Aminofluorination of Cyclopropanes: A Multifold Approach through a Common, Catalytically Generated Intermediate

One-pot C-N/C-C bond formation and oxidation of donor-acceptor cyclopropanes with tetrahydroisoquinolines: access to benzo-fused indolizines

One-pot C-N/C-C bond formation of donor-acceptor cyclopropanes (DACs) with tetrahydroisoquinolines (THIQs) has been achieved to furnish benzo-fused indolizines. These reactions involve a MgI2-catalyzed ring opening of DACs and oxidative annulation using Mn(OAc)3·2H2O. The substrate scope and functional group diversity are the important practical features.

Electrochemical Formation of Oxazolines by 1,3-Oxyfluorination of Non-activated Cyclopropanes

The C-C bond in non-activated cyclopropanes can be intramolecularly cleaved with an electrochemically generated amidyl radical forming oxazolines. In the presence of TBABF4, this provides 1,3-oxyfluorination products. C-C bond cleavage of cyclopropane proceeds with inversion of the configuration, suggesting an intramolecular homolytic substitution (SHi) mechanism. The performance of TBABF4 as an efficient fluoride source was explained by accumulation of the BF4- anion at the anode surface, at which a carbocation is formed by the oxidation of the C-centered radical.

Mechanistic insights into radical formation and functionalization in copper/N-fluorobenzenesulfonimide radical-relay reactions

Copper-catalysed radical-relay reactions that employ N-fluorobenzenesulfonimide (NFSI) as the oxidant have emerged as highly effective methods for C(sp3)-H functionalization. Herein, computational studies are paired with experimental data to investigate a series of key mechanistic features of these reactions, with a focus on issues related to site-selectivity, enantioselectivity, and C-H substrate scope. (1) The full reaction energetics of enantioselective benzylic C-H cyanation are probed, and an adduct between Cu and the N-sulfonimidyl radical (˙NSI) is implicated as the species that promotes hydrogen-atom transfer (HAT) from the C-H substrate. (2) Benzylic versus 3° C-H site-selectivity is compared with different HAT reagents: Cu/˙NSI, ˙OtBu, and Cl˙, and the data provide insights into the high selectivity for benzylic C-H bonds in Cu/NFSI-catalyzed C-H functionalization reactions. (3) The energetics of three radical functionalization pathways are compared, including radical-polar crossover (RPC) to generate a carbocation intermediate, reductive elimination from a formal CuIII organometallic complex, and radical addition to a Cu-bound ligand. The preferred mechanism is shown to depend on the ligands bound to copper. (4) Finally, the energetics of three different pathways that convert benzylic C-H bonds into benzylic cations are compared, including HAT/ET (ET = electron transfer), relevant to the RPC mechanism with Cu/NFSI; hydride transfer, involved in reactions with high-potential quinones; and sequential ET/PT/ET (PT = proton transfer), involved in catalytic photoredox reactions. Collectively, the results provide mechanistic insights that establish a foundation for further advances in radical-relay C-H functionalization reactions.

Competition between C-C and C-H Bond Fluorination: A Continuum of Electron Transfer and Hydrogen Atom Transfer Mechanisms

In 2015, we reported a photochemical method for directed C-C bond cleavage/radical fluorination of relatively unstrained cyclic acetals using Selectfluor and catalytic 9-fluorenone. Herein, we provide a detailed mechanistic study of this reaction, during which it was discovered that the key electron transfer step proceeds through substrate oxidation from a Selectfluor-derived N-centered radical intermediate (rather than through initially suspected photoinduced electron transfer). This finding led to proof of concept for two new methodologies, demonstrating that unstrained C-C bond fluorination can also be achieved under chemical and electrochemical conditions. Moreover, as C-C and C-H bond fluorination reactions are both theoretically possible on 2-aryl-cycloalkanone acetals and would involve the same reactive intermediate, we studied the competition between single-electron transfer (SET) and apparent hydrogen-atom transfer (HAT) pathways in acetal fluorination reactions using density functional theory. Finally, these analyses were applied more broadly to other classes of C-H and C-C bond fluorination reactions developed over the past decade, addressing the feasibility of SET processes masquerading as HAT in C-H fluorination literature.

