A practical synthesis of α-bromo/iodo/chloroketones from olefins under visible-light irradiation conditions

Visible-Light Induced and Iron Peroxo-Promoted Radical Difunctionalization of Alkene for the Synthesis of β-Ketosulfone and α-Chloroketone

In this work, a switchable synthesis of β-ketosulfone and α-chloroketone through a radical difunctionalization of alkenes is reported. The transformation works well under iron peroxo species/photoredox dual catalysis and an open-flask atmosphere, and the reaction is highlighted with good yields and a broad reaction scope. Mechanism studies show that the reaction is initiated by a formal [4 + 2] cyclization of the sulfonyl radical in a regioselective manner.

Radical relay cyclization/C-C bond formation of allyloxy-tethered aryl iodides with quinoxalin-2(1H)-ones via polysulfide anion photocatalysis

A visible-light-induced radical relay cyclization/C-C bond formation of quinoxalin-2(1H)-ones with allyloxy-tethered aryl iodides using polysulfide anions as a photocatalyst is described. This protocol allows efficient access to a variety of complicated molecules bearing both quinoxalin-2(1H)-one and 2,3-dihydrobenzofuran motifs in high yields under mild reaction conditions with a broad range of substrates.

Rapid and General Access to α-Haloketones Using Quaternary Ammonium Salts as Halogen Sources

A general, rapid, and scalable method for the preparation of α-halogenated ketones using N-alkenoxypyridinium salts as substrates and quaternary ammonium salts as halogen sources was developed, featuring mild reaction conditions, excellent functional group tolerance, short reaction times, and a wide substrate scope.

Visible light-mediated halogenation of organic compounds

The use of visible light and photoredox catalysis emerged as a powerful and sustainable tool for organic synthesis, showing high value for distinctly different ways of bond creation. Halogenated compounds are the cornerstone of contemporary organic synthesis: it is almost impossible to develop a route towards a pharmaceutical reagent, agrochemical, natural product, etc. without the involvement of halogen-containing intermediates. Moreover, the halogenated derivatives as final products became indispensable for drug discovery and materials science. The idea of this review is to understand and summarise the impact of visible light-promoted chemistry on halogenation and halofunctionalisation reactions.

Visible light as a sole requirement for alkylation of α-C(sp3)-H of N-aryltetrahydroisoquinolines with alkylboronic acids

An alkylation of α-C(sp3)-H at N-aryltetrahydroisoquinolines with alkylboronic acids was developed under visible-light irradiation in the absence of additional photocatalyst. The reaction proceeded well, tolerating a variety of functional groups, and featured low-cost and mild reaction conditions. A preliminary mechanistic study indicated that an electron donor-acceptor (EDA) complex between an electron-rich N-aryltetrahydroisoquinoline and an electron-poor alkylboronic acid was involved in the reaction.

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.

Electrophilic Halospirocyclization of N-Benzylacrylamides to Access 4-Halomethyl-2-azaspiro[4.5]decanes

An electrophilic spirocyclization of N-benzylacrylamides with N-halosuccinimides (NXS) as the halogenating reagents has been developed. This reaction is carried out at room temperature under simple conditions without relying on metal reagents, photochemistry, or electrochemistry, providing a fast and efficient route to synthesize a wide variety of 4-halomethyl-2-azaspiro[4.5]decanes with satisfactory yields. The approach is further highlighted through gram-scale synthesis and diverse transformations of the spiro products.

Enantioselective Chemodivergent Three-Component Radical Tandem Reactions through Asymmetric Photoredox Catalysis

Chemodivergent synthesis has been achieved in asymmetric photocatalysis. Under a dual catalyst system consisting of a chiral phosphoric acid and DPZ as a photosensitizer, different inorganic bases enabled the formation of two sets of valuable products from the three-component radical tandem transformations of 2-bromo-1-arylenthan-1-ones, styrenes, and quinoxalin-2(1H)-ones. The key to success was the distinct pKa environment, in which the radicals that formed on the quinoxalin-2(1H)-one rings after two radical addition processes underwent either single-electron oxidation or single-electron reduction. In addition, this work represents the first use of quinoxalin-2(1H)-ones in asymmetric photoredox catalysis.

