Single Electron Transfer-Induced Redox Processes Involving N-(Acyloxy)phthalimides

Photoredox/Cu-Catalyzed Decarboxylative C(sp3)-C(sp3) Coupling to Access C(sp3)-Rich gem-Diborylalkanes

gem-Diborylalkanes are highly valuable building blocks in organic synthesis and pharmaceutical chemistry due to their ability to participate in multi-step cross-coupling transformations, allowing for the rapid generation of molecular complexity. While progress has been made in their synthetic metholodology, the construction of β-tertiary and C(sp3)-rich gem-diborylalkanes remains a synthetic challenge due to substrate limitations and steric hindrance issues. An approach is presented that utilizes synergistic photoredox and copper catalysis to achieve efficient C(sp3)-C(sp3) cross-coupling of alkyl N-hydroxyphthalimide esters, which can easily be obtained from alkyl carboxylic acids, with diborylmethyl species, providing a series of C(sp3)-rich gem-diborylalkanes with 1°, 2°, and even 3° β positions. Furthermore, this approach can also be applied to complex medicinal compounds and natural products, offering rapid access to molecular complexity and late-stage functionalization of C(sp3)-rich drug candidates. Mechanistic experiments revealed that diborylmethyl Cu(I) species participated in both the photoredox process and the key C(sp3)-C(sp3) bond-forming step.

A General Iron-Catalyzed Decarboxylative Oxygenation of Aliphatic Carboxylic Acids

We report an iron-catalyzed decarboxylative C(sp3)-O bond-forming reaction under mild, base-free conditions with visible light irradiation. The transformation uses readily available and structurally diverse carboxylic acids, iron photocatalyst, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) derivatives as oxygenation reagents. The process exhibits a broad scope in acids possessing a wide range of stereoelectronic properties and functional groups. The developed reaction was applied to late-stage oxygenation of a series of bio-active molecules. The reaction leverages the ability of iron complexes to generate carbon-centered radicals directly from carboxylic acids by photoinduced carboxylate-to-iron charge transfer. Kinetic, electrochemical, EPR, UV/Vis, HRMS, and DFT studies revealed that TEMPO has a triple role in the reaction: as an oxygenation reagent, an oxidant to turn over the Fe-catalyst, and an internal base for the carboxylic acid deprotonation. The obtained TEMPO adducts represent versatile synthetic intermediates that were further engaged in C-C and C-heteroatom bond-forming reactions using commercial organo-photocatalysts and nucleophilic reagents.

Regio- and enantioselective nickel-alkyl catalyzed hydroalkylation of alkynes

The migratory insertion of metal-hydride into alkene has allowed regioselective access to organometallics, readily participating in subsequent functionalization as one conventional pathway of hydroalkylation, whereas analogous process with feedstock alkyne is drastically less explored. Among few examples, the regioselectivity of metal-hydride insertion is mostly governed by electronic bias of alkynes. To alter the regioselectivity and drastically expand the intermediate pools that we can access, one aspirational design is through alternative nickel-alkyl insertion, providing opposite regioselectivity induced by steric demand. Leveraging in situ formed nickel-alkyl species, we herein report the regio- and enantioselective hydroalkylation of alkynes with broad functional group tolerance, excellent regio- and enantioselectivity, enabling efficient route to diverse valuable chiral allylic amines motifs. Preliminary mechanistic studies indicate the aminoalkyl radical species can participate in metal-capture and lead to formation of nickel-alkyl, of which the migratory insertion is key to reverse regioselectivity observed in metal-hydride insertion.

Catalyst-Free, Zn-Mediated Decarboxylative Coupling of Chlorostibines to Access Alkylstibines with Stable C(sp3)-Sb Bonds

Herein, decarboxylative C(sp3)-Sb coupling of aliphatic carboxylic acid derivatives with chlorostibines to access alkylstibines has been achieved. This catalyst-, ligand-, and base-free approach using zinc as a reductant affords various kinds of benzyldiarylstibines and other monoalkyldiarylstibines and tolerates various functional groups, including chlorine, bromine, hydroxyl, amide, sulfone, and cyano groups. The late-stage modification and the gram-scale experiments illustrate its potential application.

Expanding the Chemical Space of Electrophilic β-Glycosyl β-Lactams through Photoinduced Diastereoselective Functionalization

Herein, we present a photoinduced diastereoselective C-3 functionalization of electrophilic β-glycosyl β-lactams. The developed protocol is simple, mild, and scalable and explores the use of 3-exomethylene β-lactams as reaction partners in a Giese type reaction. The key nucleophilic alkyl radical is generated by a photoinduced electron transfer process in the EDA complex formed by NHPI and Hantzsch esters. The diastereoselective hydrogen atom transfer to the β-lactam radical intermediate enables the synthesis of various N-phenyl β-glycosyl β-lactams.

