Alkylamination of Styrenes with Alkyl N-Hydroxyphthalimide Esters and Amines by B(C6H5)3-Facilitated Photoredox Catalysis

Photocatalysis Enables Chemodivergent Radical Polar Crossover: Ritter-Type Amidation vs Heck-Type Olefin Carbofunctionalizations

Three-component alkene difunctionalization reactions constitute an ideal platform to rapidly build molecular complexity, enabling the simultaneous introduction of two distinct, orthogonal functional groups into the C═C bond in a single step. Herein, a photoredox catalyzed Ritter-type carboamidation of electronically diverse styrenes harnessing non-stabilized, nucleophilic primary radicals generated from readily-accessible carboxylic acid-derived redox active esters is reported. Furthermore, it is found that Heck-type products are chemoselectively obtained by simply switching aryl olefin acceptors with 1,1-diarylolefins. In the context of photocatalytic chemodivergent radical polar crossover, the synthesis of various trisubstituted alkenes was achieved, simultaneously revealing a divergence in the activation of redox-active esters toward reduction. In-depth mechanistic studies demonstrated both transformation pathways, while DFT calculations indicated the origin of product switchability. Both Ritter-type and Heck-type olefin carbofunctionalizations are scalable up to 4 mmol scale in batch and continuous flow, proving the synthetic utility of the methodology.

Photosensitized Three-Component Carboimination of Alkenes Based on the Relay of Oxy Radicals to Carbon Radicals

Here, we present a metal-free photosensitized three-component reaction for the carboimination of alkenes based on oxime carbonates. Homolysis of oxime carbonates via light-mediated energy transfer enables the simultaneous generation of iminyl radicals and alkoxycarbonyloxyl radicals. The alkoxycarbonyloxyl and alkoxy radicals can act as an effective hydrogen atom transfer reagent, abstracting hydrogen atoms from alkanes and aldehydes, silanes, and phosphine oxide. This strategy exhibits broad functional group tolerance under mild reaction conditions, further broadening the diversity of alkene carboimination.

Nickel-Catalyzed, Aminoquinoline-Directed Chemo- and Regioselective Carboamination of Unactivated Olefins with Organoboronic Acids and Anthranils

A nickel-catalyzed three-component carboamination of unactivated alkenes with organoboronic acids and anthranils has been achieved for the expedient synthesis of δ-aryl and γ-amino acid derivatives. The 8-aminoquinoline (AQ) directing group is crucial for the success of the reaction, and anthranil serves as an arylnitrene precursor in this conversion. This method features mild reaction conditions, good chemo- and regioselectivity, and a broad substrate scope with good functional group tolerance.

Cu-Catalyzed Three-Component Alkene Carboamination: Mechanistic Insights and Rational Design to Overcome Limitations

Herein, we report mechanistic investigations into the Cu-catalyzed three-component carboamination of alkenes with α-halo carbonyls and aryl amines via an oxocarbenium intermediate. Monitoring the reaction reveals the formation of transient atom transfer radical addition (ATRA) intermediates with both electron-neutral and deficient vinyl arenes as well as unactivated alkenes. Based on our experimental studies and density functional theory calculations, the oxocarbenium is generated through atom transfer and subsequent intramolecular substitution. Further, mechanistic factors that dictate the regioselectivity of the nucleophilic attack onto the oxocarbenium to afford the γ-amino ester, γ-iminolactone, or γ-lactone are discussed. A strategy to overcome scope limitation with respect to unactivated alkenes is developed using the mechanistic insights gained herein. Finally, we demonstrate that under modified conditions, our Cu catalyst enables the ATRA reaction between a variety of alkyl halides and vinyl arenes/α-olefins, and we present a one-pot, two-step carbofunctionalization with an array of nucleophiles through ATRA/SN2.

Copper-Catalyzed Three-Component 1,5-Carboamination of Vinylcyclopropanes

The 1,5-copper-catalyzed carboamination of vinylcyclopropanes is presented. A carbon-centered radical, formed upon reduction of an alkyl halide by Cu(I), adds across the alkene of a vinylcyclopropane, triggering ring opening to generate a benzylic radical, which, finally, undergoes copper-mediated amination to afford a homoallylic amine. The reaction occurs with outstanding regio- and good to very good diastereoselectivities. The scope of the reaction is demonstrated with respect to all three components: alkyl halide, vinylcyclopropane, and amine nucleophile. A total of 38 examples are presented with an average yield of 60%.