Ring-Opening Fluorination of Carbo/Heterocycles and Aromatics: Construction of Complex and Diverse Fluorine-Containing Molecules

Fluorine-containing molecules have attracted much attention in medicinal, agrochemical, and materials sciences because they offer unique physical and biological properties. Therefore, many efficient fluorination reactions have been developed over the years. Recent advancements in fluorination chemistry have expanded the range of substrates, and regioselectivity/stereoselectivity control has also been achieved. Ring-opening fluorination is an efficient method to construct complex fluorine-containing molecules with diversity, starting from simple cyclic compounds. This review aims to summarize developments in ring-opening fluorination, particularly with larger-sized cyclic compounds. Fluorine introduction and bond cleavage of cyclic compounds such as carbocycles, heterocycles, and aromatics provide efficient access to fluorine-containing compounds that are difficult to be synthesized by conventional methods.

Electrochemical Oxidation Enables Regioselective 1,3-Hydroxyfunctionalization of Cyclopropanes

The direct construction of 1,3-hydroxyfunctionalized molecules is still a significant challenge, as they can currently be obtained through multiple synthetic steps. Herein, we report a general and efficient 1,3-hydroxyfunctionalization of arylcyclopropanes by electrochemical oxidation with a strategic choice of nucleophiles and H2O. 1,3-Amino alcohols, 1,3-alkynyl alcohols, 1,3-hydroxyesters, and 1,3-halo alcohols are achieved with high levels of chemo- and regio-selectivity, opening a new dimension for 1,3-difunctionalization reaction.

Divergent Fluorinations of Vinylcyclopropanes: Ring-Opening 1,5-Hydrofluorination and Ring-Retaining 1,2-Difluorination

Organofluorine compounds have been widely used in pharmaceutical, agrochemical, and material sciences. Reported herein are divergent fluorination reactions of vinylcyclopropanes with different electrophiles, which allow the facile synthesis of homoallylic monofluorides and vicinal-difluorides through ring-opening 1,5-hydrofluorination and ring-retaining 1,2-difluorination, respectively. Both protocols feature mild conditions, simple operations, good functional group tolerance, and generally good yields. The practicality of these reactions is demonstrated by their scalability, as well as the successful conversion of the formed homoallylic monofluorides into other complex fluorinated molecules.

1,3-Difunctionalization of Propargyl Silanes with Concomitant 1,2-Silyl Shift: Synthesis of Allyl Functionalized Vinyl Silanes

Terminal alkynes with a silyl group at the propargylic position upon activation with electrophiles such as N-bromosuccinimide undergo (E)-selective 1,2-silyl group migration. Subsequently, an allyl cation is formed that is intercepted by an external nucleophile. This approach provides allyl ethers and esters with stereochemically defined vinyl halide and silane handles for further functionalization. The scope of propargyl silanes and electrophile-nucleophile pairs are investigated, and various trisubstituted olefins are prepared in up to 78% yield. The obtained products have been demonstrated to serve as building blocks for transition-metal-catalyzed cross-couplings of vinyl halides, silicon-halogen exchange, and allyl acetate functionalization reactions.

Unraveling the Structure and Reactivity Patterns of the Indole Radical Cation in Regioselective Electrochemical Oxidative Annulations

Oxidation-induced strategy for inert chemical bond activation through highly active radical cation intermediate has exhibited unique reactivity. Understanding the structure and reactivity patterns of radical cation intermediates is crucial in the mechanistic study and will be beneficial for developing new reactions. In this work, the structure and properties of indole radical cations have been revealed using time-resolved transient absorption spectroscopy, in situ electrochemical UV-vis, and in situ electrochemical electron paramagnetic resonance (EPR) technique. Density functional theory (DFT) calculations were used to explain and predict the regioselectivity of several electrochemical oxidative indole annulations. Based on the understanding of the inherent properties of several indole radical cations, two different regioselective annulations of indoles have been successfully developed under electrochemical oxidation conditions. Varieties of furo[2,3-b]indolines and furo[3,2-b]indolines were synthesized in good yields with high regioselectivities. Our mechanistic insights into indole radical cations will promote the further development of oxidation-induced indole functionalizations.

