Highlyortho-Selective Chlorination of Anilines Using a Secondary Ammonium Salt Organocatalyst

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.

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.

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.

Recent Advances in Direct Regioselective C-H Chlorination at Aromatic and Aliphatic

Direct installation of key functionalities in a molecule through C-H bond activation is one of the thrust areas as well as challenging task in organic synthesis. Particularly, introduction of chlorine in a molecule imparts additional benefits for further functionalizations as well as improves the electronic behaviour such as lipophilicity and polarity towards drug development process. The chlorinated molecules have also been established as efficient biologically relevant scaffolds. Current manuscript has been focused on the direct installation of the chlorine atom at various aromatic and aliphatic positions to produce functional molecules. The key highlight of the manuscript belongs to the site selectivity (regioselectivity) for the installation of chlorine functionality. Manuscript describes the advanced methods developed for the direct C-H chlorination reactions and further simplified for the chlorination reactions at various positions including aromatic (o-, m-, and p-), benzylic, heteroaromatic, and aliphatic positions. Directing groups (DGs) and the coordination with the catalyst is the key for the enhancement of regioselectivities during direct C-H chlorination reactions.

Intramolecular chaperone-assisted dual-anchoring activation (ICDA): a suitable preorganization for electrophilic halocyclization

The halocyclization reaction represents one of the most common methodologies for the synthesis of heterocyclic molecules. Many efforts have been made to balance the relationship between structure, reactivity and selectivity, including the design of new electrophilic halogenation reagents and the utilization of activating strategies. However, discovering universal reagents or activating strategies for electrophilic halocyclization remains challenging due to the case-by-case practice for different substrates or different cyclization models. Here we report an intramolecular chaperone-assisted dual-anchoring activation (ICDA) model for electrophilic halocyclization, taking advantage of the non-covalent dual-anchoring orientation as the driving force. This protocol allows a practical, catalyst-free and rapid approach to access seven types of small-sized, medium-sized, and large-sized heterocyclic units and to realize polyene-like domino halocyclizations, as exemplified by nearly 90 examples, including a risk-reducing flow protocol for gram-scale synthesis. DFT studies verify the crucial role of ICDA in affording a suitable preorganization for transition state stabilization and X+ transfer acceleration. The utilization of the ICDA model allows a spatiotemporal adjustment to straightforwardly obtain fast, selective and high-yielding synthetic transformations.

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.

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.

Mild Synthesis of Polychlorinated Arenes for Efficient Organic Light-emitting Diodes with Dual Thermally Activated Delayed Fluorescence

Polychlorinated (hetero)arenes have shown great promise for organic optoelectronics applications. However, the harsh synthetic routes for polychlorinated compounds and the possible luminescence quenching from the compact intermolecular π-π stacking induced by chlorine atoms limit their investigations and applications in luminescent materials. Herein, two isomeric polychlorinated polycyclic aromatic hydrocarbon (PAH) compounds JY-1-Cl and JY-2-Cl consisting of rigidified aryl ketones and amine are designed and synthesized under mild conditions through nucleophilic chlorination intermediated by an electron donor-acceptor complex. Among them, as a result of the strong π-π interactions induced by chlorine atoms, JY-2-Cl exhibits bright monomer and dimer emissions with dual thermally activated delayed fluorescence (TADF) characters. Notably, compared with the non-chlorinated compounds, a high photoluminescence quantum yield is maintained after introducing multiple chlorine atoms into JY-2-Cl. The first dual-TADF organic light-emitting diodes are also successfully fabricated with maximum external quantum efficiency as high as 29.1 % by employing JY-2-Cl as emitter. This work presents a new paradigm and synthesis of polychlorinated amine-carbonyl PAHs and demonstrates the great potential of the chlorinated materials for luminescent applications.

LiBF4-Promoted Aromatic Fluorodetriazenation under Mild Conditions

An efficient and mild fluorination method through LiBF4-promoted aromatic fluorodetriazenation of 3,3-dimethyl-1-aryltriazenes is developed. The reaction proceeds smoothly and tends to complete within 2 h in the absence of a protic acid or strong Lewis acid. This method tolerates a wide range of functional groups and affords the aryl fluoride products in moderate to excellent yields.

