Visible-Light-Induced Decarboxylative Fluorination of Aliphatic Carboxylic Acids Catalyzed by Iron

Iron-Cobalt Dual Catalysis for the Synthesis of Alkenyl Amino Acids and Modification of Peptides

Herein, we report an Fe/Co dual-catalyzed strategy for synthesizing alkenyl unnatural amino acids and peptide modifications. This approach utilizes aspartic acid and glutamic acid derivatives as alkyl radical precursors. It avoids the use of expensive photoredox catalysts and substrate preactivation while preserving the chirality of the amino acids. Furthermore, this strategy enables both modification of peptides and the synthesis of amino-acid-based drug candidates for boron-neutron capture therapy (BNCT).

Photoexcited Transition-Metal Catalyzed Carbon-Halogen Bond Formation

Transition-metal catalysis has proven useful in facilitating carbon-halogen (C-X) bond formation. Despite the vast number of methodologies reported to furnish these bonds, limitations have remained, warranting continued development. The recent surge of metallaphotoredox-based transformations has provided a novel gateway to bypass these limitations. Through the use of photoexcited species, the formation of C-X bonds arise through new mechanistic pathways, finding alternatives to high reaction temperatures and stoichiometric additives. The discovery of this novel strategy has provided access to molecular space that has not been previously attainable. Herein, we report the recent advances on transition-metal photocatalyzed C-X bond formation, in hopes of easing the synthetic endeavours for chemists in industrial and academic laboratories.

Decarboxylative Alkylation of Morita-Baylis-Hillman Acetates with Aliphatic Acids via Photochemical Iron-Mediated Ligand-to-Metal Charge Transfer

Carboxylic acids are bench-stable and readily available chemical feedstocks that function as optimal and fundamental synthetic platforms for the construction of C(sp3)-C(sp3) bonds via decarboxylation processes. We present a novel and practical protocol for the decarboxylative alkylation of Morita-Baylis-Hillman acetates with various carboxylic acids via a photoinduced iron-mediated ligand-to-metal charge transfer (LMCT) process under redox-neutral conditions. This method exhibits remarkable tolerance to a wide array of carboxylic acids, including primary, secondary, and tertiary carboxylic acids, obviating the requirement for preactivated radical precursors. The preliminary mechanistic analyses indicate that a radical pathway is involved in this catalytic transformation.

Visible-Light-Driven Synergistic Se/Fe Catalysis for the Synthesis of 2-Aminoquinoline Derivatives

A visible-light-driven Se/Fe synergistic catalytic reaction between isocyanides and amines has been developed, employing air as the terminal oxidant. This efficient strategy offers facile access to 2-aminoquinoline scaffolds. The use of cost-effective and readily available Fe(OTf)3 modulates the selenium catalyst's oxidation state, enhancing the method's environmental friendliness and practicality. The protocol is further distinguished by its good yields, broad substrate scope, and mild reaction conditions.

Iron-Photocatalyzed Decarboxylative Alkylation of Carboxylic Acids with Morita-Baylis-Hillman Acetates

We present an iron-photocatalyzed decarboxylative alkylation strategy involving carboxylic acids and Morita-Baylis-Hillman (MBH) acetates to synthesize E-type tri- and tetrasubstituted alkenes with moderate to excellent stereoselectivity (E/Z ratio up to >19:1). This method is applicable to a broad range of structurally diverse primary, secondary, and tertiary alkyl carboxylic acids, as well as complex pharmaceutical and natural carboxylic acids, achieving efficient alkylation of various MBH acetates under mild conditions (>60 examples, with yields up to 96%). This approach offers a powerful strategy for streamlined alkylation.

Iron-Catalyzed SO2-Retaining Smiles Rearrangement through Decarboxylation

Radical Smiles rearrangements have emerged as powerful methodologies for constructing carbon-carbon bonds through intramolecular radical addition and fragmentation under milder conditions, with SO2 released as a byproduct. However, SO2-retaining Smiles rearrangements, which can yield valuable alkyl sulfone derivatives, have been scarcely explored. In this study, we present an unprecedented iron-catalyzed SO2-retaining Smiles rearrangement initiated by the decarboxylation of aliphatic carboxylic acids. This approach provides a mild, cost-effective, and versatile pathway to sulfone-containing compounds, demonstrating broad substrate scope and functional group tolerance. It offers a promising strategy for synthesizing γ- and δ-aryl substituted alkyl sulfones, which are traditionally challenging to produce.

