Metal-Free Photochemical Imino-Alkylation of Alkenes with Bifunctional Oxime Esters

1,3-Difunctionalization of Alkenes by Cobaloxime Photocatalysis

Represented herein is the first 1,3-difunctionalization of alkenes via photocatalysis. A single cobaloxime is used to carry out two catalytic cycles in which cobaloxime is used not only as a photocatalyst to initiate the reaction but also as a metal catalyst for the β-H elimination process. Electron-deficient alkenes, electron-rich alkenes, and unactivated alkenes could be directly converted to 1,3-bisphosphorylated products, even unsymmetric 1,3-bisphosphorylated products, with only H2 as a byproduct under extremely mild reaction conditions.

Photoinduced EnT-mediated sulfonamidylimination of alkenes and (hetero)arenes with iminophenylacetic acid oxime esters

A photoinduced EnT-mediated generation of sulfonamidyl radicals has been accomplished using rationally designed iminophenylacetic acid oxime ester reagents under metal-free conditions. This approach offers a mild, regio- and diastereoselective synthesis of N-sulfonyl diamines via diamination of alkenes and (hetero)arenes.

Two-step synthesis of vicinal trifluoromethyl primary amines from α-(trifluoromethyl)styrenes and phthalimide

A novel two-step synthesis of β-trifluoromethyl primary amines from readily available α-(trifluoromethyl)styrenes and phthalimide is developed. The first step involves a hydroamination between α-(trifluoromethyl)styrenes and phthalimide (PhthNH) with the assistance of a base. Next, the hydrazinolysis of the resulting N-(β-trifluoromethyl-β-arylethyl)phthalimides with hydrazine hydrate affords the desired N-(β-trifluoromethyl-β-arylethyl)amines.

Copper-Catalyzed [3 + 2] Annulation of O-Acyl Oximes with 4-Sulfonamidophenols for the Synthesis of 5-Sulfonamidoindoles and 2-Amido-5-sulfonamidobenzofuran-3(2H)-ones

A copper-catalyzed [3 + 2] annulation of O-acyl oximes with 4-sulfonamidophenols is developed. The advantage of this method lies in the concurrent double activation of two substrates to form nucleophilic enamines and electrophilic quinone monoimines. The substituent on the α-carbon of O-acyl oxime determines two different reaction pathways, thereby leading to the selective generation of 5-sulfonamidoindoles and 2-amido-5-sulfonamidobenzofuran-3(2H)-ones.

Difunctionalization of gem-difluoroalkenes for amination and heteroarylation via metal-free photocatalysis

gem-Difluoroalkenes are widely used building blocks in fluorine chemistry. Herein, a metal-free photocatalytic amination and heteroarylation method of gem-difluoroalkenes with heteroaryl carboxylic acid oxime esters as substrates is reported. This environmentally benign reaction proceeds via radical-radical cross-coupling in energy-transfer-mediated photocatalysis and can be used in the rapid construction of heteroaryl difluoroethylamine scaffolds and late-stage modification of complex pharmaceutical structures.

γ-Amino Alcohols via Energy Transfer Enabled Brook Rearrangement

In the long-standing quest to synthesize fundamental building blocks with key functional group motifs, photochemistry in the recent past has comprehensively established its attractiveness. Amino alcohols are not only functionally diverse but are ubiquitous in the biologically active realm of compounds. We developed bench-stable bifunctional reagents that could then access the sparsely reported γ-amino alcohols directly from feedstock alkenes through energy transfer (EnT) photocatalysis. A designed 1,3-linkage across alkenes is made possible by the intervention of a radical Brook rearrangement that takes place downstream to the EnT-mediated homolysis of our reagent(s). A combination of experimental mechanistic investigations and detailed computational studies (DFT) indicates a radical chain propagated reaction pathway.

Strain-enabled radical spirocyclization cascades: rapid access to spirocyclobutyl lactones and - lactams

Spirocyclobutane derivatives have gained significant attention in drug discovery programs due to their broad spectrum of biological activities and clinical applications. Ring-strain in organic molecules is a powerful tool to promote reactivity by releasing strain energy, allowing the construction of complex molecules selectively and efficiently. Herein, we report the first strain-enabled radical spirocyclization cascades for the synthesis of functionalized spirocyclobutyl lactones and - lactams, which are finding increasing applications in medicinal chemistry. The reaction of interelement compounds with bicyclobutane (BCB) allyl esters and - amides proceeds with high chemoselectivity under simple, catalyst-free conditions using blue light irradiation. The reaction has been successfully extended to synthesize bis-spirocycles. To introduce a more diverse set of functional groups, we have developed a dual photoredox/nickel catalytic system capable of mediating the carbosulfonylation of BCB allyl amides. The reaction shows broad applicability across various (hetero)aryl halides, aryl sulfinates, and BCB allyl amides, operates under mild conditions and demonstrates excellent functional group compatibility. The functional groups introduced during the cascade reactions served as versatile handles for further synthetic elaboration.

