorine source in the paired-electrolysis reaction has a lower global warming potential than HCl. Overall, this method should inspire other strategies for repurposing waste PVC and related polymers using electrosynthetic reactions, including those that take advantage of existing polymer additives.

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.

Catalyst-free electrosynthesis of benzothiophenes from 2-alkenylaryl disulfides

The synthesis of benzothiophenes through electrosynthesis under oxidant- and metal-free conditions has been discovered. Electrolysis of symmetrical 2-alkenylaryl disulfides using an undivided cell leads to the formation of the corresponding benzothiophenes in good to moderate yields with good functional group tolerance. The usefulness of this methodology was further investigated with a scale-up experiment, which delivered a similar result to that of the small scale reaction. Several mechanistic investigations including DFT calculations were carried out to elucidate the reaction mechanism.

Recent advances in metal catalyst- and oxidant-free electrochemical C-H bond functionalization of nitrogen-containing heterocycles

The C-H functionalization of nitrogen-containing heterocycles has emerged as a powerful strategy for the construction of carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds. In order to achieve efficient and selective C-H functionalization, electrochemical synthesis has attracted increasing attention. Because electrochemical anodic oxidation is ideal for replacing chemical reagents in C-H functionalization reactions. This mini-review summarizes the current knowledge and recent advances since 2017 in the synthetic utility of electrochemical transformations for the C-H functionalization of nitrogen-containing heterocycles.

Photo‐Induced Halogen‐Atom Transfer: Generation of Halide Radicals for Selective Hydrohalogenation Reactions

The first photo‐mediated process enabling the generation of halide radicals by Halogen‐Atom Transfer (XAT) is described. Contrary to radical transformations involving XAT reactivity, which exploit stable carbon radicals, this unique approach uses 1,2‐dihaloethanes for the generation of unstable carbon radicals by XAT. These transient radicals then undergo β‐scission with release of ethylene and formation of more stable halide radicals which have been used in selective hydrohalogenations of a large number of unsaturated hydrocarbons, including Michael acceptors, unactivated alkenes and alkynes. This hydrohalogenation is tolerant of a broad range of functionalities and is believed to proceed through a radical‐chain manifold that propagates by the use of silane derivatives.

Nanoscale zerovalent copper (nZVC) catalyzed environmental remediation of organic and inorganic contaminants: A review

Over the past decade, the nano zerovalent copper has emerged as an effective nano-catalyst for the environment remediation processes due to its ease of synthesis, low cost, controllable particle size and high reactivity despite its release during the remediation process and related concentration dependent toxicities. However, the improvised techniques involving the use of supports or immobilizer for the synthesis of Cu⁰ has significantly increased its stability and motivated the researchers to explore the applicability of Cu⁰ for the environment remediation processes, which is evident from access to numerous reports on nano zerovalent copper mediated remediation of contaminants. Initially, this review allows the understanding of the various resources used to synthesize zerovalent copper nanomaterial and the structure of Cu⁰ nanoparticles, followed by focus on the reaction mechanism and the species involved in the contaminant remediation process. The studies comprehensively presented the application of nano zerovalent copper for remediation of organic/inorganic contaminants in combination with various oxidizing and reducing agents under oxic and anoxic conditions. Further, it was evaluated that the immobilizers or support combined with various irradiation sources originates a synergistic effect and have a significant effect on the stability and the redox properties of nZVC in the remediation process. Therefore, the review proposed that the future scope of research should include rigorous focus on deriving an exact mechanism for synergistic effect for the removal of contaminants by supported nZVC.

Electrochemical collective synthesis of labeled pyrroloindoline alkaloids with Freon-type methanes as functional C1 synthons

Utilization of Freon-type methanes as functional one-carbon synthons in the synthesis of various deuterated indoline alkaloids was demonstrated here. A series of halomethyl radicals were generated from electro-reductive C-X cleavage of Freon-type methanes and captured efficiently by acrylamides to provide various halogenated oxindoles via radical cyclization. This reaction features good functional group tolerance, and deuterium and fluorine atoms could be introduced facilely from Freon-type methanes. Further transformation of halogenated oxindoles enabled the synthesis of many (labeled) bioactive drug molecules and skeletons, such as deuterated (±)-physostigmine, deuterated (±)-esermethole and deuterated (±)-lansai B.

Electrosynthesis of (hetero)aryl nitriles from α-imino-oxy acids via oxidative decarboxylation/N-O cleavage

A new method for the synthesis of (hetero)aryl nitriles via iminyl radicals has been developed through the electrochemical oxidative decarboxylation of α-imino-oxy acids. This protocol provides an efficient approach to nitriles with a broad range of functional-group tolerance under ambient conditions and can be applied for one-pot gram-scale synthesis.