Decarboxylative Alkylation of Carboxylic Acids with Easily Oxidizable Functional Groups Catalyzed by an Imidazole-Coordinated Fe3 Cluster under Visible Light Irradiation

Decarboxylative alkylation of carboxylic acids with easily oxidizable functional groups such as phenol and indole functionalities was achieved using a catalytic amount of basic iron(III) acetate, Fe(OAc)2(OH), in the presence of benzimidazole under 427 nm LED irradiation. Kinetic analyses of this catalytic reaction revealed that the reaction rate is first-order in alkenes and is linearly correlated with the light intensity; the faster reaction rate for the benzimidazole-ligated species was consistent with the increased absorbance in the visible light region. Wide functional group tolerance for the easily oxidizable groups is ascribed to the weak oxidation ability of the in situ-generated oxo-bridged iron clusters compared with other iron(III) species.

Substitution, Elimination, and Integration of Methyl Groups in Terpenes Initiated by C-H Bond Functionalization

Methyl groups are ubiquitous in natural products and biologically active compounds, but methods for their selective transformation in such structures are limited. For example, terpenoids contain many methyl groups, due to their biosynthetic pathways, but few reactions of these groups in such structures have been reported. We demonstrate that the combination of methyl C-H silylation and oxidation proximal to native hydroxyl or carbonyl groups occurs in a range of terpenoids and show that the installed hydroxyl group serves as a toehold to enable substitution, elimination, or integration of the methyl carbon into the terpenoid skeleton by the cleavage of C-C bonds. In one case, substitution of the entire methyl group occurs by further oxidation and decarboxylative coupling. In a second, substitution of the methyl group with hydrogen occurs by photochemical hydrodecarboxylation or epimerization by retro-Claisen condensation. In a third, photocatalytic decarboxyolefination formally eliminates methane from the starting structure to generate a terminal olefin for further transformations. Finally, a Dowd-Beckwith-type rearrangement cleaves a nearby C-C bond and integrates the methyl group into a ring, forming derivatives with unusual and difficult-to-access expanded rings. This strategy to transform a methyl group into a synthon marks a distinct approach to restructuring the skeletons of complex architectures and adding functional groups relevant to medicinal chemistry.

Decarboxylative Cross-Coupling Enabled by Fe and Ni Metallaphotoredox Catalysis

Decarboxylative cross-coupling of carboxylic acids and aryl halides has become a key transformation in organic synthesis to form C(sp2)-C(sp3) bonds. In this report, a base metal pairing between Fe and Ni has been developed with complementary reactivity to the well-established Ir and Ni metallaphotoredox reactions. Utilizing an inexpensive FeCl3 cocatalyst along with a pyridine carboxamidine Ni catalyst, a range of aryl iodides can be preferentially coupled to carboxylic acids over boronic acid esters, triflates, chlorides, and even bromides in high yields. Additionally, carboxylic acid derivatives containing heterocycles, N-protected amino acids, and protic functionality can be coupled in 23-96% yield with a range of sterically hindered, electron-rich, and electron-deficient aryl iodides. Preliminary catalytic and stoichiometric reactions support a mechanism in which Fe is responsible for the activation of carboxylic acid upon irradiation with light and a NiI alkyl intermediate is responsible for activation of the aryl iodide coupling partner followed by reductive elimination to generate product.

Visible-Light-Promoted Iron(II)/Lewis Base Catalysis for the Alkylation of Morita-Baylis-Hillman Acetates Using Carboxylic Acids

A novel photoinduced Fe(OTf)2/2,4,6-collidine-catalyzed alkylation of Morita-Baylis-Hillman (MBH) acetates using carboxylic acids in a regio- and stereoselective manner is reported. This method demonstrates a broad scope, encompassing various carboxylic acids and MBH acetates, including drugs and bioactive molecules, to synthesize densely functionalized cinnamates and acrylates. The reactions are performed in the green solvent dimethyl carbonate under oxidant-free conditions. Based on control experiments, a plausible mechanism involving an Fe(II)-Fe(III)-Fe(II) cycle is proposed.

Synthesis of 5-Trifluoromethyl-1,3-thiazin-4-one Compounds using Trifluoromethyl Acrylic Acid as a Synthon

A novel method has been developed for the synthesis of 1,3-thiazin-4-one compounds containing trifluoromethyl groups utilizing 2-trifluoromethyl acrylic acid and thioamides as key starting materials. This protocol is characterized by its simplicity, practicality, and tolerance toward various functional groups. Given the straightforward nature of the procedure, the ready availability of both starting materials, and the significance of drugs containing trifluoromethyl, it is anticipated that this reaction will have wide-ranging applications.