Zinc Promoted Cross-Electrophile Sulfonylation to Access Alkyl-Alkyl Sulfones

The transition metal-catalyzed multi-component cross-electrophile sulfonylation, which incorporates SO2 as a linker within organic frameworks, has proven to be a powerful, efficient, and cost-effective means of synthesizing challenging alkyl-alkyl sulfones. Transition metal catalysts play a crucial role in this method by transferring electrons from reductants to electrophilic organohalides, thereby causing undesirable side reactions such as homocoupling, protodehalogenation, β-hydride elimination, etc. It is worth noting that tertiary alkyl halides have rarely been demonstrated to be compatible with current methods owing to various undesired side reactions. In this work, a zinc-promoted cross-electrophile sulfonylation is developed through a radical-polar crossover pathway. This approach enables the synthesis of various alkyl-alkyl sulfones, including 1°-1°, 2°-1°, 3°-1°, 2°-2°, and 3°-2° types, from inexpensive and readily available alkyl halides. Various functional groups are well tolerated in the work, resulting in yields of up to 93%. Additionally, this protocol has been successfully applied to intramolecular sulfonylation and homo-sulfonylation reactions. The insights gained from this work shall be useful for the further development of cross-electrophile sulfonylation to access alkyl-alkyl sulfones.

Catalytic tert-alkylation of enamides via C-C bond cleavage under photoredox conditions

Efficient C-C bond cleavage is recognized as a persistent challenge in the field of synthetic methodology. In this study, we found that tertiary alkyl radicals are smoothly formed from tertiary alkylated dienones (BHT adducts) via SET, using PDI as a photocatalyst. Resulting tert-alkyl radicals could be applied to the tert-alkylation of enamides. The driving force of this C-C bond cleavage reaction is the mesolytic cleavage of the BHT adducts. The mechanistic study revealed that PDI anion radical is the key active species during the catalytic cycle.

Visible-Light-Induced Hydrodifluoromethylation of Unactivated Alkenes with Difluoroacetic Anhydride

We herein described a practical and efficient protocol for hydrodifluoromethylation of unactivated alkenes using readily available difluoroacetic anhydride as a difluoromethyl source by merging photocatalysis and N-hydroxyphthalimide activation. This method features a wide substrate scope and excellent compatibility with various functional groups, as demonstrated by more than 50 examples, including bioactive molecules and pharmaceutical derivatives. Mechanism investigation indicated that N-hydroxyphthalimide may also serve as the hydrogen atom donor.

Direct C-H alkylation of 3,4-dihydroquinoxaline-2-ones with N-(acyloxy)phthalimides via radical-radical cross coupling

We present an organophotoredox-catalyzed direct Csp3-H alkylation of 3,4-dihydroquinoxalin-2-ones employing N-(acyloxy)pthalimides to provide corresponding products in good yields. A broad spectrum of NHPI esters (1°, 2°, 3°, and sterically encumbered) participates in the photoinduced alkylation of a variety of 3,4-dihydroquinoxalin-2-ones. In general, mild conditions, broad scope with good functional group tolerance, and scalability are the salient features of this direct alkylation process.

Zn-Mediated Fragmentation of N-Alkoxyphthalimides Enabling the Synthesis of gem-Difluoroalkenes

Zn-mediated generation of alkoxyl radicals from N-alkoxyphthalimides emerged as an efficient approach for forming diverse and valuable alkyl radicals through β-scission or a hydrogen atom transfer process. The alkyl radical species can be further trapped by α-trifluoromethyl alkenes to construct a series of gem-difluoroalkenes.

Synthesis of N-Hydroxysuccinimide Esters, N-Acylsaccharins, and Activated Esters from Carboxylic Acids Using I2/PPh3

A method for the syntheses of isolable, active esters is described in which carboxylic acids are treated with triphenylphosphine, iodine, and triethylamine. Active esters accessible in this way include N-hydroxysuccinimide esters, N-hydroxyphthalimide esters (N-(acyloxy)phthalimides), N-acylsaccharins, pentafluorophenol esters, pentachlorophenol esters, N-hydroxybenzotriazole esters, and hexafluoro-2-propanol esters. The approach can be similarly applied toward the formation of N-acylsaccharins and N-acylimidazoles. The method is suitable for the formation of isolable active esters of aromatic and aliphatic activated acids as well as α-amino acid derivatives. These products are widely used reagents in organic synthesis, peptide synthesis, medicinal chemistry, and chemical biology (e.g., for bioconjugations). The method has broad substrate scope, uses simple and inexpensive reagents, avoids the use of carbodiimides or other coupling agents, and occurs at room temperature. Additionally, the diastereomers of compound Boc-Ala-NHCHPh are demonstrated to be distinguishable by 1H NMR (in DMSO-d6), allowing for a straightforward NMR method to establish the degree of racemization of activated esters of Boc-Ala or amide bond formations using Boc-Ala.

