Energy-transfer-induced [3+2] cycloadditions of N-N pyridinium ylides

Stabilized Carbon Radical-Mediated Assembly of Arylthianthrenium Salts, Alkenes and Amino Acid/Peptide Derivatives

Efficiently assembling amino acids and peptides with bioactive molecules facilitates the modular and streamlined synthesis of a diverse library of peptide-related compounds. Particularly notable is their application in pharmaceutical development, leveraging site-selective late-stage functionalization. Here, a visible light-induced three-component reaction involving arylthianthrenium salts, amino acid/peptide derivatives, and alkenes are introduced. This approach utilizes captodatively-stabilized carbon radicals to enable radical-radical C─C coupling, effectively constructing complex bioactive molecules. This method offers a promising alternative route for modular synthesis of peptide-derived bio-relevant compounds.

Regio- and Stereoselective β-Sulfonylamination of Alkynes via Photosensitized Bifunctional N-S Bond Homolysis

Nitrogen central radicals (NCRs) are versatile synthetic intermediates for creating functional nitrogen-containing molecules. Herein, a photosensitized β-sulfonylamination of terminal alkynes as well as acetylene has been established by employing N-sulfonyl heteroaromatics as bifunctional reagents (BFRs) to efficiently deliver versatile (E)-β-sulfonylvinylamines with excellent regio- and stereoselectivities. Mechanistic studies suggest a base-accelerated energy transfer (EnT) photocatalysis involving aromatic NCR formation, radical addition to alkynes, and sulfonylation processes.

Photocatalytic 1,3-oxyheteroarylation of aryl cyclopropanes with azine N-oxides

Cyclopropanes, valuable C3 building blocks in organic synthesis, possess high strain energy and inherent stability. We present an efficient, environmentally benign 1,3-oxyheteroarylation of aryl cyclopropanes using azine N-oxides as bifunctional reagents under visible light irradiation. This metal-free method yields β-pyridyl ketones under mild conditions. Mechanistic studies reveal a photo-induced radical pathway involving single-electron oxidation of both aryl cyclopropanes and azine N-oxides, followed by stepwise ring opening. The dual oxidation mechanism accommodates diverse cyclopropane and azine N-oxide combinations based on their oxidation potentials. This green chemistry method enhances the synthetic utility of aryl cyclopropanes while introducing an efficient strategy for their difunctionalization. The methodology aligns with sustainable organic synthesis principles, offering an environmentally conscious route to valuable synthetic intermediates.

Aziridine Group Transfer via Transient N-Aziridinyl Radicals

Aziridines are the smallest nitrogen-containing heterocycles. Strain-enhanced electrophilicity renders aziridines useful synthetic intermediates and gives rise to biological activity. Classical aziridine syntheses─based on either [2 + 1] cycloadditions or intramolecular substitution chemistry─assemble aziridines from acyclic precursors. Here, we introduce N-aziridinyl radicals as a reactive intermediate that enables the transfer of intact aziridine fragments in organic synthesis. Transient N-aziridinyl radicals are generated by the reductive activation of N-pyridinium aziridines and are directly characterized by spin-trapped EPR spectroscopy. In the presence of O2, N-aziridinyl radicals are added to styrenyl olefins to afford 1,2-hydroxyaziridination products. These results establish aziridinyl radicals as new reactive intermediates in synthetic chemistry and demonstrate aziridine group transfer as a viable synthetic disconnection.

Oxy-pyridinium Ylides Mediated 1,4-Pyridyl/Aryl Translocation

Molecular rearrangement via carbene transfer is a powerful tool to access molecular diversity. Herein, we describe an efficient approach to selective pyridyl/aryl relocation via a rhodium-catalyzed aminoarylation of diazo compounds, providing a promising strategy to access ortho-pyridyl N-alkylated pyridone scaffolds in a single operation. This reaction features the novel reactivity of oxy-pyridinium ylide, rhodium-associated five-membered transition state, and 1,4-pyridyl/aryl relocation. A computational study discloses the initial oxy-pyridinium ylide formation, keto-enol tautomerization, and 1,4-pyridyl migration to complete the whole rearrangement.

Arylthianthrenium Salts for Triplet Energy Transfer Catalysis

Sigma bond cleavage through electronically excited states allows synthetically useful transformations with two radical species. Direct excitation of simple aryl halides to form both aryl and halogen radicals necessitates UV-C light, so undesired side reactions are often observed and specific equipment is required. Moreover, only aryl halides with extended π systems and comparatively low triplet energy are applicable to synthetically useful energy transfer catalysis. Here we show the conceptual advantages of arylthianthrenium salts (ArTTs) for energy transfer catalysis with high energy efficiency compared to conventional aryl (pseudo)halides and their utility in arylation reactions of ethylene. The fundamental advance is enabled by the low triplet energy of ArTTs that may originate in large part from the electronic interplay between the distinct sulfur atoms in the tricyclic thianthrene scaffold, which is not accessible in either simple (pseudo)halides or other conventional sulfonium salts.