Divergent Copper-salt-controlled Reactions of Donor-Acceptor Cyclopropanes and N-Fluorobenzene Sulfonimide: Access to the 1,3-Haloamines and Aminoindanes

We herein report a method for divergent copper salt controlled reactions of donor-acceptor cyclopropanes and N-fluorobenzene sulfonimide (NFSI). Specifically, in the presence of CuX₂ (X=Cl, Br), the cyclopropanes underwent formal umpolung 1,3-aminohalogenation bifunctionalization via a free radical mediated ring-opening process to afford 1,3-aminochlorination and 1,3-aminobromination products in moderate to good yields. In addition, by using CuI as a catalyst, we synthesized various aminoindane derivatives via 1,3-aminoarylation cyclization of D-A cyclopropanes, the reactions involved a free radical mediated ring-opening and subsequent ring expansion via C-H bond activation.

Dimerization/Elimination of β-Styrylmalonates under Action of TiCl4

A new type of dimerization of dimethyl (β-styryl)malonates in the presence of TiCl₄ accompanied by elimination of a methanol molecule was discovered. Selective methods for the synthesis of substituted trimethyl 4-hydroxy-[1,1'-biaryl]-3,3,5(2H)-tricarboxylates and trimethyl 7-hydroxy-9,10-dihydro-5,9-methanobenzo[8]annulene-6,8,8(5H)-tricarboxylates were developed. The regularities of the occurring processes were determined and a similar reaction of β-styrylmalonate with benzylidenemalonate in the presence of TiCl₄ was performed in the scope of the suggested mechanism.

Visible light-initiated radical 1,3-difunctionalization of β,γ-unsaturated ketones

Radical-mediated 1,2-difunctionalization of olefins is a well-established synthetic technique widely used in the rapid construction of structurally diverse molecular entities. However, radical-mediated 1,3-difunctionalization reactions are rare, and the substrates are generally limited to strained skeletons. Here, we report a practical approach for 1,3-difunctionalization of available β,γ-unsaturated ketones via a radical cascade process including visible light-irradiated radical addition, thermodynamic stability-driven 1,2-carbonyl migration from unactivated all-carbon quaternary center, and terminal C-radical varied transformations. Various highly functionalized alkyl skeletons with different valuable functional groups at positions 1 and 3 and the carbonyl group at position 2 have been synthesized through a radical chain pathway or Cu-catalyzed Ritter-type reaction. Moreover, this protocol provides a real case of diversity-oriented radical rearrangement for drug discovery. We identified a previously unknown chemotype of dual inhibitors for hypoxia-inducible factor (HIF) and WNT signaling pathways from products. These small-molecule inhibitors could suppress HIF and WNT signaling-dependent HCT116 cell growth in 2D and 3D culture systems.

A copper-catalyzed four-component reaction of arylcyclopropanes, nitriles, carboxylic acids and N-fluorobenzenesulfonimide: facile synthesis of imide derivatives

An unprecedented copper-catalyzed four-component reaction of arylcyclopropanes, nitriles, carboxylic acids and N-fluorobenzenesulfonimide (NFSI) has been successfully developed, which represents the first example of a four-component reaction of non-donor-acceptor cyclopropanes. A wide range of imide derivatives were efficiently synthesized in excellent yields under mild conditions.

Substrate-controlled C-H or C-C alkynylation of cyclopropanes: generation of aryl radical cations by direct light activation of hypervalent iodine reagents

We report the first oxidative C-H alkynylation of arylcyclopropanes. Irradiation of ethynylbenziodoxolone (EBX) reagents with visible light at 440 nm promoted the reaction. By the choice of the aryl group on the cyclopropane, it was possible to completely switch the outcome of the reaction from the alkynylation of the C-H bond to the oxyalkynylation of the C-C bond, which proceeded without the need for a catalyst, in contrast to previous works. The oxyalkynylation could also be extended to aminocyclopropanes as well as styrenes. Computations indicated that the C-H activation became a favoured nearly barrierless process in the presence of two ortho methyl groups on the benzene ring.