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

Photodecarboxylative Radical Cascade Involving N-(Acyloxy)phthalimides for the Synthesis of Pyrazolones

We disclose N'-arylidene-N-acryloyltosylhydrazides as novel skeletons for the synthesis of biologically relevant alkylated pyrazolones through a photoinduced radical cascade with N-(acyloxy)pthalimides as readily available alkyl surrogates. The reaction proceeds through the formation of a photoactivated electron donor-acceptor (EDA) complex between alkyl N-(acyloxy)phthalimide (NHPI) esters and LiI/PPh3 as a commercially available donor system. The reaction exhibits a broad scope and scalability, thereby enabling synthesis of a broad spectrum of functionally orchestrated alkylated pyrazolones under mild and transition-metal-free conditions.

Dual Acridine/Decatungstate Photocatalysis for the Decarboxylative Radical Addition of Carboxylic Acids to Azomethines

A concept for the dual use of acridine and tetrabutylammonium decatungstate photocatalysts in the reactions of carboxylic acids is proposed. Imines generated in situ from aldehydes and p-methoxyaniline, as well as other azomethines, were used as radical acceptors. The role of the decatungstate is believed to facilitate the turnover of the acridine photocatalyst by means of hydrogen atom transfer.

Multicomponent Synthesis of α-Branched Amines via a Zinc-Mediated Carbonyl Alkylative Amination Reaction

Methods for the synthesis of α-branched alkylamines are important due to their ubiquity in biologically active molecules. Despite the development of many methods for amine preparation, C(sp3)-rich nitrogen-containing compounds continue to pose challenges for synthesis. While carbonyl reductive amination (CRA) between ketones and alkylamines is the cornerstone method for α-branched alkylamine synthesis, it is sometimes limited by the sterically demanding condensation step between dialkyl ketones and amines and the more restricted availability of ketones compared to aldehydes. We recently reported a "higher-order" variant of this transformation, carbonyl alkylative amination (CAA), which utilized a halogen atom transfer (XAT)-mediated radical mechanism, enabling the streamlined synthesis of complex α-branched alkylamines. Despite the efficacy of this visible-light-driven approach, it displayed scalability issues, and competitive reductive amination was a problem for certain substrate classes, limiting applicability. Here, we report a change in the reaction regime that expands the CAA platform through the realization of an extremely broad zinc-mediated CAA reaction. This new strategy enabled elimination of competitive CRA, simplified purification, and improved reaction scope. Furthermore, this new reaction harnessed carboxylic acid derivatives as alkyl donors and facilitated the synthesis of α-trialkyl tertiary amines, which cannot be accessed via CRA. This Zn-mediated CAA reaction can be carried out at a variety of scales, from a 10 μmol setup in microtiter plates enabling high-throughput experimentation, to the gram-scale synthesis of medicinally-relevant compounds. We believe that this transformation enables robust, efficient, and economical access to α-branched alkylamines and provides a viable alternative to the current benchmark methods.

Catalyst-free decarboxylative cross-coupling of N-hydroxyphthalimide esters with tert-butyl 2-(trifluoromethyl)acrylate and its application

Here, we demonstrate a practical method toward the facile synthesis of CF3-containing amino acids through visible light promoted decarboxylative cross-coupling of a redox-active ester with tert-butyl 2-(trifluoromethyl)acrylate. The reaction was driven by the photochemical activity of electron donor-acceptor (EDA) complexes that were formed by the non-covalent interaction between a Hantzsch ester and a redox-active ester. The advantages of this protocol are its synthetic simplicity, rich functional group tolerance, and a cost-effective reaction system.

Photoinduced Triphenylphosphine and Iodide Salt Promoted Reductive Decarboxylative Coupling

The transient electron donor-acceptor (EDA) complex has been an emerging area in the photoinduced organic synthesis field, generating radicals without exogenous transition-metal or organic dye-based photoredox catalysts. The catalytic platform to form suitable photoactive EDA complexes for photochemical reduction reactions remains underdeveloped. Herein, a general photoinduced reductive alkylation via the EDA complex strategy is described. A simple yet multifunctional system, triphenylphosphine and iodide salt, promotes the photoinduced decarboxylative hydroalkylation, and reductive defluorinative decarboxylative alkylation of trifluoromethyl alkenes, to access trifluoromethyl alkanes and gem-difluoroalkenes. Moreover, decarboxylative hydroalkylation can be applied to more kinds of electron-deficient alkenes as a general Giese addition reaction.

Mechanisms for radical reactions initiating from N-hydroxyphthalimide esters

Due to their ease of preparation, stability, and diverse reactivity, N-hydroxyphthalimide (NHPI) esters have found many applications as radical precursors. Mechanistically, NHPI esters undergo a reductive decarboxylative fragmentation to provide a substrate radical capable of engaging in diverse transformations. Their reduction via single-electron transfer (SET) can occur under thermal, photochemical, or electrochemical conditions and can be influenced by a number of factors, including the nature of the electron donor, the use of Brønsted and Lewis acids, and the possibility of forming charge-transfer complexes. Such versatility creates many opportunities to influence the reaction conditions, providing a number of parameters with which to control reactivity. In this perspective, we provide an overview of the different mechanisms for radical reactions involving NHPI esters, with an emphasis on recent applications in radical additions, cyclizations and decarboxylative cross-coupling reactions. Within these reaction classes, we discuss the utility of the NHPI esters, with an eye towards their continued development in complexity-generating transformations.