Electron donor-acceptor complex enabled photocascade strategy for the synthesis of trans-dihydrofuro[3,2-c]chromen-4-one scaffolds via radical conjugate addition of pyridinium ylide

A visible-light-induced photocascade strategy is disclosed for the synthesis of trans-dihydrofuro[3,2-c]chromen-4-one scaffolds. The photocascade consists of electron donor-acceptor (EDA) complex enabled formation of arylidene coumarinone, followed by 1,4-radical conjugate addition (1,4-RCA) of an in situ generated pyridinium ylide radical (PyYR) towards diastereoselective formation of the trans-dihydrofuro[3,2-c]chromen-4-one scaffold in good to excellent yield. Thorough mechanistic investigations comprising photophysical, spectroscopic, electrochemical and DFT studies provide further insights into the reaction mechanism.

External photocatalyst-free photocycloaddition between triplet vinylnitrenes with 1,3-biradical character and activated olefins under 420 nm LEDs

Herein, we report that triplet vinylnitrenes with 1,3-biradical character can directly participate in photocycloaddition reactions with olefins to produce single diastereomers of the corresponding 1-pyrrolines under 420 nm LEDs in acetonitrile solvent. Moreover, a one-pot method has been developed to produce pyrroles directly through photocycloaddition and oxidation sequences. The excited state of the substrate olefin can sensitize vinyl azide via energy transfer, eliminating the need for an external photocatalyst or sensitizer.

Zwitterionic Acridinium Amidate: A Nitrogen-Centered Radical Catalyst for Photoinduced Direct Hydrogen Atom Transfer

The development of small organic molecules that can convert light energy into chemical energy to directly promote molecular transformation is of fundamental importance in chemical science. Herein, we report a zwitterionic acridinium amidate as a catalyst for the direct functionalization of aliphatic C-H bonds. This organic zwitterion absorbs visible light to generate the corresponding amidyl radical in the form of excited-state triplet diradical with prominent reactivity for hydrogen atom transfer to facilitate C-H alkylation with a high turnover number. The experimental and theoretical investigations revealed that the noncovalent interactions between the anionic amidate nitrogen and a pertinent hydrogen-bond donor, such as hexafluoroisopropanol, are crucial for ensuring the efficient generation of catalytically active species, thereby fully eliciting the distinct reactivity of the acridinium amidate as a photoinduced direct hydrogen atom transfer catalyst.

Visible light-mediated organocatalyzed 1,3-aminoacylation of cyclopropane employing N-benzoyl saccharin as bifunctional reagent

The carboamination of unsaturated molecules using bifunctional reagents is considered an attractive approach for the synthesis of nitrogen-containing compounds. However, bifunctional C-N reagents have never been employed in the carboamination of cyclopropane. In this study, we use an N-heterocyclic carbene (NHC), N-benzoyl saccharin, as a bifunctional reagent and a photoredox catalyst for a dual-catalyzed 1,3-aminoacylation of cyclopropane. NHCs play multiple roles, functioning as Lewis base catalysts to activate C-N bonds, promoting the oxidative quenching process of PC*, and acting as efficient acyl radical transfer catalysts for the formation of C-C bonds. The oxidative quenching process between the excited-state PC* and acyl NHC adduct is the key to the photooxidation generality of aryl cyclopropanes.

Pyridinium-Based Fluorosulfonamide Reagents Enabled Photoredox-Catalyzed Radical Fluorosulfonamidation

Sulfamoyl fluorides, as a crucial building block of SuFEx, have garnered extensive research interest due to their unique properties. However, the direct radical fluorosulfonamidation process for the synthesis of sulfamoyl fluorides has been overlooked. We herein disclosed a practical procedure for constructing a redox-active fluorosulfonamide radical reagent named fluorosulfonyl-N-pyridinium tetrafluoroborate (PNSF) from SO2F2. These reagents can facilitate a range of reactions, including the N-(fluorosulfonyl) sulfonamidation of (hetero)arenes, sequential radical stereoselective fluorosulfonamidation, and 1,2-difunctionalization of alkenes.

Visible light-induced chemoselective 1,2-diheteroarylation of alkenes

Visible-light photocatalysis has evolved as a powerful technique to enable controllable radical reactions. Exploring unique photocatalytic mode for obtaining new chemoselectivity and product diversity is of great significance. Herein, we present a photo-induced chemoselective 1,2-diheteroarylation of unactivated alkenes utilizing halopyridines and quinolines. The ring-fused azaarenes serve as not only substrate, but also potential precursors for halogen-atom abstraction for pyridyl radical generation in this photocatalysis. As a complement to metal catalysis, this photo-induced radical process with mild and redox neutral conditions assembles two different heteroaryl groups into alkenes regioselectively and contribute to broad substrates scope. The obtained products containing aza-arene units permit various further diversifications, demonstrating the synthetic utility of this protocol. We anticipate that this protocol will trigger the further advancement of photo-induced alkyl/aryl halides activation.