Photocatalytic Oxo-Amination of Aryl Cyclopropanes through an Unusual SN2-Like Ring-Opening Pathway: Won >99% ee

One-pot oxo-amination of unactivated cyclopropanes with safe, green dioxygen as an oxidant and low-cost amines as nitrogen sources has generated interest since this can directly result in uniform β-located difunctional units. Formation of the three-electron cation radical followed by the nucleophilic attack of amines to open the strained ring of cyclopropanes catalyzed by classic noble-complex photocatalysts was a promising strategy. However, this ring-opening pathway could not maintain the entire second-order nucleophilic substitution (S_N2) conversion, which generally led to unsatisfactory enantioselectivity (enantiomeric excess (ee) value ∼60%). Here, we demonstrate that for such a one-step oxo-amination of cyclopropanes with benign dioxygen and pyrazoles, a highly uniform inversion of configuration could be first accomplished through a TiO₂ photocatalyst. This strategy features low-cost, semiheterogeneous photocatalysis and environmentally friendly reaction conditions, without using any sacrificial reagent or additive. Importantly, our protocol not only provides a relatively broad substrate scope tolerant to a certain range of substituted cyclopropanes and pyrazoles, resulting in various β-amino ketone products (∼50 examples) with excellent conversions and yields, but also retains excellent enantioselectivity (ee value ∼99%). A concerted S_N2 ring opening raised from an oxetane cation intermediate rather than a conventional three-electron cation radical prior to attaching to dioxygen was proposed.

C-C Bond Activation of Cyclopropanes Enabled by Phosphine-Catalyzed In Situ Formation of High-Strain Methylenecycopropane Intermediate

An effective strategy for the ring-opening/elaboration of cyclopropanes by phosphine catalyst is documented, providing the 2,4-pentadiene sulfonamides and isoindolines in moderate to good yields. The key to the success of this reaction is phosphine-catalyzed introduction of a trigonal center into cyclopropanes, which results in the formation of higher ring strain cyclopropylidenemethyl phosphonium salt. Moreover, this methodology is employed as the key step for the synthesis of bioactive molecules.

Regio- and Enantioselective Intermolecular Aminofluorination of Alkenes via Iodine(I)/Iodine(III) Catalysis

The regio‐ and enantio‐selective, intermolecular vicinal fluoroamination of α‐trifluoromethyl styrenes has been achieved by enantioselective I^I/I^III catalysis. Leveraging C₂‐symmetric resorcinol‐based aryl iodide catalysts, it has been possible to intercept the transient iodonium intermediate using simple nitriles, which function as both the solvent and nucleophile. In situ Ritter reaction provides direct access to the corresponding amides (up to 89 % yield, e.r. 93 : 7). This main group catalysis paradigm inverts the intrinsic regioselectivity of the uncatalyzed process, thereby providing facile access to tertiary, benzylic stereocenters bearing both CF₃ and F groups. Privileged phenethylamine pharmacophores can be generated in which there is complete local partial charge inversion (CF₃^δ−/F^δ− versus CH₃^δ+/H^δ+). Crystallographic analyses of representative β‐fluoroamide products reveal highly pre‐organized conformations that manifest the stereoelectronic gauche effect.

Visible-Light-Accelerated Copper-Catalyzed [3 + 2] Cycloaddition of N-Tosylcyclopropylamines with Alkynes/Alkenes

Copper-catalyzed [3 + 2] cycloadditions of N-tosylcyclopropylamine with alkynes and alkenes have been accomplished under visible light irradiation. The developed approach is compatible with a range of functionalities and allows the synthesis of diversified aminated cyclopentene and cyclopentane derivatives being relevant for drug synthesis. The protocol is operationally simple and economically affordable as it does not require any ligand, base, or additives. As the key step, the one-electron oxidation of the N-tosyl moiety by visible light-induced homolysis of a transient Cu(II)-tosylamide complex is proposed, providing a facile entry for N-centered radicals.