Metal-free photocatalytic cross-electrophile coupling enables C1 homologation and alkylation of carboxylic acids with aldehydes

In contemporary drug discovery, enhancing the sp3-hybridized character of molecular structures is paramount, necessitating innovative synthetic methods. Herein, we introduce a deoxygenative cross-electrophile coupling technique that pairs easily accessible carboxylic acid-derived redox-active esters with aldehyde sulfonyl hydrazones, employing Eosin Y as an organophotocatalyst under visible light irradiation. This approach serves as a versatile, metal-free C(sp3)-C(sp3) cross-coupling platform. We demonstrate its synthetic value as a safer, broadly applicable C1 homologation of carboxylic acids, offering an alternative to the traditional Arndt-Eistert reaction. Additionally, our method provides direct access to cyclic and acyclic β-arylethylamines using diverse aldehyde-derived sulfonyl hydrazones. Notably, the methodology proves to be compatible with the late-stage functionalization of peptides on solid-phase, streamlining the modification of intricate peptides without the need for exhaustive de-novo synthesis.

Decarboxylative Radical Sulfilimination via Photoredox, Copper, and Brønsted Base Catalysis

Sulfilimines, the aza-variants of sulfoxides, are key structural motifs in natural products, pharmaceuticals, and agrochemicals; and sulfilimine synthesis is therefore important in organic chemistry. However, methods for radical sulfilimination remain elusive, and as a result, the structural diversity of currently available sulfilimines is limited. Herein, we report the first protocol for decarboxylative radical sulfilimination reactions between sulfenamides and N-hydroxyphthalimide esters of primary, secondary, and tertiary alkyl carboxylic acids, which were achieved via a combination of photoredox, copper, and Brønsted base catalysis. This novel protocol provided a wide variety of sulfilimines, in addition to serving as an efficient route for the synthesis of S-alkyl/S-aryl homocysteine sulfilimines and S-(4-methylphenyl) homocysteine sulfoximine. Moreover, it could be used for late-stage introduction of a sulfilimine group into structurally complex molecules, thereby avoiding the need to preserve labile organosulfur moieties through multistep synthetic sequences. A mechanism involving photocatalytic substrate transformation and copper-mediated C(sp3 )-S bond formation is proposed.

Atom Transfer Radical Addition Reactions of Quinoxalin-2(1H)-ones with CBr4 and Styrenes Using Mes-Acr-MeClO4 Photocatalyst

The atom transfer radical addition (ATRA) reaction is defined as a method for introducing halogenated compounds into alkenes via a radical mechanism. In this study, we present an ATRA approach for achieving regioselective functionalization of quinoxalin-2(1H)-ones by activating C-Br bonds of CBr4 and subsequent trihaloalkyl-carbofunctionalization of styrenes employing the 9-mesityl-10-methylacridinium perchlorate (Fukuzumi) photocatalyst under 3W blue LED (450-470 nm) irradiation. This three-component radical cascade process demonstrates remarkable efficiency in the synthesis of 1-methyl-3-(3,3,3-tribromo-1-(4-chlorophenyl)propyl)quinoxalin-2(1H)-one derivatives.

Visible-light-induced decarboxylative cascade cyclization of acryloylbenzamides with N-hydroxyphthalimide esters via EDA complexes

A visible-light-induced decarboxylative cascade reaction of acryloylbenzamides with alkyl N-hydroxyphthalimide (NHP) esters for the synthesis of various 4-alkyl isoquinolinediones mediated by triphenylphosphine (PPh3) and sodium iodide (NaI) was developed. This operationally simple protocol proceeded via the photoactivation of electron donor-acceptor (EDA) complexes between N-hydroxyphthalimide esters and NaI/PPh3, resulting in multiple carbon-carbon bond formations without the use of precious metal complexes or synthetically elaborate organic dyes, which provided an alternative practical approach to synthesize diverse isoquinoline-1,3(2H,4H)-dione derivatives.

Nickel-Catalyzed Reductive Arylation of Redox Active Esters for the Synthesis of α-Aryl Nitriles: Investigation of a Chlorosilane Additive

A nickel-catalyzed reductive cross-coupling of redox active N-hydroxyphthalimide (NHP) esters and iodoarenes for the synthesis of α-aryl nitriles is described. The NHP ester substrate is derived from cyanoacetic acid, which allows for a modular synthesis of substituted α-aryl nitriles, an important scaffold in the pharmaceutical sciences. The reaction exhibits a broad scope, and many functional groups are compatible under the reaction conditions, including complex highly functionalized medicinal agents. Mechanistic studies reveal that reduction and decarboxylation of the NHP ester to the reactive radical intermediate are accomplished by a combination of a chlorosilane additive and Zn dust. We demonstrate that stoichiometric chlorosilane is essential for product formation and that chlorosilane plays a role beyond activation of the metal reductant.

