Cyclic Bifunctional Reagents Enabling a Strain-Release-Driven Formal [3 + 2] Cycloaddition of 2H-Azirines by Cascade Energy Transfer

The energy transfer (EnT)-catalyzed ring opening and further decarboxylation of isoxazole-5(4H)-ones enables the in situ generation of strained 2H-azirines. Subsequent selective C(sp2)-C(sp3) bond cleavage of the azirine intermediate allows a formal [3 + 2] cycloaddition with a wide range of electrophiles, unlocking access to valuable pyrroline-type moieties. Mechanistic experiments in combination with density functional theory (DFT) calculations revealed the unique nature of the EnT-cascade process for the generation and ring opening of the three-membered aza-cycle while providing insight into the regio- and diastereoselectivity of the annulation. This mild and straightforward method ensures the rapid construction of highly substituted cyclic imines, which can be easily converted into pyrrolidines, fused oxaziridines, and biologically relevant β-amino acid precursors.

Energy transfer-mediated carbonylimination of alkenes using a bifunctional reagent

A photoinduced EnT-mediated generation of acyl radicals has been accomplished using rationally designed acyl oxime ether reagents under metal-free conditions. This approach offers a mild and practical method for the synthesis of valuable β-aminoketones.

Synthesis of α-Chloroboronic Esters via Photoredox-Catalyzed Chloro-Alkoxycarbonylation of Vinyl Boronic Esters

α-Chloroboronic esters are a class of stable multifunctional molecules that show unique applications in pharmaceutical science and organic chemistry. Despite their apparent utility, the synthetic methods of these compounds remain limited. Herein, a novel strategy for the efficient synthesis of α-chloroboronic esters is developed via photoredox-catalyzed chloro-alkoxycarbonylation of vinyl boronic esters. This strategy features the advantages of high atom economy, environmental friendliness, and excellent functional group compatibility and was verified by the cross-coupling of a variety of free alcohols, oxalyl chlorides, and vinyl boronic esters. Control experiments and mechanistic studies indicate that the alkoxycarbonyl radical and α-boryl carbocation are key intermediates in this transformation.

Sequential Photocatalysis for Homologative Diversification of α-Amino Acids to β-Amino Acids Via Phosphonium Ylide Linchpin Strategy

β-Amino acids serve as crucial building blocks for a broad range of biologically active molecules and peptides with potential as peptidomimetics. While numerous methods have been developed for the synthesis of β-amino acids, most of them require multistep preparation of specific reagents and substrates, which limits their synthetic practicality. In this regard, a homologative transformation of abundant and readily available α-amino acids would be an attractive approach for β-amino acid synthesis. Herein, we disclose the development of a sequential process to provide diverse β-amino acids from α-amino acid derivatives and commercially available phosphonium ylides via visible light photoredox catalysis. In this two-step protocol, phosphonium ylides function as a bifunctional linchpin: they act as a carbon nucleophile to forge a C-C bond in the first step and as a carbon-centered radical source for diverse modifications of the β-amino acid scaffold in the second step. The orthogonal activation of these reactivities under mild photocatalytic conditions enables a modular three-component assembly to access β-amino acids and dipeptides with high structural diversity.

Photocatalytic synthesis of β-amino acid derivatives from alkenes with alkyl formates

The aminocarbonylation of alkenes is an efficient approach to synthesize important β-amino acid motifs. However, simple and convenient methods are still rare. Herein, we present a novel visible-light-mediated controllable three-component radical relay coupling of alkenes, alkyl formates and oxime esters. By the combination of hydrogen atom transfer and energy transfer processes, a series of β-amino esters could be obtained smoothly in one step under mild conditions. We expect that the approach can complement current methodologies for the synthesis of β-amino esters.

Direct C(sp3)-H functionalization with aryl and alkyl radicals as intermolecular hydrogen atom transfer (HAT) agents

Recent years have witnessed the emergence of direct intermolecular C(sp3)-H bond functionalization using in situ generated aryl/alkyl radicals as a unique class of hydrogen atom transfer (HAT) agents. A variety of precursors have been exploited to produce these radical HAT agents under photocatalytic, electrochemical or thermal conditions. To date, viable aryl radical precursors have included aryl diazonium salts or aryl azosulfones, diaryliodonium salts, O-benzoyl oximes, aryl sulfonium salts, aryl thioesters, and aryl halides; and applicable alkyl radical sources have included tetrahalogenated methanes (e.g., CCl3Br, CBr4 and CF3I), N-hydroxyphthalimide esters, alkyl bromides, and acetic acid. This review summarizes the current advances in direct intermolecular C(sp3)-H functionalization through key HAT events with in situ generated aryl/alkyl radicals and categorizes the procedures by the specific radical precursors applied. With an emphasis on the reaction conditions, mechanisms and representative substrate scopes of these protocols, this review aims to demonstrate the current trends and future challenges of this emerging field.