Ring-Opening Fluorination of Isoxazoles

A ring-opening fluorination of isoxazoles has been developed. Upon treatment of isoxazoles with an electrophilic fluorinating agent (Selectfluor), fluorination followed by deprotonation leads to tertiary fluorinated carbonyl compounds. This method features mild reaction conditions, good functional group tolerance, and a simple experimental procedure. Diverse transformations of the resulting α-fluorocyanoketones were also demonstrated, furnishing a variety of fluorinated compounds.

anti-Markovnikov Iodofluorination of Alkenes

The fluorination of alkenes with electrophilic N-F type reagents usually occurs through a Markovnikov-type addition, and the anti-Markovnikov-type addition may require the use of a transition metal catalyst or an expensive catalyst. Herein we describe a convenient anti-Markovnikov iodofluorination of alkenes with Selectfluor/ nBu4NI. A wide substrate scope and good functional group tolerance were observed. The process allows for the construction of various C-F bonds, especially tertiary C-F bonds. The remarkable features make this protocol attractive, including convenient operations, simple reaction conditions, and the installation of an iodine atom which provides possibilities for further transformations.

Electrochemical Ring-Opening Dicarboxylation of Strained Carbon-Carbon Single Bonds with CO2: Facile Synthesis of Diacids and Derivatization into Polyesters

Diacids are important monomers in the polymer industry to construct valuable materials. Dicarboxylation of unsaturated bonds, such as alkenes and alkynes, with CO₂ has been demonstrated as a promising synthetic method. However, dicarboxylation of C─C single bonds with CO₂ has rarely been investigated. Herein we report a novel electrochemical ring-opening dicarboxylation of C─C single bonds in strained rings with CO₂. Structurally diverse glutaric acid and adipic acid derivatives were synthesized from substituted cyclopropanes and cyclobutanes in moderate to high yields. In contrast to oxidative ring openings, this is also the first realization of an electroreductive ring-opening reaction of strained rings, including commercialized ones. Control experiments suggested that radical anions and carbanions might be the key intermediates in this reaction. Moreover, this process features high step and atom economy, mild reaction conditions (1 atm, room temperature), good chemoselectivity and functional group tolerance, low electrolyte concentration, and easy derivatization of the products. Furthermore, we conducted polymerization of the corresponding diesters with diols to obtain a potential UV-shielding material with a self-healing function and a fluorine-containing polyester, whose performance tests showed promising applications.

Ring-opening fluorination of bicyclic azaarenes

We have discovered a ring-opening fluorination of bicyclic azaarenes. Upon treatment of bicyclic azaarenes such as pyrazolo[1,5-a]pyridines with electrophilic fluorinating agents, fluorination of the aromatic ring is followed by a ring-opening reaction. Although this overall transformation can be classified as an electrophilic fluorination of an aromatic ring, it is a novel type of fluorination that results in construction of tertiary carbon-fluorine bonds. The present protocol can be applied to a range of bicyclic azaarenes, tolerating azines and a variety of functional groups. Additionally, mechanistic studies and enantioselective fluorination have been examined.

Catalytic alkene skeletal modification for the construction of fluorinated tertiary stereocenters

Herein we describe the first construction of fluorinated tertiary stereocenters based on an alkene C(sp²)–C(sp²) bond cleavage. The new process, that takes advantage of a Rh-catalyzed carbyne transfer, relies on a branched-selective fluorination of tertiary allyl cations and is distinguished by a wide scope including natural products and drug molecule derivatives as well as adaptability to radiofluorination.

We report a previously unknown disconnection approach to valuable fluorinated tertiary stereocenters based on the skeletal modification of 1,1-disubstituted alkenes by a Rh-catalyzed carbyne transfer.

Cooperative NHC/Photoredox Catalyzed Ring-Opening of Aryl Cyclopropanes to 1-Aroyloxylated-3-Acylated Alkanes

Cyclopropanes are an important class of building blocks in organic synthesis. Herein, a ring‐opening/arylcarboxylation/acylation cascade reaction for the 1,3‐difunctionalization of aryl cyclopropanes enabled by cooperative NHC and organophotoredox catalysis is reported. The cascade works on monosubstituted cyclopropanes that are in contrast to the heavily investigated donor–acceptor cyclopropanes more challenging to be difunctionalized. The key step is a radical/radical cross coupling of a benzylic radical generated in the photoredox catalysis cycle with a ketyl radical from the NHC catalysis cycle. The transformation features metal‐free reaction conditions and tolerates a diverse range of functionalities.