Photocatalytic Decarboxylative Fluorination by Quinone-Based Isoreticular Covalent Organic Frameworks

The strategic incorporation of fluorine atoms into molecules has become a cornerstone of modern pharmaceuticals, agrochemicals, and materials science. Herein, we have developed a covalent organic framework (COF)-based, robust photocatalyst that enables the photofluorodecarboxylation reaction of diverse carboxylic acids, producing alkyl fluorides with remarkable efficiency. The catalytic activity of an anthraquinone-based COF catalyst TpAQ outperforms other structurally analogous β-ketoenamine COFs. Through comprehensive control experiments, photoluminescence, and electrochemical studies, we have elucidated the unique features of the material and the mechanistic pathway. This in-depth understanding has paved the way for optimizing the reaction conditions and achieving high yields of alkyl fluorides. The versatility of this protocol extends to a broad range of aliphatic acids with diverse functional groups and heterocycles. It also enabled the late-stage diversification of anti-inflammatory drugs and steroid derivatives. This opens up exciting possibilities for synthesizing novel pharmaceuticals and functionalized molecules. The methodology was also generalized to other light-mediated decarboxylative halogenation reactions. Furthermore, our method demonstrates scalability under both batch and continuous flow conditions, offering a promising approach for large-scale production. Additionally, the TpAQ catalyst exhibits exceptional durability and can be reused multiple times without significant activity loss (>80% yield after the eighth cycle), making it a sustainable and cost-effective solution. This work lays the foundation for developing efficient and sustainable light-driven synthesis methods using COFs as photocatalysts with potential applications beyond alkyl halide synthesis.

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.

Iron-Catalyzed Multicomponent C-H Alkylation of in Situ Generated Imines via Photoinduced Ligand-to-Metal Charge Transfer

Herein, we describe a novel photoinduced iron-catalyzed strategy for multicomponent C-H alkylation of in situ generated imines. By utilizing the alkyl radicals generated through iron-mediated photocatalytic C-H activation, the imines formed in situ are further subjected to addition reactions, resulting in the synthesis of various secondary and tertiary amine products. This method is simple to operate and does not require additional oxidants. It is applicable to inert alkane substrates such as cyclic alkanes, cyclic ethers, toluene, and ketones. The reaction is also compatible with various aromatic amines, alkyl amines, halogenated aromatic amines, as well as aromatic aldehydes, alkyl aldehydes, and cinnamaldehyde, among other different types of aldehydes.

Iron-Catalyzed, Light-Driven Decarboxylative Alkoxyamination

An iron-catalyzed visible-light driven decarboxylative alkoxyamination is disclosed. In the presence of FeBr2 and TEMPO, a large array of carboxylic acids including marketed drugs and biobased molecules is turned into the corresponding alkoxyamine derivatives. The versatility of the latter offers an entry towards molecular diversity generation from abundant starting materials and catalyst. Overall, this method proposes a unified and general approach for LMCT-based iron-catalyzed decarboxylative functionalization.

Light-induced ligand-to-metal charge transfer of Fe(III)-OR species in organic synthesis

Light-induced ligand-to-metal charge transfer (LMCT) has been utilized as a powerful strategy in various organic reactions. First-row transition metals, especially iron complexes, show good applications in this process. Fe(III)-Cl and Fe(III)-OR species are two key intermediates involved in the LMCT of iron complexes. This review highlights studies on LMCT of Fe(III)-OR species, including carboxylate-iron and alkoxy-iron species, in organic transformations. Reaction conditions, substrate scope and related mechanisms are discussed.

Photoinduced Ligand-to-Metal Charge Transfer in Base-Metal Catalysis

The absorption of light by photosensitizers has been shown to offer novel reactive pathways through electronic excited state intermediates, complementing ground state mechanisms. Such strategies have been applied in both photocatalysis and photoredox catalysis, driven by generating reactive intermediates from their long-lived excited states. One developing area is photoinduced ligand-to-metal charge transfer (LMCT) catalysis, in which coordination of a ligand to a metal center and subsequent excitation with light results in the formation of a reactive radical and a reduced metal center. This mini review concerns the foundations and recent developments in ligand-to-metal charge transfer in transition metal catalysis focusing on the organic transformations made possible through this mechanism.

Iron-catalyzed decarboxylative radical addition to chiral azomethine imines upon visible light

Herein, we disclose an eco-efficient redox-neutral iron-catalyzed decarboxylative radical addition to chiral azomethine imines upon visible light (427 nm) giving cyclic hydrazine derivatives with dr ranging from 82 : 18 to >96 : 4. This earth-abundant metal promoted sequence proceeds efficiently under ligand-free conditions based on a LMCT process and opens a route to new chiral heterocycles.