Ruthenium-Catalyzed Difunctionalization of Vinyl Cyclopropanes for Double m-C(sp2)-H/C-5(sp3)-H Functionalization

With in-depth research on 1,2-difunctionalization, remote difunctionalization has garnered widespread attention for achieving multifunctionality. Herein, we report a strategy for achieving remote difunctionalization under mild conditions. This strategy exhibited good substrate suitability and functional group tolerance. In addition, the significance of this method is further evidenced by its successful application in scaling up and conducting additional transformations of target compounds. Mechanistic studies showed that a radical might be involved in this process.

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.

Visible Light Induced Three-Component 1,2-Dicarbofunctionalization of Alkenes and Alkynes

Harnessing visible-light in organic synthesis is one of the most effective methods that aligns with green and sustainable chemistry principles and hence skyrocketed in the last two decades. Similarly, three-component 1,2-dicarbofunctionalization of alkenes and alkynes has recently been a great choice to construct complex molecular systems in an easy and rapid manner. Therefore, light-induced reactions can be an excellent alternative to carry out 1,2-dicarbofunctionalization reactions, and very recently, organic chemists across the globe have fascinated us with their interesting articles. In this present review, we have summarized the recent advancements in the area of visible light induced three-component 1,2-dicarbofunctionalization of alkenes and alkynes till March 2023. We have categorized the discussion based on the catalysts used to carry out the transformations for better understanding and different important aspects of these transformations have also been covered.

Cu-Catalyzed Three-Component Carboamination of Electron Deficient Olefins

The copper-catalyzed three-component carboamination of atropates for the synthesis of α-aryl amino acid derivatives is presented. The scope of the reaction is explored with respect to all three coupling partners: the alkyl halide, the atropate, and the aryl amine. A total of 41 examples are included, with yields of ≤92%. Both primary and secondary aryl amines participate in the carboamination along with α-haloesters, nitriles, and perfluoroiodoalkanes. Mechanistic investigations support a radical mechanism involving Cu-mediated C-N bond formation with the radical adduct.

Visible-Light-Promoted Decarboxylative Alkylation/Cyclization of Vinylcycloalkanes

An efficient strategy for visible-light-promoted decarboxylative alkylation of vinylcyclopropanes with alkyl N-(acyloxy)phthalimide esters through the dual C-C bond and single N-O bond cleavage, employing triphenylphosphine and lithium iodide as the photoredox system to synthesize 2-alkylated 3,4-dihydronaphthalenes, has been established. This alkylation/cyclization involves a radical process and undergoes a sequence of N-(acyloxy)phthalimide ester single-electron reduction, N-O bond cleavage, decarboxylative, alkyl radical addition, C-C bond cleavage, and intramolecular cyclization. Moreover, using the photocatalyst Na₂-Eosin Y instead of triphenylphosphine and lithium iodide, the vinyl transfer products are acquired when vinylcyclobutanes or vinylcyclopentanes are utilized as alkyl radical receptors.

Cu-Catalyzed Polychloromethylamination of Styrenes through C(sp3 )-H Bond Cleavage

A copper-catalyzed three-component coupling reaction has been developed allowing the rapid building of valuable complex highly functionalized β-polychloromethyl amines from simple styrenes, arylamines, and dichloromethane/chloroform. Using aryldiazonium salts as a radical initiator, a series of corresponding products are obtained with moderate to good yields under a carbon dioxide or nitrogen atmosphere (50 psi). In addition, good functional group tolerance can be observed.

Photoinduced Remote C(sp3)-H Cyanation and Oxidation Enabled by a Vinyl Radical-Mediated 1,5-HAT Strategy

We report a vinyl radical-mediated 1,5-hydrogen atom transfer (1,5-HAT) strategy for the remote C(sp³)-H functionalization reaction, which includes cyanation, oxidation, and etherification under visible-light-induced photochemical conditions. This reaction is achieved using readily available alkyl N-hydroxyphthalimide esters as radical precursors, which can efficiently react with diverse alkynes to form key vinyl radical intermediates followed by a 1,5-HAT process. A series of structurally diverse γ-cyano, γ-carbonyl, and γ-oxygenated alkenes with excellent stereoselectivity can be efficiently constructed by this synthetic protocol.