Ring Expansion Fluorination of Unactivated Cyclopropanes Mediated by a New Monofluoroiodane(III) Reagent

Herein, we report a new strategy for carbon-carbon bond scission and intramolecular ring expansion fluorination of unactivated cyclopropanes, which was accomplished with a new hypervalent fluoroiodane(III) reagent 1. This novel method delivers medicinally relevant 4-fully substituted fluoropiperidines in moderate to high yields with excellent regio- and diastereoselectivity. Reagent 1, which has an N-acetylbenziodazole framework, was readily synthesized via three steps in 76 % overall yield and was characterized by NMR spectroscopy and X-ray crystallography. Owing to the presence of a secondary I⋅⋅⋅O bonding interaction between the λ³ -iodane atom and the carbonyl oxygen of the acetyl group of the N-acetylbenziodazole framework, 1 has excellent stability and can be stored at ambient temperature for 6 months without any detectable decomposition. Density functional theory calculations and experimental studies showed that the reaction proceeds via a carbocation intermediate that readily combines with a fluoride ion to generate the product.

Arylcyclopropane yet in its infancy: the challenges and recent advances in its functionalization

Electronically unbiased arylcyclopropane functionalization has always been a challenge to organic chemists, and the emergence of donor-acceptor cyclopropanes (DACs) has not only vehemently overshadowed them but still dominates the cyclopropane chemistry. Unlike DACs, the absence of pre-installed functional groups makes it harder for them to activate and participate in a reaction. The field has witnessed considerably slow progress since its inception due to the inherent challenges. There are only a few strategies available to open arylcyclopropanes. Therefore, this work is still in its infancy stage in spite of these materials being one of the earliest known type of cyclopropanes. This review manifests the history, endeavors, and achievements alongside the associated challenges, opportunities, and the need for concerted efforts to accomplish the long-awaited golden age of arylcyclopropanes.

Palladium-Catalyzed Fluoroalkylation via C(sp3)-S Bond Cleavage of Vinylsulfonium Salts

An interrupted Pummerer/palladium-catalyzed fluoro-alkylation strategy was developed for alkenyl C-H fluoroalkylthiolation. Palladium-catalyzed ring-opening fluoroalkylation via aliphatic C-S bond cleavage of the vinylsulfonium salts efficiently afforded fluoroalkylthiolated alkene derivatives from readily available alkene substrates and CsF. The protocol features broad substrate scopes and good functional group tolerance under an air atmosphere. The practicability of the synthetic method was demonstrated by transforming the multisubstituted alkene products to diverse fluoroalkylthiolated N-heterocycles.

Electrochemical C-C bond cleavage of cyclopropanes towards the synthesis of 1,3-difunctionalized molecules

Electrochemistry has a lot of inherent advantages in organic synthesis and many redox reactions have been achieved under electrochemical condition. However, the electrochemical C-C bond cleavage and functionalization reactions are less studied. Here we develop electrochemical C-C bond cleavage and 1,3-difuntionalization of arylcyclopropanes under catalyst-free and external-oxidant-free conditions. 1,3-difluorination, 1,3-oxyfluorination and 1,3-dioxygenation of arylcyclopropanes are achieved with a high chemo- and regioselectivity by the strategic choice of nucleophiles. This protocol has good functional groups tolerance and can be scaled up. Mechanistic studies demonstrate that arylcyclopropane radical cation obtained from the anode oxidation and the subsequently generated benzyl carbonium are the key intermediates in this transformation. This development provides a scenario for constructing 1,3-difunctionalized molecules.

Selectfluor-Promoted Intramolecular N-S Bond Formation of α-Carbamoyl Ketene Dithioacetals in the Presence of Water: Synthesis of Multifunctionalized Isothiazolones

A practical and efficient protocol toward fully substituted isothiazolones through Selectfluor-mediated intramolecular oxidative annulation of α-carbamoyl ketene dithioacetals has been developed in the presence of H₂O and metal-free conditions. Notably, the experimental results reveal that H₂O was crucial to the formation of new N-S bonds and the elimination of alkyl group from the sulfur atom. This protocol provides readily prepared substrates and possesses good functional group tolerance, mild reaction conditions, and operational simplicity, which provides potential access to applications in the pharmaceutical chemistry.