Direct conversion of carboxylic acids to free thiols via radical relay acridine photocatalysis enabled by N-O bond cleavage

Carboxylic acids and thiols are basic chemical compounds with diverse utility and widespread reactivity. However, the direct conversion of unprotected acids to thiols is hampered due to a fundamental problem - free thiols are incompatible with the alkyl radicals formed on decarboxylation of carboxylic acids. Herein, we describe a concept for the direct photocatalytic thiolation of unprotected acids allowing unprotected thiols and their derivatives to be obtained. The method is based on the application of a thionocarbonate reagent featuring the N-O bond. The reagent serves both for the rapid trapping of alkyl radicals and for the facile regeneration of the acridine-type photocatalyst.

Towards a General Access to 1-Azaspirocyclic Systems via Photoinduced, Reductive Decarboxylative Radical Cyclizations

A convenient and versatile approach to important 1-azaspirocyclic systems relevant to medicinal chemistry and natural products is reported herein. The main strategy relies on a reductive decarboxylative cyclization of redox-active esters which can be rapidly assembled from abundant cyclic azaacids and tailored acceptor sidechains, with a focus on alkyne acceptors enabling the generation of useful exo-alkene moieties. Diastereoconvergent variants were studied and could be achieved either through remote stereocontrol or conformational restriction in bicyclic carbamate substrates. Two sets of metal-free photocatalytic conditions employing inexpensive eosin Y were disclosed and studied experimentally to highlight key mechanistic divergences.

Visible Light-Induced Imide Alkylation of Azauracils with Aryl Diazoesters

A visible light-induced green and sustainable N-H functionalization of (aza)uracils with α-diazo esters leading to imide alkylation is described. The reaction does not require any catalyst or additive and proceeds under mild conditions. Moreover, an intriguing three component coupling was observed when (aza)uracils were allowed to react with α-diazo esters in cyclic ethers (e. g. 1,4-dioxane, THF) as a solvent. Both the insertion and three-component coupling features broad scope with good to excellent yields and appreciable functional group tolerance. Notably, the divergent method enables modification of natural products and pharmaceuticals, thereby facilitates access to potentially biologically active compounds.

One-Pot Transformation of Aldehydes to Ketones via Minisci-Type Reaction of Imines

A method for the conversion of aldehydes to ketones via the preliminary formation of aldiminines is described. The imines are involved in acid promoted Minisci-type reaction with alkyl radicals generated from esters of N-hydroxylphthalimide under photoredox conditions. Aminyl radical cations formed after the addition of the iminium ions are believed to be key intermediates, which determine the reaction outcome.

Visible-Light-Promoted C(sp3)-C(sp3) Cross-Coupling of Amino Acids and Aryl Trifluoromethyl Ketones Through Simultaneous Decarboxylation and Defluorination

A photoredox-catalyzed approach for the difluoroalkylation of amino acids was achieved through simultaneous decarboxylation and defluorination processes. This innovative protocol employs commonly available amino acids and trifluoroacetophenones as the primary starting materials, eliminating the necessity for preactivation. This strategy has enabled the synthesis of several difluoroketone functionalized amines in moderate to impressive yields. These synthesized compounds are presented as foundational molecules for subsequent modification. The underlying mechanism for the transformation is anchored in a single electron transfer (SET) radical pathway.

Growing Utilization of Radical Chemistry in the Synthesis of Pharmaceuticals

Our current unhealthy lifestyle and the exponential surge in the population getting affected by a variety of diseases have made pharmaceuticals or drugs an imperative part of life, making the development of innovative strategies for drug discovery or the introduction of refined, cost-effective and modern technologies for the synthesis of clinically used drugs, a need of the hour. Ever since their discovery, free radicals and radical cations or anions as reactive intermediates have captivated the chemists, resulting in an exceptional utilization of these moieties throughout the field of chemical synthesis, owing to their unprecedented and widespread reactivity. Sticking with the idea of not judging the book by its cover, despite the conventional thought process of radicals being unstable and difficult to control entities, scientists and academicians around the globe have done an appreciable amount of work utilizing both persistent as well as transient radicals for a variety of organic transformations, exemplifying them with the synthesis of significant biologically active pharmaceutical ingredients. This review truly accounts for the organic radical transformations including radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling with metal-complexes and radical cations coupling with nucleophiles, that offers fascinating and unconventional approaches towards the construction of intricate structural frameworks of marketed APIs with high atom- and step-economy; complementing the otherwise employed traditional methods. This tutorial review presents a comprehensive package of diverse methods utilized for radical generation, featuring their reactivity to form critical bonds in pharmaceutical total synthesis or in building key starting materials or intermediates of their synthetic journey, acknowledging their excellence, downsides and underlying mechanisms, which are otherwise poorly highlighted in the literature. Despite great achievements over the past few decades in this area, many challenges and obstacles are yet to be unraveled to shorten the distance between the academics and the industry, which are all discussed in summary and outlook.

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.