Energy transfer photocatalysis: exciting modes of reactivity

Excited (triplet) states offer a myriad of attractive synthetic pathways, including cycloadditions, selective homolytic bond cleavages and strain-release chemistry, isomerizations, deracemizations, or the fusion with metal catalysis. Recent years have seen enormous advantages in enabling these reactivity modes through visible-light-mediated triplet-triplet energy transfer catalysis (TTEnT). This tutorial review provides an overview of this emerging strategy for synthesizing sought-after organic motifs in a mild, selective, and sustainable manner. Building on the photophysical foundations of energy transfer, this review also discusses catalyst design, as well as the challenges and opportunities of energy transfer catalysis.

Visible-Light-Induced Decarboxylative Aminosulfonylation of (Hetero)aryl Carboxylic Oxime Esters

Sulfonamides are important structures in pharmaceuticals, agrochemicals, and organocatalysts, yet the rapid and benign synthesis of these compounds is still a great challenge. Herein we report a photoinduced method for synthesizing sulfonamides from (hetero)aryl carboxylic acid oxime esters. This reaction proceeds via one-pot cascade radical-radical cross-coupling by energy-transfer-mediated photocatalysis. A wide substrate scope including (hetero)aryl substrates and late-stage modification of pharmaceutical molecular entities reveal its generality.

Photoexcitation of (diarylmethylene)amino benziodoxolones for alkylamination of styrene derivatives with carboxylic acids

The alkylamination of alkenes using pristine carboxylic acids was achieved by the photoexcitation of (diarylmethylene)amino benziodoxolones (DABXs), which serve as both an oxidant and an aminating reagent (an iminyl radical precursor). The developed method is a simple photochemical reaction without the need for external photosensitizers and shows a broad substrate scope for aliphatic carboxylic acids leading to the formation of primary, secondary, and tertiary alkyl radicals, thus enabling the facile synthesis of various structurally complex amines. Mechanistic investigations including transient absorption spectroscopy measurements using a laser flash photolysis (LFP) method disclosed the unique photochemical reactivity of DABXs, which undergoes homolysis of their I-N bonds to give an iminyl radical and ortho-iodobenzoyloxy radical, the latter of which participates in the single-electron oxidation of carboxylates.

Photocatalyzed Amination of Alkyl Halides to Access Primary Amines

We demonstrate that oxime ester derivatives can be used as both a halogen atom transfer (XAT) agent and an imine source under photocatalytic conditions, allowing the radical amination of alkyl halides, resulting in the formation of a broad scope of imines. Hydrolysis of the latter gives direct access to the corresponding primary amines. Mechanistically, the reaction is believed to proceed through the formation of aryl radical intermediates, which are responsible for the activation of alkyl halides via XAT.

Modular access to alkylgermanes via reductive germylative alkylation of activated olefins under nickel catalysis

Carbon-introducing difunctionalization of C-C double bonds enabled by transition-metal catalysis is one of most straightforward and efficient strategies to construct C-C and C-X bonds concurrently from readily available feedstocks towards structurally diverse molecules in one step; however, analogous difunctionalization for introducing germanium group and other functionalities remains elusive. Herein, we describe a nickel-catalyzed germylative alkylation of activated olefins with easily accessible primary, secondary and tertiary alkyl bromides and chlorogermanes as the electrophiles to form C-Ge and C-Calkyl bonds simultaneously. This method provides a modular and facile approach for the synthesis of a broad range of alkylgermanes with good functional group compatibility, and can be further applied to the late-stage modification of natural products and pharmaceuticals, as well as ligation of drug fragments. More importantly, this platform enables the expedient synthesis of germanium substituted ospemifene-Ge-OH, which shows improved properties compared to ospemifene in the treatment of breast cancer cells, demonstrating high potential of our protocol in drug development.