Reaction strategies for the meta-selective functionalization of pyridine through dearomatization

The pyridine moiety is a crucial structural component in various pharmaceuticals. While the direct ortho- and para-functionalization of pyridines is relatively straightforward, the meta-selective C-H functionalization remains a significant challenge. This review highlights dearomatization strategies as a key area of interest in expanding the application of meta-C-H functionalization of pyridines. Dearomatization enables the meta-functionalization through various catalytic methods that directly generate dearomatization products, and some products can be rearomatized back to pyridine derivatives. Furthermore, this article also covers the dearomatization of multiple positions of pyridine in the synthesis of polycyclic compounds. It offers a comprehensive overview of the latest advancements in dearomatization at different positions of pyridine, aiming to provide a valuable resource for researchers in this field. It also highlights the advantages and limitations of existing technologies, aiming to inform a broader audience about this important field and foster its future development.

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 Z/E isomerization unlocking the stereodivergent construction of axially chiral alkene frameworks

The past century has witnessed a large number of reports on the Z/E isomerization of alkenes. However, the vast majority of them are still limited to the isomerization of di- and tri-substituted alkenes. The stereospecific Z/E isomerization of tetrasubstituted alkenes remains to be an underdeveloped area, thus lacking in a stereodivergent synthesis of axially chiral alkenes. Herein we report the atroposelective synthesis of tetrasubstituted alkene analogues by asymmetric allylic substitution-isomerization, followed by their Z/E isomerization via triplet energy transfer photocatalysis. In this regard, the stereodivergent synthesis of axially chiral N-vinylquinolinones is achieved efficiently. Mechanistic studies indicate that the benzylic radical generation and distribution are two key factors for preserving the enantioselectivities of axially chiral compounds.

Design of N-N ylide bond-based high energy density materials: a theoretical survey

The generally encountered contradiction between large energy content and stability poses great difficulty in designing nitrogen-rich high-energy-density materials. Although N-N ylide bonds have been classified as the fourth type of homonuclear N-N bonds (besides >N-N<, -N[double bond, length as m-dash]N-, and N[triple bond, length as m-dash]N), accessible energetic molecules with N-N ylide bonds have rarely been explored. In this study, 225 molecules with six types of novel structures containing N-N ylide bonds were designed using density functional theory and CBS-QB3 methods. To guide future synthesis, the effects of substitution on the thermal stability, detonation velocity, and detonation pressure of the structures were evaluated under the premise that the N-N ylide skeleton remains stable. The calculations show that the bond dissociation energy values of the N-N ylide bonds of the designed 225 structures were in the range of 61.21-437.52 kJ mol-1, except for N-1NNH2. Many of the designed structures with N-N ylide bonds exhibit high detonation properties, which are superior to those of traditional energetic compounds. This study convincingly demonstrates the feasibility of the design strategy of introducing an N-N ylide bond to develop new types of energetic materials.

2D-Graph of intermolecular interactions predicts radical character of anion-π* type charge-transfer complexes

The molecular orbital (MO) theory is one of the most useful methods to describe the formation of a new chemical bond between two molecules. However, it is less often employed for modelling non-bonded intermolecular interactions because of the small charge-transfer contribution. Here we introduce two simple descriptors, the energy difference (EDA) of the HOMO of an electron donor and the LUMO of an acceptor against such HOMO-LUMO overlap integral (SDA), to show that the MO theory could give a unified charge-transfer picture of both bonding and non-bonding interactions for two molecules. It is found that similar types of interactions tend to be closer to each other in this 2D graph. Notably, in a transition region from strong bonding to single-electron transfer, the interacting molecular pairs appear to present a "hybrid" between chemical bonding and a radical pair, such as anion-π* interactions. It is concluded that the number of nodes in the HOMO and LUMO play a crucial role in determining the bonding character of the molecular pair.

Photoinduced Direct Electron Transfer between Quinoxalin-2(1H)-ones and Alkyl Carboxylic Acids for C-H Alkylation

The progress of efficient and sustainable approaches for decarboxylative coupling reactions is synthetically appealing due to the structural diversity, lack of toxicity, and widespread commercial accessibility of carboxylic acids. However, the decarboxylation reaction still encounters challenges related to the utilization of oxidants, catalysts, and prefunctionalization conditions. We report herein a mild method that facilitates direct electron transfer between alkyl carboxylic acids and excited-state substrates for C-H alkylation of quinoxalin-2(1H)-ones without the involvement of any catalyst or additive.