A Chemoselective Polarity-Mismatched Photocatalytic C(sp3 )-C(sp2 ) Cross-Coupling Enabled by Synergistic Boron Activation

We report the development of a C(sp3 )-C(sp2 ) coupling reaction using styrene boronic acids and redox-active esters under photoredox catalysis. The reaction proceeds through an unusual polarity-mismatched radical addition mechanism that is orthogonal to established processes. Synergistic activation of the radical precursor and organoboron are critical mechanistic events. Activation of an N-hydroxyphthalimide (NHPI) ester by coordination to boron enables electron transfer, with decomposition leading to a nucleofuge rebound, activating the organoboron to radical addition. The unique mechanism enables chemoselective coupling of styrene boronic acids in the presence of other alkene radical acceptors. The scope and limitations of the reaction, and a detailed mechanistic investigation are presented.

Synthesis of Alkylated Polyfluorobenzenes through Decarboxylative Giese Addition of Aliphatic N-Hydroxyphthalimide Esters with Polyfluorostyrene

Polyfluoroaromatic compounds play crucial roles in medicinal and material science. However, the synthesis of alkylated polyfluoroarenes has been relatively underdeveloped. In this study, we devised a novel decarboxylative coupling reaction between aliphatic N-hydroxyphthalimide esters and polyfluorostyrene, leveraging the photochemical activity of electron donor-acceptor (EDA) complexes. This method offers simple reaction conditions, a broad substrate scope, and excellent functional group tolerance. Furthermore, we have demonstrated the practicality of this protocol through late-stage polyfluoroaryl modification of biologically active molecules using readily available carboxylic acids as starting materials, thus providing an important supplement to the current toolbox for accessing alkylated polyfluoroaryl motifs.

Recent Advances in Photochemical Asymmetric Three-Component Reactions

Over the past decades, asymmetric photochemical synthesis has garnered significant attention for its sustainability and unique ability to generate enantio-enriched molecules through distinct reaction pathways. Photochemical asymmetric three-component reactions have demonstrated significant potential for the rapid construction of chiral compounds with molecular diversity and complexity. However, noteworthy challenges persist, including the participation of high-energy intermediates such as radical species, difficulties in precise control of stereoselectivity, and the presence of competing background and side reactions. Recent breakthroughs have led to the development of sophisticated strategies in this field. This review explores the intricate mechanisms, synthetic applications, and limitations of these methods. We anticipate that it will contribute towards advancing asymmetric catalysis, photochemical synthesis, and green chemistry.

Nickel-Catalyzed Asymmetric Decarboxyarylation with NHP Esters of α-Amino Acid to Chiral Benzylamines

A nickel-catalyzed asymmetric decarboxyarylation of NHP esters via reductive cross-coupling has been established. Utilizing the NHP of amino acid esters as radical precursors furnishes a new protocol in which structurally diverse chiral benzylamines could be accessible. This method has demonstrated excellent catalytic efficiency, high enantioselective control, mild conditions, and good functional group tolerance, thus enabling the late-stage modification of bioactive molecules and pharmaceuticals.

Direct and Catalyst-Free Ester Metathesis Reaction for Covalent Adaptable Networks

Thermosetting polymers possess excellent environmental resistance and mechanical properties but cannot be reprocessed due to their covalently cross-linked structures. Recycling of thermosets via the implantation of dynamic covalent bonds offers a promising solution. Here, we report the direct and catalyst-free ester metathesis of N-acyloxyphthalimide (NAPI) at about 100 °C without the requirement of hydroxyl groups and its utilization for the fabrication of covalent adaptable networks (CANs). NAPI metathesis has interesting sigmoid kinetics with a fast exchange rate, which proceeds via a free radical chain mechanism, guaranteeing a fast associative exchange under a rather low dissociation. The bifunctional molecule of NAPI as both the radical precursor and substrate is the key to the dissociatively initiated associative (DAssociative) mechanism and kinetic behavior. Based on the efficient NAPI metathesis, polyester networks, poly(N-acyloxyphthalimides) (PNAPIs), show excellent malleability. Notably, PNAPIs exhibit exceptional solvent resistance and mechanical stability at elevated temperatures owing to the unique DAssociative mechanism, suggesting exciting opportunities for designing recyclable thermosetting polymers.

Photoredox-Catalyzed Decarboxylative Bromination, Chlorination and Thiocyanation Using Inorganic Salts

Decarboxylative halogenation reactions of alkyl carboxylic acids are highly valuable reactions for the synthesis of structurally diverse alkyl halides. However, many reported protocols rely on stoichiometric strong oxidants or highly electrophilic halogenating agents. Herein, we describe visible-light photoredox-catalyzed decarboxylative halogenation reactions of N-hydroxyphthalimide-activated carboxylic acids that avoid stoichiometric oxidants and use inexpensive inorganic halide salts as the halogenating agents. Bromination with lithium bromide proceeds under simple, transition-metal-free conditions using an organic photoredox catalyst and no other additives, whereas dual photoredox-copper catalysis is required for chlorination with lithium chloride. The mild conditions display excellent functional-group tolerance, which is demonstrated through the transformation of a diverse range of structurally complex carboxylic acid containing natural products into the corresponding alkyl bromides and chlorides. In addition, we show the generality of the dual photoredox-copper-catalyzed decarboxylative functionalization with inorganic salts by extension to thiocyanation with potassium thiocyanide, which was applied to the synthesis of complex alkyl thiocyanates.