Activation of Ethanol via Conjunction of a Photocatalyst and a HAT Reagent for the Synthesis of Benzimidazoles

The transformation of ethanol to value-added chemicals has tremendous potential. However, generally, harsh reaction conditions are needed for the functionalization of ethanol due to its high dehydrogenation energy. Herein, a metal-free photo-mediated activation of challenging ethanol and higher aliphatic alcohols for the synthesis of differently functionalized benzimidazoles under mild conditions is disclosed. The interplay of a photocatalyst and a HAT reagent facilitated the activation of aliphatic alcohols. A wide array of diamines with different functional groups were well tolerated, and the protocol was also extended to N-substituted diamines for the synthesis of industrially important benzimidazoles. A probable catalytic cycle was proposed based on various mechanistic studies.

Radical-Polar Crossover Enabled Triple Cleavage of CF2Br2: A Multicomponent Tandem Cyclization to 3-Fluoropyrazoles

The elaboration of step-economy and catalytic approaches for accessing diverse fluorinated heterocyclics is highly desirable. Described herein is a radical-polar crossover enabled three-component cyclization to polysubstituted fluoropyrazoles by using CF2Br2 as a novel C1F1 synthon. Mechanistic experiments revealed that the in situ generation of the reactive intermediate gem-difluorovinylimine ion is the key to this transformation. This protocol unlocks the novel reactivity of CF2Br2 and adds significant synthetic values to fluorine chemistry.

Merging Cu(I) and Cu(II) Photocatalysis: Development of a Versatile Oxohalogenation Protocol for the Sequential Cu(II)/Cu(I)-Catalyzed Oxoallylation of Vinylarenes

A sequential photocatalytic strategy is developed via the merger of Cu(II)/Cu(I)-catalytic cycles for the oxoallylation of vinyl arenes via α-haloketones. The initial Cu(II)-photocatalyzed oxohalogenation exploits ligand-to-metal charge transfer (LMCT) to generate halide radicals from acyl halides utilizing air as a terminal oxidant and can be employed for the late-stage modification of pharmaceuticals and agrochemicals. α-Bromoketones obtained this way can be subsequently subjected to a one-pot Cu(I)-photocatalyzed allylation. This sequential photocatalysis proceeds in a highly regio- and chemoselective fashion and is inconsequential to the electronic nature of styrenes.

Microwave-accelerated cross-dehydrogenative-coupling (CDC) of N-(quinolin-8-yl)amides with acetone/acetonitrile under metal-free conditions

A highly selective remote C(sp³)-H acetonation of N-(quinolin-8-yl)amide scaffolds at the C5-position under microwave irradiation has been developed. In the absence of a transition-metal-catalyst, benzoyl peroxide (BPO)-promoted cross-dehydrogenation coupling (CDC) of N-(quinolin-8-yl)amides with acetone/acetonitrile occurred smoothly to generate the corresponding 5-acetonated/acetonitriled N-(quinolin-8-yl)amides in good yields. The transformation is operationally simple, rapid, easily scaled-up to the gram scale, and shows a broad substrate scope.

Photoinduced Photocatalyst-Free Cascade Cyclization of Alkynes with Sodium Sulfinates for the Synthesis of Benzothiophenes and Thioflavones

The subject of this investigation is a new method for the construction of sulfonylated heterocycles which overcomes the limitations of classical approaches using a cheap feedstock sulfonylating agent, especially under photocatalyst- and metal-free conditions.

I2-mediated coupling of quinazolinone enamines with 2-aminopyridines: a new strategy to access spiroquinazolinones

Herein, we report a novel, rapid and efficient route to the spiroquinazolinone framework via an umpolung strategy mediated by molecular iodine. A library of functionalized spiroquinazolinone iodide salts was synthesized in moderate to good yields under ambient, metal-free and mild conditions. The current methodology opens up a new efficient and concise strategy for the construction of spiroquinazolinones.

Visible-light-induced C(sp3)-H thiocyanation of pyrazolin-5-ones: a practical synthesis of 4-thiocyanated 5-hydroxy-1H-pyrazoles

A direct, aerobic and visible light photocatalytic approach to synthesize 4-thiocyanated 5-hydroxy-1H-pyrazoles via cross-coupling of pyrazolin-5-ones with ammonium thiocyanate is described. Under redox-neutral and metal-free conditions, a series of 4-thiocyanated 5-hydroxy-1H-pyrazoles could be easily and efficiently obtained in good to high yields by using low-toxicity and inexpensive ammonium thiocyanate as the thiocyanate source.

Copper-Promoted Aerobic Oxidative [3+2] Cycloaddition Reactions of N,N-Disubstituted Hydrazines with Alkynoates: Access to Substituted Pyrazoles

A copper-promoted aerobic oxidative [3+2] cycloaddition reaction for the synthesis of various substituted pyrazoles from N,N-disubstituted hydrazines with alkynoates in the presence of bases is developed. This work involves a direct C(sp³)-H functionalization and the formation of new C-C/C-N bonds. In this strategy, inexpensive and easily available Cu₂O serves as the promoter and air acts as the green oxidant. The reaction exhibits the advantages of high atom and step economy, high regioselectivity, and easy operation.