Visible-Light-Induced Decarboxylative Alkylation/Ring Opening and Esterification of Vinylcyclopropanes

A visible-light-induced four-component reaction of vinylcyclopropanes, N-(acyloxy)phthalimide esters, N,N-dimethylformamide (DMF), and H₂O through an oxidative ring opening of cyclopropane is presented. This procedure provides a new and effective way to construct formate esters. DMF is employed as both a solvent and the source of CHO. This difunctionalization of vinylcyclopropanes shows good functional group tolerance under room temperature. A radical pathway is involved, and carbonyl oxygen of ester originated from water in this transformation.

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

Three-Component Ruthenium-Catalyzed meta-C-H Alkylation of Phenol Derivatives

Herein, we realized the multicomponent reactions of phenol derivatives via a six-membered cycloruthenated intermediate for the first time. This strategy exhibited good substrate suitability and functional group tolerance with various phenol derivatives and provided a potential synthetic drug approach. Mechanistic studies showed that a radical might be involved in this process. In addition, the meta alkylated phenol was obtained by further removal of the directing group.

Photoredox-Catalyzed Oxidative Radical-Polar Crossover Enables the Alkylfluorination of Olefins

The three-component alkylfluorination of olefins via an oxidative radical-polar crossover under visible light-induced photocatalysis is disclosed. A key feature of this reaction is the incorporation of two synthetically meaningful components involving a three-dimensional alkyl group and a fluorine atom using easily preparable N-hydroxyphthalimide esters as the alkyl donors and a low-cost hydrogen fluoride as the fluorine source. Furthermore, a one-step procedure using commercially available carboxylic acids demonstrated the versatility of this new method.

Copper-promoted cross-coupling of nitroarenes with 4-alkyl-1,4-dihydropyridines using a peroxide-driven radical reductive strategy

Direct radical-mediated reductive coupling of nitroarenes with 4-alkyl-1,4-dihydropyridines to build the C(sp3)–N bond using 4-alkyl-1,4-dihydropyridines as internal reducing agents and alkyl sources is presented.

An organophotoredox-catalyzed redox-neutral cascade involving N-(acyloxy)phthalimides and allenamides: synthesis of indoles

An organophotoredox-catalyzed radical cascade of allenamides and alkyl N-(acyloxy)phthalimides for the synthesis of indoles is documented. The method features mild and robust reaction conditions, and exhibits broad scope. The tandem process enriches the limited repertoire of alkyl NHPI ester addition on electron-rich π-bonds as well as radical chemistry involving allenamides.

Visible-Light-Induced Transition-Metal-Free Nitrogen-Centered Radical Strategy for the Synthesis of 2-Acylated 9H-Pyrrolo[1,2-a]indoles

A convenient and efficient visible-light-induced tandem acylation/cyclization of N-propargylindoles with aryl- or alkyl-substituted acyl oxime esters for the synthesis of 2-acyl-substituted 9H-pyrrolo[1,2-a]indoles under transition-metal-free conditions, which proceeds via nitrogen-centered radical-mediated cleavage of the C-C σ-bond in acyl oxime esters, is established. The aryl or alkyl acyl radicals, which come from acyl oxime esters, attack the C-C triple bonds in N-propargylindoles and then go through intramolecular cyclization/isomerization.

Recent Advances on Photoinduced Cascade Strategies for the Synthesis of N-Heterocycles

Over the last decade, visible-light photocatalysis has proved to be a powerful tool for the construction of N-heterocyclic frameworks, important constituents of natural products, insecticides, pharmacologically relevant therapeutic agents and catalysts. This account highlights recent developments and established methods towards the photocatalytic cascades for preparation of different classes of N-heterocycles, giving emphasis on our contribution to the field.