Generation of Aromatic N-Heterocyclic Radicals for Functionalization of Unactivated Alkenes

Nitrogen-centered radicals (NCRs) have been widely recognized as versatile synthetic intermediates for the construction of nitrogen containing molecules of high value. As such, there has been a long-standing interest in the field of organic synthesis to develop novel nitrogen-based radicals and explore their inherent reactivity. In this study, we present the generation of aromatic N-heterocyclic radicals and their application in a novel and diverse functionalization of unactivated alkenes. Bench-stable aromatic N-heterocyclic pyridinium salts were employed as crucial NCR precursors, which enabled the efficient conversion of various unactivated alkenes into medicinally relevant alkylated N-heterocyclic amines. This approach offers an unexplored retrosynthetic disconnection for the synthesis of related molecules that commonly possess therapeutic value. Furthermore, this platform can be extended to the synthesis of densely functionalized heterocyclic amines by utilizing disulfides and diethyl bromomalonate as radical quenchers. Mechanistic investigations indicate an energy transfer (EnT) pathway involving the formation of a transient aromatic N-heterocyclic radical, radical addition to unactivated alkenes, and subsequent generation of the amination product through either hydrogen atom transfer (HAT) or radical addition processes.

Chemoselective Quinoline and Isoquinoline Reduction by Energy Transfer Catalysis Enabled Hydrogen Atom Transfer

(Hetero)arene reduction is one of the key avenues for synthesizing related cyclic alkenes and alkanes. While catalytic hydrogenation and Birch reduction are the two broadly utilized approaches for (hetero)arene reduction across academia and industry over the last century, both methods have encountered significant chemoselectivity challenges. We hereby introduce a highly chemoselective quinoline and isoquinoline reduction protocol operating through selective energy transfer (EnT) catalysis, which enables subsequent hydrogen atom transfer (HAT). The design of this protocol bypasses the conventional metric of reduction reaction, that is, the reductive potential, and instead relies on the triplet energies of the chemical moieties and the kinetic barriers of energy and hydrogen atom transfer events. Many reducing labile functional groups, which were incompatible with previous (hetero)arene reduction reactions, are retained in this reaction. We anticipate that this protocol will trigger the further advancement of chemoselective arene reduction and enable the current arene-rich drug space to escape from flatland.

Photocatalytic Enantioselective Hydrosulfonylation of α,β-Unsaturated Carbonyls with Sulfonyl Chlorides

This research explores the enantioselective hydrosulfonylation of various α,β-unsaturated carbonyl compounds via the use of visible light and redox-active chiral Ni-catalysis, facilitating the synthesis of enantioenriched α-chiral sulfones with remarkable enantioselectivity (exceeding 99 % ee). A significant challenge entails enhancing the reactivity between chiral metal-coordinated carbonyl compounds and moderate electrophilic sulfonyl radicals, aiming to minimize the background reactions. The success of our approach stems from two distinctive attributes: 1) the Cl-atom abstraction employed for sulfonyl radical generation from sulfonyl chlorides, and 2) the single-electron reduction to produce a key enolate radical Ni-complex. The latter process appears to enhance the feasibility of the sulfonyl radical's addition to the electron-rich enolate radical. An in-depth investigation into the reaction mechanism, supported by both experimental observations and theoretical analysis, offers insight into the intricate reaction process. Moreover, the versatility of our methodology is highlighted through its successful application in the late-stage functionalization of complex bioactive molecules, demonstrating its practicality as a strategy for producing α-chiral sulfones.

Modular and Practical 1,2-Aryl(Alkenyl) Heteroatom Functionalization of Alkenes through Iron/Photoredox Dual Catalysis

Efficient methods for synthesizing 1,2-aryl(alkenyl) heteroatomic cores, encompassing heteroatoms such as nitrogen, oxygen, sulfur, and halogens, are of significant importance in medicinal chemistry and pharmaceutical research. In this study, we present a mild, versatile and practical photoredox/iron dual catalytic system that enables access to highly privileged 1,2-aryl(alkenyl) heteroatomic pharmacophores with exceptional efficiency and site selectivity. Our approach exhibits an extensive scope, allowing for the direct utilization of a wide range of commodity or commercially available (hetero)arenes as well as activated and unactivated alkenes with diverse functional groups, drug scaffolds, and natural product motifs as substrates. By merging iron catalysis with the photoredox cycle, a vast array of alkene 1,2-aryl(alkenyl) functionalization products that incorporate a neighboring azido, amino, halo, thiocyano and nitrooxy group were secured. The scalability and ability to rapid synthesize numerous bioactive small molecules from readily available starting materials highlight the utility of this protocol.