Visible-Light-Promoted Intermolecular β-Acyl Difunctionalization of Alkenes via Oxidative Radical-Polar Crossover

A visible-light-induced β-acyl difunctionalization of alkenes with acyl oxime esters and various nucleophiles was developed to achieve molecular complexity from readily available raw materials via oxidative radical-polar crossover. A variety of nucleophiles, including NH-sulfoximines, indoles, indazole, and trimethoxybenzene, were all effectively applicable to the sustainable reaction system. The novel synthetic strategy features mild reaction conditions, a broad substrate scope (39 examples), easy scale-up, and excellent regioselectivity.

Three-Component Photochemical 1,2,5-Trifunctionalizations of Alkenes toward Densely Functionalized Lynchpins

Radical remote 1,n-difunctionalization reactions (n > 2) of alkenes are powerful tools to efficiently introduce functional groups with selected distances into target molecules. Among these reactions, 1,5-difunctionalizations are an important subclass, leading to sought-after scaffolds, but typically suffer from tailored starting materials and strict limitations for the formed functional group in 2-position. Seeking to address these issues and to make radical 1,5-difunctionalizations of alkenes more applicable, we report a novel three-component 1,2,5-trifunctionalization reaction between imine-based bifunctional reagents and two distinct alkenes, driven by visible light energy transfer-catalysis. Key to achieving this selective one-step installation of three different functional groups via the choreographed formation of four bonds was the utilization of a 1,2-boron shift and the rigorous capitalization of radical polarities and stabilities. Thorough mechanistic studies were carried out, and the synthetic utility of the obtained products was demonstrated by various downstream modifications. Notably, in addition to the functionalization of individual functional groups, their interplay gave rise to a unique array of cyclic products.

Photochemical Oximesulfonylation of Alkenes Using Sulfonyl-Oxime-Ethers as Bifunctional Reagents

Utilization of oxime ethers as bifunctional reagents remains unknown. Herein, we present a mechanistically distinct strategy that enables oximesulfonylation of olefins using sulfonyl-oxime-ethers as bifunctional reagents under metal-free photochemical conditions. Via concomitant C-S and C-C bond formation, the process permits incorporation of oxime and sulfonyl groups into olefins in a complete atom-economic fashion, providing rapid access to multi-functionalized β-sulfonyl oxime ethers with good yields and stereoselectivity. The method is amenable to functionalization of complex bioactive molecules and is shown to be scalable. A radical chain mechanism initiated via photochemical Hydrogen Atom Transfer (HAT) mediated N-O bond cleavage is suggested for the process, based on our results on mechanistic investigations.

Photo-induced imino functionalizations of alkenes via intermolecular charge transfer

A catalyst-free photosensitized strategy has been developed for regioselective imino functionalizations of alkenes via the formation of an EDA complex. This photo-induced protocol facilitates the construction of structurally diverse β-imino sulfones and vinyl sulfones in moderate to high yields. Mechanistic studies reveal that the reaction is initiated with an intermolecular charge transfer between oximes and sulfinates, followed by fragmentation to generate a persistent iminyl radical and transient sulfonyl radical. This catalyst-free protocol also features excellent regioselectivity, broad functional group tolerance and mild reaction conditions. The late stage functionalization of natural product derived compounds and total synthesis of some bioactive molecules have been demonstrated to highlight the utility of this protocol. Meanwhile, the compatibility of different donors has proved the generality of this strategy.

The Merger of Halogen Atom Transfer (XAT) and Energy Transfer Catalysis (EnT) for the Modular 1,2-Iminylalkylation of Diazenes

The 1,2-iminylalkylation of diazenes using alkyl iodides in combination with an O-benzoyl oxime is reported. In this transformation, O-benzoyl oxime acted as a radical precursor and XAT mediator. In addition to common alkyl iodides, other alkyl iodides such as iodomethane, iodomethane-d3, trifluoroiodomethane, ethyl difluoroiodoacetate, and iodoalkanes containing unprotected hydroxyl and amide groups can also serve as C-radical precursors in the 1,2-iminylalkylation with electrophilic diazenes as radical acceptors.

Photoinduced Difunctionalization of Diazenes Enabled by N-N Radical Coupling

In this study, a metal-free difunctionalization strategy for diazenes was developed using a range of bifunctionalization reagents. This strategy involves a unique N(sp3)-N(sp2) radical coupling between the hydrazine radical and the imine radical. More than 30 triazane core motifs were constructed by installing imines and various functional groups, including alkyl, phenyl, cyanoalkyl, and sulfonyl groups, on both ends of the nitrogen-nitrogen bond of diazenes in an efficient manner.