Photoinduced 1,2-dicarbofunctionalization of alkenes with organotrifluoroborate nucleophiles via radical/polar crossover

Alkene 1,2-dicarbofunctionalizations are highly sought-after transformations as they enable a rapid increase of molecular complexity in one synthetic step. Traditionally, these conjunctive couplings proceed through the intermediacy of alkylmetal species susceptible to deleterious pathways including β-hydride elimination and protodemetalation. Herein, an intermolecular 1,2-dicarbofunctionalization using alkyl N-(acyloxy)phthalimide redox-active esters as radical progenitors and organotrifluoroborates as carbon-centered nucleophiles is reported. This redox-neutral, multicomponent reaction is postulated to proceed through photochemical radical/polar crossover to afford a key carbocation species that undergoes subsequent trapping with organoboron nucleophiles to accomplish the carboallylation, carboalkenylation, carboalkynylation, and carboarylation of alkenes with regio- and chemoselective control. The mechanistic intricacies of this difunctionalization were elucidated through Stern-Volmer quenching studies, photochemical quantum yield measurements, and trapping experiments of radical and ionic intermediates.

Visible light-induced alkylpyridylation of styrenes via a reductive radical three-component coupling

A visible light-induced and metal-free strategy for the intermolecular three-compoment alkylpyridylation of styrenes is reported. Hantzsch ester was found to be key to initiate the overall reductive radical coupling reaction. This radical process realized difunctionalization of styrenes, selectively yielding alkylated pyridines in good to excellent yields with a wide tolerance of functional groups, mild reaction conditions and simple operation. This new reaction complements existing visible light-induced variants of styrenes with NHP esters and expands the capabilities of radical-based cross-coupling reactions of pyridines.

Metal-Free Photosynthesis of Alkylated Benzimidazo[2,1-a]isoquinoline-6(5H)-ones and Indolo[2,1-a]isoquinolin-6(5H)-ones in PEG-200

A visible-light-induced decarboxylation reaction was developed for the synthesis of alkylated benzimidazo[2,1-a]isoquinoline-6(5H)-ones and indolo[2,1-a]isoquinolin-6(5H)-ones under metal-free conditions. Impressively, metal catalysts and traditionally volatile organic solvents could be effectively avoided.

Oxidative Three-Component Carboamination of Vinylarenes with Alkylboronic Acids

Three-component carboamination of alkenes is of significant interest due to the ease by which functionalized amines can be produced from readily available chemical building blocks. Previously, a variety of carbon-centered radical precursors have been studied as the carbon components for this reaction, however, the use of general alkyl sources has remained as an unsolved challenge. Herein we present our efforts to develop an oxidative carboamination protocol that utilizes alkylboronic acids as carbon-centered radical precursors. The presented work demonstrates 34 examples, ranging from 17 to 88% yields, with a broad scope in vinylarenes, amines, and alkylboronic acids. Preliminary mechanistic studies suggest that a single-electron oxidation of the alkylboronic acid generates a carbon-centered radical intermediate that adds across the olefin followed by C-N bond formation via Cu-mediated inner-sphere or carbocation-mediated pathways.

Tris(pentafluorophenyl)borane catalyzed C-C and C-heteroatom bond formation

A series of boron based Lewis acids have been reported to date, but among them, tris(pentafluorophenyl)borane (BCF) has gained the most significant attention in the synthetic chemistry community. The viability of BCF as a potential Lewis acid catalyst has been vastly explored in organic and materials chemistry due to its thermal stability and commercial availability. Most explorations of BCF chemistry in organic synthesis has occurred in the last two decades and many new catalytic reactivities are currently under investigation. This review mainly focuses on recent reports from 2018 onwards and provides a concise knowledge to the readers about the role of BCF in metal-free catalysis. The review has mainly been categorized by different types of organic transformation mediated through BCF catalysis for the C-C and C-heteroatom bond formation.

Organophotoredox-Catalyzed Three-Component Coupling of Heteroatom Nucleophiles, Alkenes, and Aliphatic Redox Active Esters

This manuscript describes a visible-light-mediated organophotoredox catalytic process for vicinal difunctionalization of alkenes using heteroatom nucleophiles and aliphatic redox active esters. A wide range of heteroatom nucleophiles including alcohols, water, carboxylic acids, amides, and halogens can be used for this reaction. This radical relay type reaction allows forging of C(sp³)-C(sp³) with a carbon-centered radical and C(sp³)-heteroatom bonds with a benzyl cation on the vinylarenes with complete regioselectivity in a single step.