Distal Ruthenaelectro-Catalyzed meta-C-H Bromination with Aqueous HBr

While electrochemical ortho-selective C-H activations are well established, distal C-H activations continue to be underdeveloped. In contrast, we herein describe the electrochemical meta-C-H functionalization. The remote C-H bromination was accomplished in an undivided cell by RuCl3 ⋅3 H2 O with aqueous HBr. The electrohalogenation proceeded under exogenous ligand- and electrolyte-free conditions. Notably, pyrazolylarenes were meta-selectively brominated at the benzenoid moiety, rather than on the electron-rich pyrazole ring for the first time. Mechanistic studies were suggestive of an initial ruthenacycle formation, and a subsequent ligand-to-ligand hydrogen transfer (LLHT) process to liberate the brominated product.

Recent Progresses in the Preparation of Chlorinated Molecules: Electrocatalysis and Photoredox Catalysis in the Spotlight

Among halogenated molecules, those containing chlorine atoms are fundamental in many areas such as pharmaceuticals, polymers, agrochemicals and natural metabolites. Despite the fact that many reactions have been developed to install chlorine on organic molecules, most of them rely on toxic and hazardous chlorinating reagents as well as harsh conditions. In an attempt to move towards more sustainable approaches, photoredox catalysis and electrocatalysis have emerged as powerful alternatives to traditional methods. In this review, we collect the most recent and significant examples of visible-light- or current-mediated chlorination published in the last five years.

Ruthenaelectro-catalyzed C-H acyloxylation for late-stage tyrosine and oligopeptide diversification

Ruthenaelectro(ii/iv)-catalyzed intermolecular C-H acyloxylations of phenols have been developed by guidance of experimental, CV and computational insights. The use of electricity bypassed the need for stoichiometric chemical oxidants. The sustainable electrocatalysis strategy was characterized by ample scope, and its unique robustness enabled the late-stage C-H diversification of tyrosine-derived peptides.

Ionic Reactivity of 2-Isocyanoaryl Thioethers: Access to 2-Halo and 2-Aminobenzothia/Selenazoles

An ionic cascade insertion/cyclization reaction of thia-/selena-functionalized arylisocyanides has been successfully developed for the efficient and practical synthesis of 2-halobenzothiazole/benzoselenazole derivatives. This synthetic protocol, incorporating a halogen atom when forming the five-membered ring of benzothia/selenazoles, is different from the existing ones, where halogenation of the preformed benzothia/selenazole precursors happens. Additionally, a facile access to 2-aminobenzothiazoles is also achieved by the one-pot cascade reaction of 2-isocyanoaryl thioethers, iodine, and amines.

Electro-oxidation induced O–S cross-coupling of quinoxalinones with sodium sulfinates for synthesizing 2-sulfonyloxylated quinoxalines

The novel C2–O sulfonylation of quinoxalinones via electro-oxidation induced O–S coupling strategy under mild conditions was reported.

Electrochemical dual-oxidation strategy enables access to α-chlorosulfoxides from sulfides

Electrochemistry contributes a strong tool for the manufacture of molecules, addressing intractable challenges in synthetic chemistry by enabling innovative reaction pathways. Herein, a bifunctional reagent, aqueous hydrochloric acid, is used to establish an electrochemical selective dual-oxidation approach that gives access to α-chlorosulfoxides from sulfides. This strategy presents broad substrate scope, high diastereoselectivity, and regioselectivity. The late-stage modification of amino acids and pharmaceutical derivatives further highlights the utility. Furthermore, detailed mechanistic studies reveal that the key success for this selective chemical transformation is the dual-oxidation process at the anode. This electrochemical dual-oxidation strategy may have wide universality; we anticipate diverse applications of this protocol across the many fields of chemistry.

Selective C3–H nitration of 2-sulfanilamidopyridines with tert-butyl nitrite

A selective C3–H nitration of bioactive 2-sulfanilamidopyridine derivatives, including corticosteroid 11-β-dehydrogenase isozyme, secretory phospholipase A2 inhibitor and human neutrophil elastase inhibitor, has been reported.

Carbene catalyzed C(sp3)–Cl activation of chlorinated solvents for benzyne chlorination

A new mode of N-heterocyclic carbene (NHC) organocatalysis was discovered, in which the normally inert chlorinated solvents were activated by carbene to realize the chlorination of hexa-dehydro-Diels-Alder derived benzynes.

Electrocatalytic three-component synthesis of 4-halopyrazoles with sodium halide as the halogen source

The first example of the electrocatalytic multicomponent synthesis of 4-chloro/bromo/iodopyrazoles from hydrazines, acetylacetones and sodium halides under chemical oxidant- and external electrolyte-free conditions has been developed.

Electrochemical Desaturative β-Acylation of Cyclic N-Aryl Amines

Herein, we disclose a straightforward, robust, and simple route to access β-substituted desaturated cyclic amines via an electrochemically driven desaturative β-functionalization of cyclic amines. This transformation is based on multiple single-electron oxidation processes using catalytic amounts of ferrocene. The reaction proceeds in the absence of stoichiometric amounts of electrolyte under mild conditions, affording the desired products with high chemo- and regioselectivity. The reaction was tolerant of a broad range of substrates and also enables late-stage β-C(sp³ )-H acylation of potentially valuable products. Preliminary mechanistic studies using cyclic voltammetry reveal the key role of ferrocene as a redox mediator in the reaction.