Photocatalytic Multicomponent 1,n-Carboimination with Alkyl Iodides and O-Benzoyl Oxime through EnT and XAT Processes

In this study, we developed a strategy using commercially available alkyl iodides and O-benzoyl oxime to efficiently introduce alkyl and iminyl groups via energy transfer and halogen-atom transfer processes. We performed three-component 1,2-carboimination of olefins and four-component 1,4-carboimination across olefins and alkynes, resulting in the synthesis of over 60 nitrogen-containing molecules. Moreover, this transformation enables the synthesis of molecules with sensitive groups that were previously difficult to achieve.

Metal-Free Amino(hetero)arylation and Aminosulfonylation of Alkenes Enabled by Photoinduced Energy Transfer

β-(Hetero)arylethylamines are privileged structural motifs found in many high-value organic molecules, including pharmaceuticals and natural products. To construct these important molecular skeletons, previous methods are mainly achieved by amino(hetero)arylation reaction with the aid of transition metals and preactivated substrates. Herein, we report a metal-free and photoinduced intermolecular amino(hetero)arylation reaction for the single-step installation of both (hetero)aryl and iminyl groups across alkenes in an efficient and regioselective manner. This method shows broad scope (up to 124 examples) and excellent tolerance of various olefins─from the simplest ethylene to complex multisubstituted alkenes can all participate in the reaction. Furthermore, aminosulfonylation of alkenes can be also conducted in the presence of sodium bisulfite as the SO2 source.

EnT-Mediated Amino-Sulfonylation of Alkenes with Bifunctional Sulfonamides: Access to β-Amino Sulfone Derivatives

β-Amino sulfones are commonly found structural motifs in biologically active compounds. Herein, we report a direct photocatalyzed amino-sulfonylation reaction of alkenes for the efficicient production of important compounds by simple hydrolysis without the need for additional oxidants and reductants. In this transformation, the sulfonamides worked as bifunctional reagents, simultaneously generating sulfonyl radicals and N-centered radicals which were added to alkene in a highly atom-economical fashion with high regioselectivity and diastereoselectivity. This approach showed high functional group tolerance and compatibility, facilitating the late-stage modification of some bioactive alkenes and sulfonamide molecules, thereby expanding the biologically relevant chemical space. Scaling up this reaction led to an efficient green synthesis of apremilast, one of the best-selling pharmceuticals, demonstrating the synthetic utility of the applied method. Moreover, mechanistic investigations suggest that an energy transfer (EnT) process was in operation.

Visible-Light-Induced Radical Cascade Cross-Coupling via C(sp3)-H Activation and C-N/N-O Cleavage: Feasible Access to Methylenebisamide Derivatives

Here we report facile and manipulable access to methylenebisamide derivatives via visible-light-driven radical cascade processes incorporating C(sp³)-H activation and C-N/N-O cleavage. Mechanistic studies reveal that a traditional Ir-catalyzed photoredox pathway and a novel copper-induced complex-photolysis pathway are both involved, contributing to activating the inert N-methoxyamides and rendering the valuable bisamides. This approach exhibits many advantages, including mild reaction conditions, broad scope and functional group tolerance, and competitive step economy. Given the mechanistic plenitude and operational simplicity, we believe this package deal paves a promising way for the synthesis of valuable nitrogen-containing molecules.

Photochemical and Atom-Economical Sulfonylimination of Alkenes with Bifunctional N-Sulfonyl Ketimine

An organo-photocatalytic sulfonylimination of alkenes was developed by employing readily available N-sulfonyl ketimines as bifunctional reagents. This transformation, featuring prominent functional group tolerance, provides a direct and atom-economic approach for the synthesis of valuable β-amino sulfone derivatives as a single regioisomer. In addition to terminal alkenes, internal alkenes participate in this reaction with high diastereoselectivity. N-Sulfonyl ketimines with aryl or alkyl substituents were found to be compatible with this reaction condition. This method could be applied in the late-stage modifications of drugs. Additionally, a formal insertion of alkene into cyclic sulfonyl imine was observed, affording a ring expansion product.