Salicylaldehyde as an SET-HAT Bifunctional Photocatalyst for the Intermolecular Transalkylation of Phthalimide

Salicylaldehyde works as an efficient photocatalyst for the intermolecular transalkylation of phthalimide. The well-designed dimethyl N-hydroxyphthalimide ester proves to be a good alkylation reagent. It inhibits the competing intramolecular alkylation of alkylating reagent, enabling the site-specific synthesis of N-substituted phthalimide.

Aryl versus Alkyl Redox-Active Diazoacetates ─ Light-Induced C-H Insertion or 1,2-Rearrangement

Diazo compounds with redox-active leaving groups are versatile reagents for orthogonal functionalizations, previously utilized in the Rh-catalyzed synthesis of highly substituted cyclopropanes. Photochemical activation of aryl-substituted diazoacetates generates carbenes, whereas redox-active esters can furnish C-radicals via the photoexcitation of EDA complexes. However, the photochemical behavior of these two functionalities, while present in one molecule, remains to be defined. We demonstrate that under light irradiation, reactions occur only on the diazo moiety, leaving the NHPI functionality intact. Not only aryl- but also alkyl-substituted NHPI diazoacetates are activated by blue light; either C-H insertion or the hydrogen/carbon 1,2-rearrangement occurs depending on the aryl/alkyl group.

Decarboxylative Cross-Electrophile Coupling of (Hetero)Aromatic Bromides and NHP Esters

Aryl bromides are known to be challenging substrates in the decarboxylative cross-electrophile coupling with redox-active NHP esters-the majority of such processes utilize aryl iodides. Herein, we describe the development of conditions that are suitable for the decarboxylative cross-electrophile coupling of NHP esters and a wide range of (hetero)aryl bromides. The key advances that allowed for the use of aryl bromides in this reaction are (1) the identification of ligand L3 as an optimal ligand for the use of electron-neutral and deficient aryl bromides and (2) the significant improvement in yield that iodide salts and excess heterogenous zinc impart to this reaction. A wide variety of NHP esters perform well under the optimized conditions, including methyl, primary, secondary, and several strained tertiary systems. Likewise, a variety of aromatic and heteroaromatic bromides relevant to medicinal chemistry perform well in this transformation, including an aryl bromide precursor to the known drug dapagliflozin.

Nickel-Catalyzed Amination of Alkyl Electrophiles

Bimolecular nucleophilic substitution SN2 is the earliest and most important means of amination of alkyl electrophiles; its practical utilization is largely limited to primary or activated substrates. Furthermore, a persistent challenge lies in establishing C(sp3)-N bonds from alkyl substrates in cross-coupling chemistry using palladium and nickel catalysts. Therefore, the methods of constructing C(sp3)-N bonds remain rare from alkyl electrophiles. The existing routes are limited to copper catalysis and photoredox catalysis. Here, we demonstrate an alternative amination strategy for rapid construction of C(sp3)-N bonds from accessible alkyl electrophiles, which were used as radical precursors under nickel catalysis by Ni (III) species reductive eliminations in high efficiency.

Practical Synthesis and Antifungal Investigation of Drimane Meroterpenoids Enabled by Nickel-Catalyzed Decarboxylative Coupling

Drimane meroterpenoids have drawn increasing attention in the discovery of novel pharmaceutical leads owing to their structural diversity and bioactivity variation, but further development is significantly impeded by the lack of an efficient modular route of preparation. A nickel-catalyzed decarboxylative cross-coupling paradigm has been established to expeditiously access a constellation of drimane meroterpenoids. The redox-active drimane precursor is a bench-stable coupling partner and is easily available from the inexpensive feedstock sclareol. This transformation features the tolerance of challenging functional groups (phenol, aldehyde, ester, etc.) and mild conditions with a low-cost nickel catalytic system. The synthetic utility is further highlighted by the direct scalable synthesis of challenging drimane meroterpenoids as diversifiable advanced intermediates for late-stage functionalizations. This method facilitated antifungal investigations and culminated in the discovery of compounds C8 and C3 as new antifungal leads against Rhizoctonia solani, with EC50 values of 4.9 and 7.2 μM, respectively.

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.