POCl3/Sulfoxide and AcCl/Sulfoxide Mediated Chlorination of Pyrrolo[2,1-a]isoquinolines

We have developed an efficient chlorination of pyrrolo[2,1-a]isoquinoline derivatives using POCl3 as the chlorine source and tetramethylene sulfoxide as a promoter. A series of pyrrolo[2,1-a]isoquinolines, polysubstituted pyrroles, and naphthols have been readily chlorinated under mild reaction conditions (26 examples, up to >99% yield). AcCl can also act as the chlorine source competently in this chlorination reaction.

"Magic Chloro": Profound Effects of the Chlorine Atom in Drug Discovery

Chlorine is one of the most common atoms present in small-molecule drugs beyond carbon, hydrogen, nitrogen, and oxygen. There are currently more than 250 FDA-approved chlorine-containing drugs, yet the beneficial effect of the chloro substituent has not yet been reviewed. The seemingly simple substitution of a hydrogen atom (R = H) with a chlorine atom (R = Cl) can result in remarkable improvements in potency of up to 100,000-fold and can lead to profound effects on pharmacokinetic parameters including clearance, half-life, and drug exposure in vivo. Following the literature terminology of the "magic methyl effect" in drugs, the term "magic chloro effect" has been coined herein. Although reports of 500-fold or 1000-fold potency improvements are often serendipitous discoveries that can be considered "magical" rather than planned, hypotheses made to explain the magic chloro effect can lead to lessons that accelerate the cycle of drug discovery.

The Rise of Trichlorides Enabling an Improved Chlorine Technology

Chlorine plays a central role for the industrial production of numerous materials with global relevance. More recently, polychlorides have been evolved from an area of academic interest to a research topic with enormous industrial potential. In this minireview, the value of trichlorides for chlorine storage and chlorination reactions are outlined. Particularly, the inexpensive ionic liquid [NEt₃ Me][Cl₃ ] shows a similar and sometimes even advantageous reactivity compared to chlorine gas, while offering a superior safety profile. Used as a chlorine storage, [NEt₃ Me][Cl₃ ] could help to overcome the current limitations of storing and transporting chlorine in larger quantities. Thus, trichlorides could become a key technique for the flexibilization of the chlorine production enabling an exploitation of renewable, yet fluctuating, electrical energy. As the loaded storage, [NEt₃ Me][Cl₃ ], is a proven chlorination reagent, it could directly be employed for downstream processes, paving the path to a more practical and safer chlorine industry.

Catalytic Electrophilic Halogenation of Arenes with Electron-Withdrawing Substituents

The electrophilic halogenation of arenes is perhaps the simplest method to prepare aryl halides, which are important structural motifs in agrochemicals, materials, and pharmaceuticals. However, the nucleophilicity of arenes is weakened by the electron-withdrawing substituents, whose electrophilic halogenation reactions usually require harsh conditions and lead to limited substrate scopes and applications. Therefore, the halogenation of arenes containing electron-withdrawing groups (EWGs) and complex bioactive compounds under mild conditions has been a long-standing challenge. Herein, we describe Brønsted acid-catalyzed halogenation of arenes with electron-withdrawing substituents under mild conditions, providing an efficient protocol for aryl halides. The hydrogen bonding of Brønsted acid with the protic solvent 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) enables this transformation and thus solves this long-standing problem.

C-H chlorination of (hetero)anilines via photo/organo co-catalysis

Chlorinated (hetero)anilines are a class of important structural motifs that are widely present in synthetic building blocks and pharmaceuticals. Despite recent advancements, direct aniline chlorination still suffers from ortho/para and mono/poly chlorination selectivity problems. Herein, we disclose a photo-redox and organo co-catalyzed chlorination method for anilines. This method has great substrate generality and excellent mono-chlorination selectivity. Another merit of this method is the late-stage modification of drug molecules, which would be useful in medicinal chemistry.