Electrochemical monofluoroalkylation cyclization of N-arylacrylamides to construct monofluorinated 2-oxindoles

An electrochemical monofluoroalkylation cyclization of N-arylacrylamides to synthesize monofluorinated 2-oxindoles has been developed, which employs common dimethyl 2-fluoromalonate as a monofluoroalkyl radical precursor and obviates the use of prefunctionalized monofluoroalkylation reagents and sacrificial oxidants. A variety of monofluorinated nitrogen-containing heterocyclic compounds were efficiently obtained with satisfactory yields from readily available materials.

Copper Cocatalyst Modulated Radical Generation for Selective Heterogeneous Photosynthesis of α-Haloketones

The α-haloketones are important precursors for synthetic chemistry and pharmaceutical applications; however, their production relies heavily on traditional synthetic methods via halogenation of ketones that are toxic and environmentally risky. Here, we report a heterogeneous photosynthetic strategy of α-haloketone production from aromatic olefins using copper-modified graphitic carbon nitride (Cu-C₃N₄) under mild reaction conditions. By employing NiX₂ (X = Cl, Br) as the halogen source, a series of α-haloketones can be synthesized using atmospheric air as the oxidant under visible-light irradiation. In comparison with pristine carbon nitride, the addition of Cu as a cocatalyst provides a moderate generation rate of halogen radicals and selective reduction of molecular oxygen into ^•OOH radicals, thus leading to a high selectivity to α-haloketones. The Cu-C₃N₄ also exhibits high stability and versatility, rendering it a promising candidate for solar-driven synthetic applications.

Visible-Light-Induced Tandem Radical Brominative Addition/Cyclization of Activated Alkynes with CBr4 for the Synthesis of 3-Bromocoumarins

A visible-light-induced tandem radical brominative addition/spiro-cyclization/1,2-ester migration of activated alkynes with CBr₄ is developed. This protocol features good functional group tolerance, operational simplicity, and mild reaction conditions without the use of catalysts and external additives, providing easy access to valuable 3-bromocoumarins in generally high yields.

CuCl2 -Catalyzed α-Chloroketonation of Aromatic Alkenes via Visible-Light-Induced LMCT

Here we report a copper-catalyzed protocol for the synthesis of α-chloroketones from aromatic alkenes including electron-deficient olefins under visible-light irradiation. Preliminary mechanistic studies show that the peroxo Cu(II) species is the key intermediate and hydroperoxyl (HOO⋅) and chlorine (Cl⋅) radicals can be generated by ligand-to-metal charge transfer (LMCT).

Visible-light-driven radical Friedländer hetero-annulation of 2-aminoaryl ketone and α-methylene carbonyl compound via organic dye fluorescein through a single-electron transfer (SET) pathway

The discoveries recommend that the photoinduced conditions of fluorescein-determined go about as impetus for photochemically combining polysubstituted quinolines in ethanol at room temperature under air environment by means of revolutionary Friedländer hetero-annulation of 2-aminoaryl ketone and α-methylene carbonyl compound. This study lays out an original capability for photochemically orchestrating fluorescein. This non-metallic organic dye is economically accessible and modest, producing great outcomes, accelerating the cycle, and achieving a high compound economy. The turnover number (TON) and turnover recurrence (TOF) of polysubstituted quinolines have been determined. This cycle will likewise run on a gram scale, demonstrating the chance of modern applications.

Photocatalytic C-H Activation and Amination of Arenes with Nonactivated N-Hydroxyphthalimides Involving Phosphine-Mediated N-O Bond Scission

Herein, we reported a metal-free photoredox/phosphine-catalyzed C-H amination of arenes. This allows for concise synthesis of highly functionalized N-arylphthalimides from readily available N-hydroxyphthalimides directly, without the preparation of activated N-hydroxyphthalimide intermediates. Mechanistic studies reveal that the radical is produced via phosphine-mediated N-O bond scission.

Visible-Light-Induced C-F and C-N Bond Cleavage for the Synthesis of gem-Difluoroalkenes

Herein, we describe a novel and efficient photoredox catalytic radical addition/defluoroalkylation coupling reaction between primary amines and trifluoromethyl-substituted alkenes. A series of gem-difluoroalkenes were synthesized via C-N bond cleavage of α-3°, α-2°, and α-1° amines under visible light irradiation. This reaction is characterized by a broad substrate scope and good functional group tolerance.