Decarboxylative Nucleophilic Fluorination of Aliphatic Carboxylic Acids

Herein, we present a decarboxylative nucleophilic fluorination of carboxylic acids with a silver catalyst. This strategy enables the synthesis of a myriad of diverse and valuable fluorinated motifs under mild conditions, demonstrating good functional-group tolerance and utility in late-stage functionalization. In contrast to traditional electrophilic fluorination, this nucleophilic method utilizes a more readily available nucleophilic fluorinating reagent, providing substantial advantages in terms of cost efficiency, broad substrate scope, and functional-group compatibility.

Silver-Catalyzed Decarboxylative Remote Fluorination via a Zwitterion-Promoted 1,4-Heteroaryl Migration

A silver-catalyzed decarboxylative remote fluorination via a zwitterion-promoted 1,4-heteroaryl migration has been developed. A variety of heteroaryl-tethered benzyl fluorides have been readily synthesized with good regioselectivity under mild conditions. The zwitterion of the substrate is suggested to accelerate the 1,4-heteroaryl migration, which determines the regioselectivity of this transformation.

Iron-Catalyzed Csp2-Csp3 Cross-Coupling via Double Decarboxylation: One Step Synthesis of Remote Polar Alkenes

Herein, we report an efficient photoinduced iron-catalyzed strategy for cross-couplings of alkyl carboxylic and acrylic acids, which provides a powerful tool for the synthesis of a variety of alkenes with polar functional groups. This novel synthetic methodology can also be applied to the preparation of ketones by using α-keto acids. Mechanistic experiments revealed preliminary mechanistic details. Diverse functionalization could be achieved, which may help streamline the synthesis of complex analogues for drug discovery.

Decarboxylative halogenation of aliphatic carboxylic acids catalyzed by iron salts under visible light

In this article, we report a general protocol for the direct decarboxylative chlorination, iodination, and bromination of aliphatic carboxylic acids catalyzed by iron salts under visible light. This method enjoys a broad substrate scope with good functional group compatibility, including complex natural products. Benzylic and allylic C(sp3)-H bonds can be retained under the oxidative halogenation conditions. This method also shows application potential for late-stage functionalization.

Radical-Mediated Decarboxylative C-C and C-S Couplings of Carboxylic Acids via Iron Photocatalysis

Despite the significant success of decarboxylative radical reactions, the catalytic systems vary considerably upon different radical acceptors, requiring renewed case-by-case reaction optimization. Herein, we developed an iron catalytic condition that enables the highly efficient decarboxylation of various carboxylic acids for a range of radical transformations. This operationally simple protocol was amenable to a wide array of radical acceptors, delivering structurally diverse oxime ethers, alkenylation, alkynylation, thiolation, and amidation products in useful to excellent yields (>40 examples, up to 95% yield).

Selective α-oxidation of amides via visible-light-driven iron catalysis

Hydroxyl radicals (˙OH) as one of the highly reactive species can react unselectively with a wide range of chemicals. The ˙OH radicals are typically generated under harsh conditions. Herein, we report hydroxyl radical-induced selective N-α C(sp3)-H bond oxidation of amides under greener and mild conditions via an Fe(NO3)3·9H2O catalyst inner sphere pathway upon irradiation with a 30 W blue LED light strip (λ = 455 nm) using NaBrO3 as the oxidant. This protocol exhibited high chemoselectivity and excellent functional group tolerance. A preliminary mechanism investigation demonstrated that the iron catalyst afforded hydroxyl radicals via the visible-light-induced homolysis (VLIH) of iron complexes followed by a hydrogen atom transfer (HAT) process to realize this transformation.

Photocatalytic hydrofluoroalkylation of alkenes with carboxylic acids

Incorporation of fluoroalkyl motifs in pharmaceuticals can enhance the therapeutic profiles of the parent molecules. The hydrofluoroalkylation of alkenes has emerged as a promising route to diverse fluoroalkylated compounds; however, current methods require superstoichiometric oxidants, expensive/oxidative fluoroalkylating reagents and precious metals, and often exhibit limited scope, making a universal protocol that addresses these limitations highly desirable. Here we report the hydrofluoroalkylation of alkenes with cheap, abundant and available fluoroalkyl carboxylic acids as the sole reagents. Hydrotrifluoro-, difluoro-, monofluoro- and perfluoroalkylation are all demonstrated, with broad scope, mild conditions (redox neutral) and potential for late-stage modification of bioactive molecules. Critical to success is overcoming the exceedingly high redox potential of feedstock fluoroalkyl carboxylic acids such as trifluoroacetic acid by leveraging cooperative earth-abundant, inexpensive iron and redox-active thiol catalysis, enabling these reagents to be directly used as hydroperfluoroalkylation donors without pre-activation. Preliminary mechanistic studies support the radical nature of this cooperative process.