Radical-mediated alkoxypolyhaloalkylation of styrenes with polyhaloalkanes and alcohols via C(sp3)–H bond cleavage

A radical-mediated alkoxypolyhaloalkylation of styrenes with polychloroalkanes and alcohols for the facile synthesis of complex polyhaloalkanes with excellent functional-group compatibility and a broad substrate scope.

Three-component photoredox 1,2-alkylamination of styrenes with alkanes and nitrogen nucleophiles via C(sp3)–H bond cleavage

A three-component photoredox 1,2-alkylamination of styrenes involving functionalization of C(sp3)–H bonds in alkyl halides instead of functionalization of C-halogen bonds is disclosed.

Photocatalytic dual decarboxylative alkenylation mediated by triphenylphosphine and sodium iodide

An efficient photocatalytic dual decarboxylative alkenylation of α,β-unsaturated carboxylic acids and alkyl N-hydroxyphthalimide (NHP) esters mediated by triphenylphosphine and sodium iodide has been developed. This protocol proceeds under 456-nanometer irradiation by visible blue light in the absence of transition metals or organic dye based photoredox catalysts. The reaction is successfully applied to a wide range of redox-active esters derived from aliphatic carboxylic acids (1°, 2° and 3°) and α-amino acids, enabling transformations of diverse α,β-unsaturated carboxylic acids to α,β-alkylated styrenes with high efficiency and excellent selectivity under mild conditions.

Intermolecular radical carboamination of alkenes

Vicinal alkene carboamination is a highly efficient and practical synthetic strategy for the straightforward preparation of diverse and valuable amine derivatives starting from simple compounds. During the last decade that approach has found continuous research interests and various practical methods have been developed using transition-metal catalysis. Driven by the renaissance of synthetic radical chemistry, intermolecular radical alkene carboamination comprising a C-C bond and a C-N bond forming step has been intensively investigated recently culminating in novel strategies and improved protocols which complement existing methodologies. Radical alkene carboamination can be achieved via three different reaction modes. Such cascades can proceed through N-radical addition to an alkene with subsequent C-C bond formation leading to 2,1-carboamination products. Alternatively, the C-C bond can be installed prior to the C-N bond via initial C-radical addition to the alkene with subsequent β-amination resulting in 1,2-carboamination. The third mode comprises initial single electron oxidation of the alkene to the corresponding alkene radical cation that gets trapped by an N-nucleophile and the cascade is terminated by radical C-C bond formation. In this review, the three different conceptual approaches will be discussed and examples from the recent literature will be presented. Further, the reader will get insights into the mechanism of the different transformations.

Reductive coupling of imines with redox-active esters by visible light photoredox organocatalysis

The direct alkylation of imines with redox-active esters by visible light photoorganocatalysis provides a direct way for accessing α-branched secondary amines which are found in numerous bioactive molecules.

Transition-metal-free synthesis of CMe2CF3-containing chroman-4-ones via decarboxylative trifluoroalkylation

(NH₄)₂S₂O₈-mediated decarboxylative trifluoroalkylation of alkenes with 3,3,3-trifluoro-2,2-dimethylpropanoic acid under metal-free conditions has been described.

The photoredox alkylarylation of styrenes with alkyl N-hydroxyphthalimide esters and arenes involving C-H functionalization

The In(OTf)₃-promoted three-component photoredox alkylarylation of styrenes with alkyl NHP esters and arenes to access alkylated arene derivatives through C-C bond cleavage and C-H functionalization is reported. By utilizing visible-light photoredox catalysis, alkyl N-hydroxyphthalimide esters serving as alkyl carbon-centered radicals and a wide range of arenes (e.g., indoles, pyrrole, and electron-rich arenes) as nucleophiles were used to enable the introduction of various alkyl groups and aryl groups across the C[double bond, length as m-dash]C bonds with excellent selectivity and functional group tolerance.

Decarboxylative Alkylation of Heteroarenes Using N-Hydroxybenzimidoyl Chloride Esters

Functionalized N-heteroarenes are highly desired motifs in medicinal chemistry and pharmaceutical industry. Minisci-type reactions usually require a protonated N-heteroarene for the alkyl radical to attack. This work describes a leaving-group-assisted redox-active ester to enable direct coupling of an amino acid with N-heteroarenes. The efficient and sustainable photoredox strategy provides rapid access to an alkylated heterocyclic manifold.