Oxidative Fluorination of Cyclopropylamides through Organic Photoredox Catalysis

Silver-Catalyzed Carbofluorination of Olefins and α-Fluoroolefins with Carbamoyl Radicals

The reactivity of carbamoyl radicals, generated in situ from sodium oxamate salts, has been investigated in the context of radical carbofluorination reactions of olefins and α-fluoroolefins, respectively. Both transformations are catalyzed by silver salts and required the presence of potassium persulfate (K2S2O8) and SelectfluorTM as a radicophilic fluorine source. The reported methods provide a direct access to β-fluoroamides and β,β-difluoroamides.

Cooperative Photoredox and Cobalt-Catalyzed Acceptorless Dehydrogenative Functionalization of Cyclopropylamides towards Allylic N,O-Acyl-acetal Derivatives

We disclose herein a novel photoredox and cobalt co-catalyzed ring-opening/acceptorless dehydrogenative functionalization of mono-donor cyclopropanes. This sustainable and atom-economic approach allows the rapid assembly of a wide range of allylic N,O-acyl-acetal derivatives. The starting materials are readily available and the reaction features mild conditions, broad substrate scope, and excellent functional group compatibility. The optimized conditions accommodate assorted cycloalkylamides and primary, secondary, and tertiary alcohols, with applications in late-stage functionalization of pharmaceutically relevant compounds, stimulating further utility in medicinal chemistry. Moreover, selective nucleophilic substitutions with various carbon nucleophiles were achieved in a one-pot fashion, offering a reliable avenue to access some cyclic and acyclic derivatives.

Integrating Olefin Carboamination and Hofmann-Löffler-Freytag Reaction by Radical Deconstruction of Hydrazonyl N-N Bond

As a type of elementary organic compounds containing N-N single bond, hydrazone involved chemical conversions are extremely extensive, but they are mainly limited to N2-retention and N2-removal modes. We report herein an unprecedented protocol for the realization of division utilization of the N2-moiety of hydrazone by a radical facilitated N-N bond deconstruction strategy. This new conversion mode enables the successful combination of alkene carboamination and Hofmann-Löffler-Freytag reaction by the reaction of N-homoallyl mesitylenesulfonyl hydrazones with ethyl difluoroiodoacetate under photocatalytic redox neutral conditions. Mechanism studies reveal that the reaction undergoes a radical relay involving addition, crucial remote imino-N migration and H-atom transfer. Consequently, a series of structurally significant ϵ-N-sulphonamide-α,α-difluoro-γ-amino acid esters are efficiently produced via continuous C-C bond and dual C-N bonds forging.

Photocatalytic (3 + 3) Annnulation of Vinyldiazo Compounds and Aminocyclopropanes

A (3 + 3) annulation of aminocyclopropanes and vinyldiazo compounds enabled by organo-photocatalysis is described. The reaction allows the regioselective synthesis of cyclohexenes bearing adjacent amino and carbonyl groups with broad functional group tolerance. In a departure from previous reports, our work demonstrated that a distonic radical cation can be preferentially intercepted by weakly nucleophilic vinyldiazo compounds, followed by an exclusive 6-endo radical cyclization for ring closure. Based on the interaction between adjacent amino and ester groups, the products can be further converted to cyclohexene-fused 1,3-oxazinane and azetidine.

Iron(II)-Catalyzed Radical [3 + 2] Cyclization of N-Aryl Cyclopropylamines for the Synthesis of Polyfunctionalized Cyclopentylamines

A facile iron(II)-catalyzed radical [3 + 2] cyclization of N-aryl cyclopropylamines with various alkenes to access the structurally polyfunctionalized cyclopentylamine scaffolds has been developed. Using low-cost FeCl2·4H2O as catalyst, N-aryl cyclopropylamines could be utilized to react with a wide range of alkenes including exocyclic/acyclic terminal alkenes, cycloalkenes, alkenes from the natural-occurring compounds (Alantolactone, Costunolide), and known drugs (Ibuprofen, l-phenylalanine, Flurbiprofen) to obtain a variety of cyclopentylamines fused with different useful motifs in generally good yields and diastereoselectivities. The highlight of this protocol is also featured by no extra oxidant, no base, EtOH as the solvent, gram-scale synthesis, and further diverse transformations of the synthetic products. More importantly, an iron(II)-mediated hydrogen radical dissociation pathway was proposed based on the mechanism research experiments.

Electrochemical Ring-Opening of Cyclopropylamides with Alcohols toward the Synthesis of 1,3-Oxazines

An electrochemical method is presented to construct 1,3-oxazines by the oxidative ring-opening of cyclopropylamides with alcohols. This method avoids the use of external oxidants and thus shows good functional group tolerance. The substrate scope covers primary, secondary, and tertiary alcohols as well as (hetero)aryl amide-substituted cyclopropanes.