Halogen bond-induced electrophilic aromatic halogenations

In recent years, there has been increasing interest in utilising halogen bonds in organic synthesis, especially in aromatic halogenation reactions. N-Halosuccinimides and 1,3-dihalo-5,5-dimethylhydantoins are popular sources of halonium ions due to their ease of handling and low toxicities. Traditionally, these N-haloimides are activated by electrophiles, namely Brønsted and Lewis acids. The recent discovery of possible activation by nucleophilic Lewis base catalysts led to a paradigm shift in aromatic halogenation. Active functional motifs in Lewis base catalysts such as CS, R-S-R¹, Ar-S-S-Ar, SO, Ar-NH₂, and R₂NH⁺Cl^- form halogen bonds with the positively charged σ-hole of the halogen atoms: an essential interaction to produce halonium ions. This review highlights the evolution of the two modes of activation. Evidence of halogen bond formation from mechanistic studies of nucleophilic activation is also discussed herein.

Oxoammonium salts are catalysing efficient and selective halogenation of olefins, alkynes and aromatics

Electrophilic halogenation reactions have been a reliable approach to accessing organohalides. During the past decades, various catalytic systems have been developed for the activation of haleniums. However, there is still a short of effective catalysts, which could cover various halogenation reactions and broad scope of unsaturated compounds. Herein, TEMPO (2,2,6,6-tetramethylpiperidine nitroxide) and its derivatives are disclosed as active catalysts for electrophilic halogenation of olefins, alkynes, and aromatics. These catalysts are stable, readily available, and reactive enough to activate haleniums including Br⁺, I⁺ and even Cl⁺ reagents. This catalytic system is applicable to various halogenations including haloarylation of olefins or dibromination of alkynes, which were rarely realized in previous Lewis base catalysis or Lewis acid catalysis. The high catalytic ability is attributed to a synergistic activation model of electrophilic halogenating reagents, where the carbonyl group and the halogen atom are both activated by present TEMPO catalysis.

Organohalides are widely used as synthetic precursors and target products, but for various halogenation reactions there is a need for effective catalysts to activate commercially available haleniums. Here, the authors report that TEMPO and its derivatives are active catalysts for electrophilic halogenation of olefins, alkynes and aromatics, under mild reaction conditions and with good functional group tolerance.

C-5 selective chlorination of 8-aminoquinoline amides using dichloromethane

An oxidant-free electrochemical regioselective chlorination of 8-aminoquinoline amides at ambient temperature in batch and continuous-flow was achieved. Inert DCM was used as the chlorinating reagent. Owing to the continuous-flow setup, the reaction scale up can be achieved conveniently with higher productivity. Moreover, this method has good position-control, and water and air tolerance. Costly quaternary ammonium salts were avoided. Radical-trapping, H/D exchange, KIE and cyclic voltammetry experiments were conducted to gain insight into the reaction mechanism.

Catalyst-Controlled Regioselective Chlorination of Phenols and Anilines through a Lewis Basic Selenoether Catalyst

We report a highly efficient ortho-selective electrophilic chlorination of phenols utilizing a Lewis basic selenoether catalyst. The selenoether catalyst resulted in comparable selectivities to our previously reported bis-thiourea ortho-selective catalyst, with a catalyst loading as low as 1%. The new catalytic system also allowed us to extend this chemistry to obtain excellent ortho-selectivities for unprotected anilines. The selectivities of this reaction are up to >20:1 ortho/para, while the innate selectivities for phenols and anilines are approximately 1:4 ortho/para. A series of preliminary studies revealed that the substrates require a hydrogen-bonding moiety for selectivity.

Complementary Site-Selective Halogenation of Nitrogen-Containing (Hetero)Aromatics with Superacids

Site-selective functionalization of arenes that is complementary to classical aromatic substitution reactions remains a long-standing quest in organic synthesis. Exploiting the generation of halenium ion through oxidative process and the protonation of the nitrogen containing function in HF/SbF₅ , the chlorination and iodination of classically inert Csp² -H bonds of aromatic amines occurs. Furthermore, the superacid-promoted (poly)protonation of the molecules acts as a protection, favoring the late-stage selective halogenation of natural alkaloids and active pharmaceutical ingredients.