Cu(II)-Mediated aerobic oxidative synthesis of sulfonated chromeno[4,3-c]pyrazol-4(2H)-ones

Herein, starting with propiolates and sulfonyl hydrazides, we developed a concise and facile synthesis of 2-sulfonylated chromeno [4,3-c]pyrazol-4(2H)-ones or 2,5-dihydro-4H-pyrazolo[4,3-c]quinolin-4-ones via Cu(II)-promoted oxidative cascade C-C/C-N bond formation. This protocol has the advantages of atom economy and good functional group tolerance. The primary mechanism studies indicate that the reaction involves a free-radical process as well as terminal alkyne C-H activation.

Visible-light-promoted decarboxylative radical cascade cyclization to acylated benzimidazo/indolo[2,1-a]isoquinolin-6(5H)-ones in water

A metal-free visible-light-induced decarboxylative radical addition/cyclization procedure at room temperature was described for the synthesis of acylated benzimidazo/indolo[2,1-a]isoquinolines. The procedure was prepared in water via a reaction of functionalized 2-arylbenzoimidazoles or 2,3-diarylindoles and α-oxocarboxylic acids in the presence of phenyliodine(iii) diacetate (PIDA) in one step under mild reaction conditions. In this procedure, traditional heating and metal reagents could be effectively avoided to access 1,4-dicarbonyl-containing benzimidazo/indolo[2,1-a]isoquinoline-6(5H)-ones in satisfactory yields.

imethyl 2-Benzylmalonate

An electrochemical dearomatizing spirocyclization of alkynes with dimethyl 2-benzylmalonates for the preparation of spiro[4.5]deca-trienones has been developed. This approach adopts ferrocene (Cp₂Fe) as an electrocatalyst to produce carbon-centered radical intermediates from C-H-based malonates, which obviates the forthputting of noble-metal reagents, sacrificial chemical oxidants and 2-bromomalonates. A wide variety of spiro compounds are efficiently prepared with satisfactory results under mild conditions.

Photocatalyzed Defluorinative Dichloromethylation of α-CF3 Alkenes Using CHCl3 as the Radical Source

A visible-light-induced defluorinative dichloromethylation of α-CF₃ alkenes was developed with cheap and readily accessible chloroform simultaneously as a dichloromethylation reagent and reaction medium, leading to the facile preparation of new polyhalogenated scaffolds. Notably, the change from CHCl₃ to CDCl₃ offers a straightforward pathway for accessing the deuterated analogues with excellent degrees of D incorporation. Mechanistic studies suggested the reaction underwent a radical addition of the dichloromethyl radical with alkenes, followed by sequential single-electron transfer and defluorination. This protocol features mild conditions, easy operation, facile scalability, and high efficiency, allowing convenient access to dichloronated gem-difluoroalkenes.

Palladium-Catalyzed Highly Chemo- and Stereoselective Bisthiolation of Terminal Alkynes with Allyl Phenyl Sulfides via C-S Bond Cleavage

A facile and general method for palladium-catalyzed stereoselective bisthiolation of terminal alkynes with allyl phenyl sulfides has been developed. The scope and versatility of the reaction have been demonstrated, and a broad range of substrates bearing electron-donating and -withdrawing groups on the aromatic rings were all compatible with this reaction, providing the desired (Z)-1,2-dithio-1-alkenes in moderate to good yields. Preliminary mechanistic studies demonstrated that the sulfur source of the desired products may be successively incorporated into alkynes via C-S bond cleavage of two molecules of allyl phenyl sulfides and ruled out the possibility of vinyl sulfides, alkynyl sulfides, and disulfide intermediates being involved in this reaction.

Synthetic Access to Aromatic α-Haloketones

α-Haloketones play an essential role in the synthesis of complex N-, S-, O-heterocycles; of which some exhibit a remarkable biological activity. Research further illustrated that α-bromo-, α-chloro-, and α-iodoketones are key precursors for blockbuster pharmacological compounds. Over the past twenty years, substantial advances have been made in the synthesis of these industrially relevant building blocks. Efforts have focused on rendering the synthetic protocols greener, more effective and versatile. In this survey, we summarised and thoroughly evaluated the progress of the field, established in the past two decades, in terms of generality, efficacy and sustainability.

Catalyst- and Oxidizing Reagent-Free Electrochemical Benzylic C(sp3)-H Oxidation of Phenol Derivatives

A site-selective electrochemical approach for the benzylic C(sp³)-H oxidation reaction of phenol derivatives along with hydrogen evolution has been developed. The protocol proceeds in an easily available undivided cell at room temperature under catalyst- and oxidizing reagent-free conditions. The corresponding aryl aldehydes and ketones are obtained in satisfactory yields, and the gram-scale synthesis is easy to be carried out.