Photochemical Alkene Trifluoromethylimination Enabled by Trifluoromethylsulfonylamide as a Bifunctional Reagent

Herein, we disclose a facile and versatile trifluoromethylimination of alkene with a rationally designed N-(diphenylmethylene)-1,1,1-trifluoromethanesulfonamide as a bench-stable and readily accessible carboamination reagent. Enabled by an energy transfer (EnT) process, an array of alkenes were able to be facilely CF₃-iminated under metal-free photocatalytic conditions. The mild reaction conditions and good functional group compatibility render this protocol highly valuable for the difunctionalization of olefins with structural complexity and diversity.

Photosensitized Vicinal Sulfonylamination of Alkenes with Oxime Ester and DABCO·(SO2)2

A metal-free photosensitized three-component reaction of oxime esters, alkenes, and DABCO·(SO₂)₂ was developed. This protocol could accommodate a wide substrate scope, including activated and unactivated alkenes and aryl and aliphatic carboxylic acid oxime esters, delivering a broad range of β-amino sulfones in moderate to high yields. The insertion of SO₂ as a linker moiety allows the manipulation of the functionality in the reaction process, expanding the utility of oxime esters as bifunctional reagents.

Metal-free photosensitized radical relay 1,4-carboimination across two distinct olefins

Intermolecular carboamination of olefins offers a powerful platform for the rapid construction of structurally complex amines from abundant feedstocks. However, these reactions often require transition-metal catalysis, and are mainly limited to 1,2-carboamination. Herein, we report a novel radical relay 1,4-carboimination across two distinct olefins with alkyl carboxylic acid-derived bifunctional oxime esters via energy transfer catalysis. The reaction is highly chemo- and regioselective, and multiple C-C and C-N bonds were formed in a single orchestrated operation. This mild and metal-free method features a remarkably broad substrate scope with excellent tolerance of sensitive functional groups, therefore providing easy access to structurally diverse 1,4-carboiminated products. Moreover, the obtained imines could be easily converted into valuable biologically relevant free γ-amino acids.

EnT-Mediated N-S Bond Homolysis of a Bifunctional Reagent Leading to Aliphatic Sulfonyl Fluorides

Sulfur(VI) fluoride exchange (SuFEx) gives rise to a plethora of high-valent sulfur linkages; however, the availability of (aliphatic) sulfonyl fluoride manifolds lag behind, owing to the limited sources of introducing the SO₂F moiety via a classical two-electron approach. Recently, radical-based methodologies have emerged as a complementary strategy to increase the diversity of accessible click partners. In this work, synthesis of a bench-stable sulfamoyl fluoride reagent is presented, which may undergo sigma-bond homolysis upon visible-light-induced sensitization to form protected β-amino sulfonyl fluorides from alkene feedstocks. Notably, this offers an appealing strategy to access various building blocks for peptido sulfonyl fluorides, relevant in a medicinal chemistry context, as well as an intriguing entry to β-ammonium sulfonates and β-sultams, from alkenes. Densely functionalized 1,3-sultones were obtained by employing allyl alcohols as substrates. Surprisingly, allyl chloride-derived β-imino sulfonyl fluoride underwent S-O bond formation and ring closure to yield rigid cyclopropyl β-imino sulfonate ester under SuFEx conditions. Furthermore, by engaging a thiol-based hydrogen atom donor in the reaction, the reactivity of the same reagent can be tuned toward the direct synthesis of aliphatic sulfonyl fluorides. Mechanistic experiments indicate an energy transfer (EnT)-mediated process. The transient sulfonyl fluoride radical adds to the alkene and product formation occurs upon either radical-radical coupling or hydrogen atom transfer (HAT), respectively.

Highly Selective Radical Relay 1,4-Oxyimination of Two Electronically Differentiated Olefins

Radical addition reactions of olefins have emerged as an attractive tool for the rapid assembly of complex structures, and have plentiful applications in organic synthesis, however, such reactions are often limited to polymerization or 1,2-difunctionalization. Herein, we disclose an unprecedented radical relay 1,4-oxyimination of two electronically differentiated olefins with a class of bifunctional oxime carbonate reagents via an energy transfer strategy. The protocol is highly chemo- and regioselective, and three different chemical bonds (C-O, C-C, and C-N bonds) were formed in a single operation in an orchestrated manner. Notably, this reaction provides rapid access to a large variety of structurally diverse 1,4-oxyimination products, and the obtained products could be easily converted into valuable biologically relevant δ-hydroxyl-α-amino acids. With a combination of experimental and theoretical methods, the mechanism for this 1,4-oxyimination reaction has been investigated. Theoretical calculations reveal that a radical chain mechanism might operate in the reaction.