Silver-Catalyzed Decarboxylative Remote Fluorination via a Zwitterion-Promoted 1,4-Heteroaryl Migration

A silver-catalyzed decarboxylative remote fluorination via a zwitterion-promoted 1,4-heteroaryl migration has been developed. A variety of heteroaryl-tethered benzyl fluorides have been readily synthesized with good regioselectivity under mild conditions. The zwitterion of the substrate is suggested to accelerate the 1,4-heteroaryl migration, which determines the regioselectivity of this transformation.

Synthesis of 3-aminotetrahydro-1H-carbazols by visible-light photocatalyzed cycloaddition of cyclopropylanilines with 2-alkenylarylisocyanides

A visible-light-induced cycloaddition between 2-alkenylarylisocyanides and cyclopropylanilines is reported. This cascade radical reaction constructs two new C-C bonds and two rings to afford 3-aminotetrahydro-1H-carbazols with high atom and step economy. The mechanism is rationalized as involving sequential distonic radical cation formation/isocyanide insertion/5-exo-trig cyclization/intramolecular iminium ion addition/tautomerization.

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.

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.

Photochemical Intermolecular [3σ + 2σ]-Cycloaddition for the Construction of Aminobicyclo[3.1.1]heptanes

The development of synthetic strategies for the preparation of bioisosteric compounds is a demanding undertaking in medicinal chemistry. Numerous strategies have been developed for the synthesis of bicyclo[1.1.1]pentanes (BCPs), bridge-substituted BCPs, and bicyclo[2.1.1]hexanes. However, progress on the synthesis of bicyclo[3.1.1]heptanes, which serve as meta-substituted arene bioisosteres, has not been previously explored. Herein, we disclose the first photoinduced [3σ + 2σ] cycloaddition for the synthesis of trisubstituted bicyclo[3.1.1]heptanes using bicyclo[1.1.0]butanes and cyclopropylamines. This transformation not only uses mild and operationally simple conditions but also provides unique meta-substituted arene bioisosteres. The applicability of this method is showcased by simple derivatization reactions.

Synthesis of remote fluoroalkenyl ketones by photo-induced ring-opening addition of cyclic alkoxy radicals to fluorinated alkenes

Fluoroalkenyl moieties are often used as carbonyl mimics in medicine preparation, and thus the development of facile routes for the synthesis of such compounds is of great importance. In this work, we report a photocatalytic ring-opening addition of cyclic alcohols to α-(trifluoromethyl)styrenes, which underwent a proton-coupled electron transfer and β-scission process, delivering a great variety of remote gem-difluoroalkenyl ketone derivatives. This methodology can also be applied in the reaction of gem-difluorostyrenes and 1,1,2-trifluorostyrenes to access monofluoro- and 1,2-difluoroalkenyl ketones.

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.

Activation of aminocyclopropanes via radical intermediates

Aminocyclopropanes are versatile building blocks for accessing high value-added nitrogen-containing products. To control ring-opening promoted by ring strain, the Lewis acid activation of donor-acceptor substituted systems is now well established. Over the last decade, alternative approaches have emerged proceeding via the formation of radical intermediates, alleviating the need for double activation of the cyclopropanes. This tutorial review summarizes key concepts and recent progress in ring-opening transformations of aminocyclopropanes via radical intermediates, divided into formal cycloadditions and 1,3-difunctionalizations.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

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.

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.

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.

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.

Diamine Synthesis via the Nitrogen-Directed Azidation of σ- and π-C-C Bonds

Diamines are essential building blocks for the synthesis of agrochemicals, drugs, and organic materials, yet their synthesis remains challenging, as both nitrogens need to be differentiated and diverse substitution patterns (1,2, 1,3, or 1,4) are required. We report herein a new strategy giving access to 1,2, 1,3, and 1,4 amido azides as orthogonally protected diamines based on the nitrogen-directed diazidation of alkenes, cyclopropanes, and cyclobutanes. Commercially available copper thiophene-2-carboxylate (CuTc, 2 mol %) as catalyst promoted the diazidation of both π and σ C-C bonds within 10 min in the presence of readily available oxidants and trimethylsilyl azide. Selective substitution of the formed α-amino azide by carbon nucleophiles (electron-rich aromatic, malonate, organosilicon, organoboron, organozinc, and organomagnesium compounds) was then achieved in a one-pot fashion, leading to the formation of 1,2-, 1,3-, and 1,4-diamines with the amino groups protected orthogonally as an amide/carbamate and an azide.