Proton Transfer Can Govern Regioselectivity Assisted by Iron Catalysis

Ortho-selective aromatic C-H functionalization is frequently used in organic synthesis and chemical/pharmaceutical industries. However, this reaction relies heavily on the use of directing groups suffering from limited substrate scope and extra steps to put on and remove the directing/protecting groups. Herein we present the previously neglected concept that enables good to nearly complete selective ortho position. Proton transfer was utilized to tune the electron density on the aryl ring and determine the positional selectivity of electrophilic substitution. Consistently with deuteration experiments and DFT studies, this work demonstrates that acid-promoted proton transfer directs accelerated ortho-selective halogenation of NH/OH contained aromatic amines/phenols with excellent selectivity (>40 examples; up to 98:2 ortho/para selectivity). The application potential of this Fe-catalyzed method is demonstrated by the convenient synthesis of three alkaloids and tizanidine. This report raises the possibility that proton transfer could serve as the basis of developing new selective C-H functionalization reactions.

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Highly ortho-selective halogenations of anilines and carbazoles

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Lewis acids being able to accelerate EAS reactions

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Proton shift found to be crucial for the regioselectivity

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Practical iron sulfonate catalysis being scaled up to 100 g

U.S. Food and Drug Administration-Certified Food Dyes as Organocatalysts in the Visible Light-Promoted Chlorination of Aromatics and Heteroaromatics

Seven FDA-certified food dyes have been investigated as organocatalysts. As a result, Fast Green FCF and Brilliant Blue FCF have been discovered as catalysts for the chlorination of a wide range of arenes and heteroarenes in moderate to excellent yields and high regioselectivity. Mechanistic investigations of the separate systems indicate that different modes of activation are in operation, with Fast Green FCF being a light-promoted photoredox catalyst that is facilitating a one-electron oxidation of N-chlorosuccinimide (NCS) and Brilliant Blue FCF serving as a chlorine-transfer catalyst in its sulfonphthalein form with 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) as stoichiometric chlorine source. Dearomatization of naphthol and indole substrates was observed in some examples using the Brilliant Blue/DCDMH system.

Triptycenyl Sulfide: A Practical and Active Catalyst for Electrophilic Aromatic Halogenation Using N-Halosuccinimides

A Lewis base catalyst Trip-SMe (Trip = triptycenyl) for electrophilic aromatic halogenation using N-halosuccinimides (NXS) is introduced. In the presence of an appropriate activator (as a noncoordinating-anion source), a series of unactivated aromatic compounds were halogenated at ambient temperature using NXS. This catalytic system was applicable to transformations that are currently unachievable except for the use of Br₂ or Cl₂: e.g., multihalogenation of naphthalene, regioselective bromination of BINOL, etc. Controlled experiments revealed that the triptycenyl substituent exerts a crucial role for the catalytic activity, and kinetic experiments implied the occurrence of a sulfonium salt [Trip-S(Me)Br][SbF₆] as an active species. Compared to simple dialkyl sulfides, Trip-SMe exhibited a significant charge-separated ion pair character within the halonium complex whose structural information was obtained by the single-crystal X-ray analysis. A preliminary computational study disclosed that the π system of the triptycenyl functionality is a key motif to consolidate the enhancement of electrophilicity.

Organocatalytic, Enantioselective Dichlorination of Unfunctionalized Alkenes

The use of a new class of unsymmetrical cinchona-alkaloid-based, phthalazine-bridged organocatalysts enabled the highly enantioselective dichlorination of unfunctionalized alkenes. In combination with the electrophilic chlorinating agent 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) and triethylsilyl chloride (TES-Cl) as the source of nucleophilic chloride, 1-aryl-2-alkyl alkenes were dichlorinated with enantioselectivities of up to 94:6 er. Initial mechanistic investigations suggest that no free chlorine is formed, and by replacement of the chloride by fluoride, enantioselective chlorofluorinations of alkenes are possible.