A Straightforward Approach to Fluorinated Pyrimido[1,2-b]indazole Derivatives via Metal/Additive-Free Annulation with Enaminones, 3-Aminoindazoles, and Selectfluor

A novel and efficient three-component reaction with two C-N bonds and one C-F bond formation has been reported, which provides a straightforward route to a variety of fluorinated pyrimido[1,2-b]indazole derivatives. This transformation has the advantage of excellent functional group compatibility, including aliphatic and aromatic substituents enaminones. Moreover, metal and additives are not necessary for this reaction, which is of great significance for the synthesis and application of fluorinated heterocycles.

Employing Visible-Light Photoredox Catalysis in Multicomponent-Multicatalyst Reactions: One-Pot Synthesis of Spiroquinazolin-2-(thi)ones

The multicomponent-multicatalyst reaction ((MC)²R) and visible-light catalysis have emerged as green and powerful strategies for achieving ideal syntheses. Here, we report the first example of a visible-light-induced approach toward spiroquinazolin(thi)ones. This (MC)²R features an eco-friendly energy source and solvent, metal-free catalysts, step- and atom-economy, a relay catalysis strategy, air as green oxidant, mild conditions, and easily accessible starting materials.

Regio- and stereoselective electrochemical selenoalkylation of alkynes with 1,3-dicarbonyl compounds and diselenides

A regio- and stereoselective electrochemical approach for the selenoalkylation of alkynes with 1,3-dicarbonyl compounds and diselenides has been developed.

Electrochemical formal [3 + 2] cycloaddition of azobenzenes with hexahydro-1,3,5-triazines

A catalyst-free electrochemical [3 + 2] cycloaddition of azobenzenes with hexahydro-1,3,5-triazines without an external oxidant has been developed for constructing the 1,2,4-triazolidine skeleton.

Selective polychloromethylation and halogenation of alkynes with polyhaloalkanes

We disclosed a selective polychloromethylation and halogenation reaction of alkynes via a radical addition/spirocyclization cascade sequence, in which applying polyhaloalkanes as the precursor of polyhalomethyl and halogen radical. Across this...

Efficient synthesis of spiro diheterocycles via multi-component dicyclization reaction

Spiro diheterocycles with one valuable quaternary carbon were constructed in one pot via an in situ cyclization–respiroannulation strategy.

Selective electrophilic di- and mono-fluorinations for the synthesis of 4-difluoromethyl and 4-fluoromethyl quinazolin(thi)ones by Selectfluor-triggered multi-component reaction

A simple and efficient domino protocol for the selective synthesis of 4-difluoromethyl and 4-fluoromethyl quinazolin(thi)ones was established from readily available 2-aminoacetophenones and iso(thio)cyanates mediated by Selectfluor. The reaction outcomes are...

Visible-Light-Induced Catalytic Selective Halogenation with Photocatalyst

Halide moieties are essential structures of compounds in organic chemistry due to their popularity and wide applications in many fields such as natural compounds, agrochemicals, and pharmaceuticals. Thus, many methods have been developed to introduce halides into various organic molecules. Recently, visible-light-driven reactions have emerged as useful methods of organic synthesis. Particularly, halogenation strategies using visible light have significantly improved the reaction efficiency and reduced toxicity, as well as promoted reactions under mild conditions. In this review, we have summarized recent studies in visible-light-mediated halogenation (chlorination, bromination, and iodination) with photocatalysts.

Merging cobalt and photoredox catalysis for the C8-H alkoxylation of 1-naphthylamine derivatives with alcohols

A combined cobalt and photoredox catalysis system to realize the C8-H alkoxylation of 1-naphthylamine derivatives with alcohols was developed. Using commercially available alkyl alcohols as raw materials and Co(OAc)₂ and rose bengal as catalysts, 1-naphthylamine derivatives reacted with alcohols to generate the corresponding C8-H alkoxylation products in good yields.

Visible-light-induced C(sp3)-H functionalizations of piperidines to 3,3-dichloro-2-hydroxy-piperidines with N-chlorosuccinimide

A visible-light-induced synthesis of 3,3-dichloro-2-hydroxy-piperidines via site-selective functionalizations of C(sp³)-H in N-substituted piperidines using easily available N-chlorosuccinimide as chlorine source was developed. Mechanistic investigations suggest that chlorine radical is involved in this transformation.