Electrochemical oxidative Z-selective C(sp2)-H chlorination of acrylamides

An electrochemical method for the oxidative Z-selective C(sp²)-H chlorination of acrylamides has been developed. This catalyst and organic oxidant free method is applicable across various substituted tertiary acrylamides, and provides access to a broad range of synthetically useful Z-β-chloroacrylamides in good yields (22 examples, 73% average yield). The orthogonal derivatization of the products was demonstrated through chemoselective transformations and the electrochemical process was performed on gram scale in flow.

Iminyl-radicals by electrochemical decarboxylation of α-imino-oxy acids: construction of indole-fused polycyclics

Iminyl radicals are reactive intermediates that can be used for the construction of various valuable heterocycles. Herein, the electrochemical decarboxylation of α-imino-oxy acids for the generation of iminyl radicals has been accomplished under exogenous-oxidant- and metal-free conditions through the use of nBu₄NBr as a mediator. The resulting iminyl radicals undergo intramolecular cyclization smoothly with the adjacent (hetero)arenes to afford a series of indole-fused polycyclic compounds.

Ozone-Activated Cataluminescence Sensor System for Dichloroalkanes Based on Silica Nanospheres

The detection and monitoring of dichloroalkanes, which are typical chlorinated volatile organic compounds (CVOCs) with obvious biological toxicity, is of significance for environmental pollution and public health. Herein, a novel ozone-activated cataluminescence (CTL) sensor system based on silica nanospheres was developed for highly sensitive and fast quantification of dichloroalkanes. A typical CTL system coupled with a plasma-ozone-assist unit was designed for promoting the CTL response of dichloroalkanes. The ozone generated by plasma provides a new pathway of catalytic oxidation process, which accompanied by the CTL signal amplification of dichloroalkanes results in an enhanced CTL sensor system with improved limit of detection (1,2-dichloroethane: 0.04 μg mL^-1, 1,2-dichloropropane: 0.03 μg mL^-1) and benign selective performance under the interference of CO₂, H₂O, NO, NO₂, SO₂, CS₂, and other common CVOCs. Moreover, a segmented CTL mechanism including co-adsorption of ozone and dichloroalkanes, thermal elimination, the ozonation route, and a luminous step was ratiocinated based on multiple characterizations and discussion. The proposed methodology and theory open up an attractive perspective for the analysis of less active volatile organic compounds.

Electrochemical C-H Halogenations of Enaminones and Electron-Rich Arenes with Sodium Halide (NaX) as Halogen Source for the Synthesis of 3-Halochromones and Haloarenes

Without employing an external oxidant, the simple synthesis of 3-halochromones and various halogenated electron-rich arenes has been realized with electrode oxidation by employing the simplest sodium halide (NaX, X = Cl, Br, I) as halogen source. This electrochemical method is advantageous for the simple and mild room temperature operation, environmental friendliness as well as broad substrate scope in both C-H bond donor and halogen source components.

Electrooxidative dearomatization of biaryls: synthesis of tri- and difluoromethylated spiro[5.5]trienones

Radical spirocyclization via dearomatization has emerged as an attractive strategy for the rapid synthesis of structurally diverse spiro molecules. We report the use of electrochemistry to perform an oxidative dearomatization of biaryls leading to tri- and difluoromethylated spiro[5.5]trienones in a user friendly undivided cell set-up and a constant current mode. The catalyst- and chemical oxidant-free dearomatization procedure features ample scope, and employs electricity as the green and sole oxidant.

Metalloenzyme-Inspired Ce-MOF Catalyst for Oxidative Halogenation Reactions

The structure of UiO-66(Ce) is formed by CeO_2-x defective nanoclusters connected by terephthalate ligands. The initial presence of accessible Ce³⁺ sites in the as-synthesized UiO-66(Ce) has been determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR)-CO analyses. Moreover, linear scan voltammetric measurements reveal a reversible Ce⁴⁺/Ce³⁺ interconversion within the UiO-66(Ce) material, while nanocrystalline ceria shows an irreversible voltammetric response. This suggests that terephthalic acid ligands facilitate charge transfer between subnanometric metallic nodes, explaining the higher oxidase-like activity of UiO-66(Ce) compared to nanoceria for the mild oxidation of organic dyes under aerobic conditions. Based on these results, we propose the use of Ce-based metal-organic frameworks (MOFs) as efficient catalysts for the halogenation of activated arenes, as 1,3,5-trimethoxybenzene (TMB), using oxygen as a green oxidant. Kinetic studies demonstrate that UiO-66(Ce) is at least three times more active than nanoceria under the same reaction conditions. In addition, the UiO-66(Ce) catalyst shows an excellent stability and can be reused after proper washing treatments. Finally, a general mechanism for the oxidative halogenation reaction is proposed when using Ce-MOF as a catalyst, which mimics the mechanistic pathway described for metalloenzymes. The superb control in the generation of subnanometric CeO_2-x defective clusters connected by adequate organic ligands in MOFs offers exciting opportunities in the design of Ce-based redox catalysts.