Practical and regioselective amination of arenes using alkyl amines

The formation of carbon-nitrogen bonds for the preparation of aromatic amines is among the top five reactions carried out globally for the production of high-value materials, ranging from from bulk chemicals to pharmaceuticals and polymers. As a result of this ubiquity and diversity, methods for their preparation impact the full spectrum of chemical syntheses in academia and industry. In general, these molecules are assembled through the stepwise introduction of a reactivity handle in place of an aromatic C-H bond (that is, a nitro group, halogen or boronic acid) and a subsequent functionalization or cross-coupling. Here we show that aromatic amines can be constructed by direct reaction of arenes and alkyl amines using photocatalysis, without the need for pre-functionalization. The process enables the easy preparation of advanced building blocks, tolerates a broad range of functionalities, and multigram scale can be achieved via a batch-to-flow protocol. The merit of this strategy as a late-stage functionalization platform has been demonstrated by the modification of several drugs, agrochemicals, peptides, chiral catalysts, polymers and organometallic complexes.

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

Although the application of 1,2-dichloroethane (DCE) as a chlorinating reagent in organic synthesis with the concomitant release of vinyl chloride as a useful byproduct is a fantastic idea, it still presents a tremendous challenge and has not yet been achieved because of the harsh dehydrochlorination conditions and the sluggish C-H chlorination process. Here we report a bifunctional electrocatalysis strategy for the catalytic dehydrochlorination of DCE at the cathode simultaneously with anodic oxidative aromatic chlorination using the released HCl as the chloride source for the efficient synthesis of value-added (hetero)aryl chlorides. The mildness and practicality of the protocol was further demonstrated by the efficient late-stage chlorination of bioactive molecules.

Site-Selective C-H Functionalization of (Hetero)Arenes via Transient, Non-Symmetric Iodanes

A strategy for C-H functionalization of arenes and heteroarenes has been developed to allow site-selective incorporation of various anions, including Cl, Br, OMs, OTs, and OTf. This approach is enabled by in situ generation of reactive, non-symmetric iodanes by combining anions and bench-stable PhI(OAc)₂. The utility of this mechanism is demonstrated via para-selective chlorination of medicinally relevant arenes, as well as site-selective C-H chlorination of heteroarenes. Spectroscopic, computational, and competition experiments describe the unique nature, reactivity, and selectivity of these transient, unsymmetrical iodanes.

Revisiting 1-chloro-1,2-benziodoxol-3-one: efficient ortho-chlorination of aryls under aqueous conditions

The ortho-selective amide directing group assisted chlorination of aryls with the easily accessible 1-chloro-1,2-benziodoxol-3-one as oxidant as well as chlorinating reagent is reported in the absence of a radical initiator. The open air, aqueous conditions and recyclable reagent demonstrate the green principles involved in the reaction.

Cp*Rh(iii) catalyzed ortho-halogenation of N-nitrosoanilines by solvent-controlled regioselective C–H functionalization

A highly efficient rhodium-catalyzed, solvent-controlled regioselective C–H halogenation of anilines by using N-nitroso as a suitable and removable directing group was achieved under mild reaction conditions.

Activation of Remote meta-C-H Bonds in Arenes with Tethered Alcohols: A Salicylonitrile Template

Palladium-catalyzed activation of remote meta-C-H bonds in arenes containing tethered alcohols was achieved with high regioselectivity by using a nitrile template. Computational studies on the macrocyclic transition state of the regioselectivity-determining C-H activation steps revealed that both the C-N-Ag angles and gauche comformations of phenyl ether play an extremely important role in the meta selectivity.

Room temperature C(sp2)-H oxidative chlorination via photoredox catalysis

Photoredox catalysis has been developed to achieve room temperature oxidative C–H chlorination of aromatic compounds using NaCl as the chlorine source and Na₂S₂O₈ as the oxidant.

Photoredox catalysis has been developed to achieve oxidative C–H chlorination of aromatic compounds using NaCl as the chlorine source and Na₂S₂O₈ as the oxidant. The reactions occur at room temperature and exhibit exclusive selectivity for C(sp²)–H bonds over C(sp³)–H bonds. The method has been used for the chlorination of a diverse set of substrates, including the expedited synthesis of key intermediates to bioactive compounds and a drug.