Photocatalyzed Imino-Difluoromethylation of Alkenes with Bifunctional Oxime Esters

Herein, we report a simple and versatile difluoromethylene-imide reaction in which a series of olefins can undergo a difluoromethylenimine reaction under photocatalytic conditions through an energy transfer (EnT) process. The reaction has mild conditions and a wide range of applicability. We successfully synthesized 27 molecules containing difluoromethylene units, featuring easily accessible starting materials and operational simplicity.

Energy-Transfer Enabled Divergent Synthesis of Polycyclic γ-Sultines

A visible-light-initiated energy-transfer enabled radical cyclization for the divergent synthesis of polycyclic γ-sultine derivatives has been developed. The reaction provides an alternative and expeditious access to benzofused γ-sultine frameworks, the analogues of γ-lactones and γ-sultones, and features good functional group compatibility, mild reaction conditions and excellent diastereoselectivity. The robustness and application potential of this method have also been successfully displayed by two gram-scale reactions and the synthesis of polycyclic sultones. Mechanistic studies indicated the transformations through a possible energy-transfer enabled intramolecular radical homolytic substitution or hydrogen atom transfer process mainly.

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.

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.

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.

Photoredox/Nickel Dual Catalysis-Enabled Cross-Dehydrogenative C-H Amination of Indoles with Unactivated Amine

Herein, a direct cross-dehydrogenative C-H amination of indoles has been successfully achieved, enabled by the merger of photocatalysis with nickel catalysis. This developed process does not require stoichiometric oxidants and prefunctionalization of amine partners, providing a concise platform for C-N bond formation. Moreover, the synthetic practicality of this transformation was well revealed by its high step- and atom-economy, high reaction efficiency, and broad functional group tolerance.

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.

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.

A Photosensitizer for N-O Bond Activation: 2,7-Br-4CzIPN-Catalyzed Difunctionalization of Alkenes with Oxime Esters

We developed 2,4,5,6-tetrakis(2,7-dibromo-9H-carbazol-9-yl)isophthalonitrile (2,7-Br-4CzIPN) as a new photosensitizer for the energy-transfer-driven N-O bond dissociation of oxime esters. In the presence of 2,7-Br-4CzIPN, difunctionalization of alkenes with oxime esters, including oxyimination, aminocarboxylation, and amidylimination, could afford a variety of versatile molecules in good yields with excellent regioselectivity, which widely occur in natural products and drugs. Our theoretical investigations and experiments have demonstrated that 2,7-Br-4CzIPN has unique photophysical properties, favorable triplet energy, and excellent photocatalytic activity.

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.

Visible-Light-Induced 1,7-Enyne Dicyclization: Synthesis of Ester-Substituted Benzo[j]phenanthridines

A novel alkoxycarbonyl-radical-triggered cascade cyclization of 1,7-enynes, with alkyloxalyl chlorides as the ester units, for the synthesis of benzo[j]phenanthridines is described. The reaction conditions exhibit excellent compatibility with a broad range of alkoxycarbonyl radical sources and realize the installation of an ester group in the polycyclic compound. This radical cascade cyclization reaction features excellent functional group tolerance, mild reaction conditions, and good to excellent yields.

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.

Photocatalytic 1,2-Iminosulfonylation and Remote 1,6-Iminosulfonylation of Olefins

A new class of iminosulfonylation reagents were developed and extensively used in the 1,2-iminosulfonylation of various olefins. Olefins containing bioactive molecules, such as indomethacin, gemfibrozil, clofibrate, and fenbufen, afforded the desired iminosulfonylation products in synthetically useful yields. Furthermore, the first remote 1,6-iminosulfonylation of alkenes was realized by using oxime ester bifunctionalization reagents. Overall, more than 40 structurally diverse β-imine sulfones were obtained in moderate to excellent yields.

Photoinduced Remote C(sp3)-H Imination Enabled by Vinyl Radical-Mediated 1,5-Hydrogen Atom Transfer

A vinyl radical-mediated 1,5-hydrogen atom transfer strategy for remote C(sp³)-H imination under visible-light-induced photochemical metal-free conditions afforded diverse γ-imino alkenes with excellent stereoselectivity. Oxime ester-based bifunctional reagents provided not only nucleophilic alkyl radicals for radical addition reactions with electron-deficient alkynes but also long-lived steady-state imine radicals for trapping alkyl radicals following the intramolecular 1,5-hydrogen migration of unstable olefin radicals.

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.

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.