Radical Decarboxylative Carbon-Nitrogen Bond Formation

The carbon-nitrogen bond is one of the most prevalent chemical bonds in natural and artificial molecules, as many naturally existing organic molecules, pharmaceuticals, agrochemicals, and functional materials contain at least one nitrogen atom. Radical decarboxylative carbon-nitrogen bond formation from readily available carboxylic acids and their derivatives has emerged as an attractive and valuable tool in modern synthetic chemistry. The promising achievements in this research topic have been demonstrated via utilizing this strategy in the synthesis of complex natural products. In this review, we will cover carbon-nitrogen bond formation via radical decarboxylation of carboxylic acids, Barton esters, MPDOC esters, N-hydroxyphthalimide esters (NHP esters), oxime esters, aryliodine(III) dicarboxylates, and others, respectively. This review aims to bring readers a comprehensive survey of the development in this rapidly expanding field. We hope that this review will emphasize the knowledge, highlight the proposed mechanisms, and further disclose the fascinating features in modern synthetic applications.

Photodecarboxylative C-H Alkylation of Azauracils with N-(Acyloxy)phthalimides

We disclose a transition-metal-free NaI/PPh₃-mediated direct C-H alkylation of azauracils using N-(acyloxy)pthalimides (NHPIs) as readily available alkyl surrogates under visible light irradiation. Detailed mechanistic studies reveal formation of a photoactivated electron donor-acceptor (EDA) complex between NaI/PPh₃, TMEDA, and alkyl NHPI ester and establish the crucial role of TMEDA in increasing the activity of the photoredox system. The reaction demonstrates a broad scope, scalability, and appreciable functional group tolerance. A variety of azauracils are shown to undergo alkylation by primary, secondary, and tertiary NHPI esters under mild conditions, furnishing the desired products in good to excellent yields.

Copper-Catalyzed Enantioselective Decarboxylative Cyanation of Benzylic Acids Promoted by Hypervalent Iodine(III) Reagents

Copper-catalyzed asymmetric radical cyanation reactions have emerged as a powerful strategy for rapid construction of α-chiral nitriles. However, the directly decarboxylative cyanation reactions of common alkyl carboxylic acids remain largely elusive. Herein, we report a protocol for copper-catalyzed direct and enantioselective decarboxylative cyanation of benzylic acids. The in situ activation of acid substrates by a commercially inexpensive hypervalent iodine(III) reagent promoted the yield of the alkyl radicals under mild reaction conditions without prefunctionalization. The structurally diverse chiral alkyl nitriles were produced in good yields with high enantioselectivities. In addition, the chiral products can be readily converted to other useful chiral compounds via further transformations.

Photoinduced Alkyl/Aryl Radical Cascade for the Synthesis of Quaternary CF3-Containing Oxindoles and Indoline Alkaloids

Metal- and additive-free, photoinduced decarboxylative radical alkylation-cyclization of CF₃-acrylamides with alkyl redox-active esters provided the corresponding quaternary CF₃-oxindole derivatives in good yields. Notably, diaryliodonium salts also efficiently participated in the arylation-cyclization of CF₃-acrylamides in environmentally benign H₂O as a solvent. The present approach has been extended for the concise synthesis of CF₃-attached indoline alkaloid analogues, i.e., CF₃-(±)-desoxyeseroline, CF₃-(±)-esermethole, and CF₃-(±) progesterone receptor antagonists. The preliminary mechanistic studies revealed that the reaction is likely to proceed through initial photoexcitation of redox-active ester/diaryliodonium salts followed by the SET process with acrylamide.

Synthesis of cyclopent-1-enecarbonitriles via a tandem Giese/HWE reaction initiated by visible light

In the manuscript, a novel method for the preparation of cyclopent-1-enecarbonitriles via tandem Giese/HWE reaction initiated by visible light in the presence of fac-Ir(ppy)₃ as a photocatyst has been described. The cascade reactivity combining radical and polar processes has proven applicable for a wide range of N-(acyloxy)phthalimides (which serve as precursors of the corresponding radicals) as well as diethyl (E)-(1-cyano-2-arylvinyl)phosphonates. The key parameters responsible for the success of the described strategy are: visible light, 1 mol% of photoredox catalyst, base, anhydrous solvent and inert atmosphere. The reaction results in new sp³-sp³ and sp²-sp² carbon-carbon bonds formation under mild conditions.

Construction of 3-Oxazolin-5-one via Visible Light-Induced Nondecarboxylative Coupling and Sequential Reactions

3-Oxazolin-5-ones are precursors of nitrile ylides and represent valuable 1,3-dipoles for constructing cyclic imines or pyrrole compounds. Harnessing the power of photocatalysis, we accomplished a straightforward synthesis of 3-oxazolin-5-ones from redox-active esters and secondary nitro compounds. Visible light-induced nondecarboxylative coupling of a redox-active ester, nitro aldol condensation, and subsequent visible light-induced N-oxide deoxygenation were accomplished within 2 h. The reaction mechanism was supported by experimental data and DFT calculations.

Pd-Catalyzed Photochemical Alkylative Functionalization of C═C and C═N Bonds

The development of excited-state palladium-catalyzed alkylative cyclization of acrylamides and the alkylation of quinoxalinones is described. The application of a variety of primary, secondary, and tertiary unactivated alkyl halides as alkyl radical precursors and the use of a simple catalyst system are the highlights of this reactivity manifold. The reactions exhibit wide scope, occur under mild conditions, and furnish the products in excellent yields.