Copper-Catalyzed Ring-Opening 1,3-Aminotrifluoromethylation of Arylcyclopropanes

The copper-catalyzed reaction of arylcyclopropanes, N-fluorobis(arenesulfonyl)imides, and (bpy)Zn(CF₃)₂ (bpy = 2,2'-bipyridine) at room temperature affords the corresponding ring-opening 1,3-aminotrifluoromethylation products in satisfactory yields. The protocol is highly regioselective, providing a convenient entry to γ-trifluoromethylated amines. A mechanism involving the trifluoromethylation of benzyl radicals is proposed.

Reactivities of electrophilic N-F fluorinating reagents

Electrophilic fluorination represents one of the most direct and useful methods available for the selective introduction of fluorine into organic compounds. Electrophilic fluorinating reagents of the N-F class have revolutionised the incorporation of fluorine atoms into both pharmaceutically- and agrochemically-important substrates. Since the earliest N-F reagents were commercialised in the 1990s, their reactivities have been investigated using qualitative and, more recently, quantitative methods. This review discusses the different experimental approaches employed to determine reactivities of N-F reagents, focussing on the kinetics studies reported in recent years. We make critical evaluations of the experimental approaches against each other, theoretical approaches, and their applicability towards practical problems. The opportunities for achieving more efficient synthetic electrophilic fluorination processes through kinetic understanding are highlighted.

Dirhodium(ii)-catalyzed diamination reaction via a free radical pathway

Unlike C–N bond formation through the well-known dirhodium(ii)-nitrenoid pathway, dirhodium(ii)-catalyzed 1,2- and 1,3-diamination reactions are realized by a free radical mechanism.

Photo-induced transition-metal and external photosensitizer-free organic reactions

Photoreactions have become powerful synthetic tools with a broad scope of applications. This review mainly focuses on photoreactions in the absence of transition-metals and photosensitizers, and highlights the mechanisms of such reactions.

Quinoline-Fused Lactones via Tandem Oxidation Cyclization: Metal-Free sp3 C-H Functionalization

A unique lactonization of 2-methyl-3-acyl-4-phenylquinolines using PhIO as the oxidant and selectfluor as an additive is reported. The reaction occurs under ambient conditions through tandem oxidation and cyclization of sp³ C-H bonds under metal-free conditions. The heterocycle-fused lactones are obtained in moderate to good yield.

Copper-catalysed radical reactions of alkenes, alkynes and cyclopropanes with N-F reagents

The mild generation of nitrogen-centred radicals from N-F reagents has become a convenient synthetic tool. This methodology provides access to the aminative difunctionalisation of alkenes and alkynes and the radical ring-opening of cyclopropanes, among other similar transformations. This review article aims to provide an overview of recent developments of such processes involving radical reactions and N-F reagents using copper-based catalysts.

Photosensitized direct C-H fluorination and trifluoromethylation in organic synthesis

The importance of fluorinated products in pharmaceutical and medicinal chemistry has necessitated the development of synthetic fluorination methods, of which direct C-H fluorination is among the most powerful. Despite the challenges and limitations associated with the direct fluorination of unactivated C-H bonds, appreciable advancements in manipulating the selectivity and reactivity have been made, especially via transition metal catalysis and photochemistry. Where transition metal catalysis provides one strategy for C-H bond activation, transition-metal-free photochemical C-H fluorination can provide a complementary selectivity via a radical mechanism that proceeds under milder conditions than thermal radical activation methods. One exciting development in C-F bond formation is the use of small-molecule photosensitizers, allowing the reactions i) to proceed under mild conditions, ii) to be user-friendly, iii) to be cost-effective and iv) to be more amenable to scalability than typical photoredox-catalyzed methods. In this review, we highlight photosensitized C-H fluorination as a recent strategy for the direct and remote activation of C-H (especially C(sp3)-H) bonds. To guide the readers, we present the developing mechanistic understandings of these reactions and exemplify concepts to assist the future planning of reactions.