Photocatalytic Modular Cyanoalkylamination of Alkenes Involving Two Different Iminyl Radicals

The modular cyanoalkylamination of alkenes using bench-stable and easy-to-handle α-imino-oxy acid oxime esters as difunctional reagents creates new synthetic avenues. A metal-free photosensitization protocol for the installation of both amino and cyanoalkyl functionalities onto alkene feedstocks in a single step via two differently reactive nitrogen-centered radicals was developed via energy-transfer catalysis. Excellent functional group tolerance and mild reaction conditions also render this protocol suitable for the cyanoalkylamination of pharmaceutically relevant molecule-derived alkenes.

Metal-free photosensitized intermolecular carboimination of alkenes: a green and direct access to both β-amino acids and β-amino ketones

β-Amino carbonyl substructures are privileged motifs in natural products and active pharmaceutical compounds. Here, we report a photoinduced metal-free and highly regioselective intermolecular carboimination method via the simultaneous introduction of amino and carbonyl groups into the CC double bond in one step, providing straightforward, green and general access to both β-amino acid and β-amino ketone motifs from readily available alkene feedstocks. The mild reaction conditions, excellent functional group tolerance and product diversity should make this a broadly applicable carboimination approach of very broad interest to organic and medicinal chemists.

N-O Bond Activation by Energy Transfer Photocatalysis

A radical shift toward energy transfer photocatalysis from electron transfer photocatalysis under visible-light photoirradiation is often due to the greener prospects of atom and process economy. Recent advances in energy transfer photocatalysis embrace unique strategies for direct small-molecule activation and sometimes extraordinary chemical bond formation in the absence of additional/sacrificial reagents. Selective energy transfer photocatalysis requires careful selection of substrates and photocatalysts for a perfect match with respect to their triplet energies while having incompatible redox potentials to prevent competitive electron transfer pathways. Substrates containing labile N-O bonds are potential targets for generating reactive key intermediates via photocatalysis to access a variety of functionalized molecules. Typically, the differential electron densities of N and O heteroatoms have been exploited for generation of either N- or O-centered radical intermediates from the functionalized substrates by the electron transfer pathway. However, the latest developments involve direct N-O bond homolysis via energy transfer to generate both N- and O-centered radicals for their subsequent utilization in diverse organic transformations, also in the absence of sacrificial redox reagents. In this Account, we highlight our key contributions in the field of N-O bond activation via energy transfer photocatalysis to generate reactive radical intermediates, with coverage of useful mechanistic insights. More specifically, well-designed N-O bond-containing substrates such as 1,2,4-oxadiazolines, oxime esters, N-indolyl carbonates, and N-enoxybenzotriazoles were successfully utilized in versatile transformations involving selective energy transfer over electron transfer from photocatalysts with high triplet state energy. Direct access to reactive N-, O-, and C-centered (if decarboxylation follows) radical intermediates was achieved for diverse cross-couplings and rearrangement processes. In particular, a variety of open-shell nitrogen reactive intermediates, including N(sp²) and N(sp³) radicals and nitrenes, have been utilized. Notably, diversified transformations of identical substrates have been achieved through careful control of the reaction conditions. 1,2,4-Oxadiazolines were converted into spiro-azolactams through iminyl intermediates in the presence of ¹O₂, benzimidazoles, or sulfoximines with external sulfoxide reagent through triplet nitrene intermediates under inert conditions. Besides, oxime esters underwent either intramolecular C(sp³)-N radical-radical coupling or intermolecular C(sp³)-N radical-radical coupling by a combined energy transfer-hydrogen atom transfer strategy. Furthermore, a series of electrochemical and photophysical experiments as well as computational studies were performed to substantiate the proposed selective energy-transfer-driven reaction pathways. We hope that this Account will serve as a guide for the rational design of selective energy transfer processes through the activation of further labile chemical bonds.

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

The concurrent installation of C-C and C-N bonds across alkene frameworks represents a powerful tool to prepare motifs that are ubiquitous in pharmaceuticals and bioactive compounds. To construct such prevalent bonds, most alkene difunctionalization methods demand the use of precious metals or activated alkenes. We report a metal-free, photochemically mediated imino-alkylation of electronically diverse alkenes to install both alkyl and iminyl groups in a highly efficient manner. The exceptionally mild reaction conditions, broad substrate scope, excellent functional group tolerance, and facile one-pot reaction protocol highlight the utility of this method to prepare privileged motifs from readily available alkene and acid feedstocks. One key and striking feature of this transformation is that an electrophilic trifluoromethyl radical is equally efficient with both electron-deficient and electron-rich alkenes. Additionally, dispersion-corrected density functional theory (DFT) and empirical investigations provide detailed mechanistic insight into this reaction.

Synthesis of γ-amino acids via photocatalyzed intermolecular carboimination of alkenes

We report a direct approach to achieve the energy transfer-driven carboimination of alkenes for the synthesis of a diverse collection of valuable γ-amino acids.