Visible light-mediated halogenation of organic compounds

The use of visible light and photoredox catalysis emerged as a powerful and sustainable tool for organic synthesis, showing high value for distinctly different ways of bond creation. Halogenated compounds are the cornerstone of contemporary organic synthesis: it is almost impossible to develop a route towards a pharmaceutical reagent, agrochemical, natural product, etc. without the involvement of halogen-containing intermediates. Moreover, the halogenated derivatives as final products became indispensable for drug discovery and materials science. The idea of this review is to understand and summarise the impact of visible light-promoted chemistry on halogenation and halofunctionalisation reactions.

Synthesis of Tertiary Fluorides through an Acid-Mediated Deoxyfluorination of Tertiary Alcohols

The combination of methanesulfonic acid and potassium bifluoride is reported for the deoxyfluorination of tertiary alcohols. Under metal-free conditions that use readily available, cheap, and easy-to-handle reagents, a range of tertiary alcohols could be converted into the corresponding fluorides in excellent yields (average yields of 85% for 23 examples). Mechanistic investigation showed that the reaction proceeds at 0 °C, in part, through an elimination/hydrofluorination pathway, but no residual alkenes are observed. The application of these conditions for the fluorination of ether and ester is also demonstrated.

Iron-mediated modular decarboxylative cross-nucleophile coupling

Carboxylic acids are valuable building blocks for pharmaceutical discovery because of their chemical stability, commercial availability, and structural diversity. Decarboxylative coupling reactions enable versatile functionalization of these feedstock chemicals, but many of the most general methods require prefunctionalization of carboxylic acids with redox-active moieties. These internal oxidants can be costly, their installation impedes rapid library synthesis, and their use results in environmentally problematic organic byproducts. We report herein a method for the direct decarboxylative cross-coupling of native carboxylic acids with nucleophilic coupling partners mediated by inexpensive, terrestrially abundant, and nontoxic Fe(III) salts. This method involves an initial photochemical decarboxylation followed by radical-polar crossover, which enables the construction of diverse carbon-carbon, carbon-oxygen, and carbon-nitrogen bonds with remarkable generality.

Ferrocenophanium Stability and Catalysis

Ferrocenium catalysis is a vibrant research area, and an increasing number of ferrocenium-catalyzed processes have been reported in the recent years. However, the ferrocenium cation is not very stable in solution, which may potentially hamper catalytic applications. In an effort to stabilize ferrocenium-type architectures by inserting a bridge between the cyclopentadienyl rings, we investigated two ferrocenophanium (or ansa-ferrocenium) cations with respect to their stability and catalytic activity in propargylic substitution reactions. One of the ferrocenophanium complexes was characterized by single crystal X-ray diffraction. Cyclic voltammetry experiments of the ferrocenophane parent compounds were performed in the absence and presence of alcohol nucleophiles, and the stability of the cations in solution was judged based on the reversibility of the electron transfer. The experiments revealed a moderate stabilizing effect of the bridge, albeit the effect is not very pronounced or straightforward. Catalytic propargylic substitution test reactions revealed decreased activity of the ferrocenophanium cations compared to the ferrocenium cation. It appears that the somewhat stabilized ferrocenophanium cations show decreased catalytic activity.

NIS-initiated photo-induced oxidative decarboxylative sulfoximidation of cinnamic acids

N-Iodosuccinimide catalyzed, visible-light-induced oxidative decarboxylative cross-coupling between cinnamic acids and NH-sulfoximines is presented. This strategy results in the formation of α-keto-N-acyl sulfoximines via the construction of two new CO bonds and one C-N bond. The in situ-generated N-iodosulfoximine serves as the light-absorbing species in the absence of any external photosensitizer. The keto carbonyl and amidic carbonyl oxygen in the resulting product originate from dioxygen and water respectively.

Sustainable Photoinduced Decarboxylative Chlorination Mediated by Halogen Atom Transfer

Chlorinated organic backbones constitute important components in existing biologically active chemicals, and they are extraordinary useful intermediates in organic synthesis. Herein, an operationally simple and sustainable halodecarboxylation protocol via halogen-atom transfer (XAT) as a key step is presented. The method merges a metal-free photoredox system with (diacetoxyiodo)benzene (PIDA) as a hypervalent iodine reagent using 1,2-dihaloethanes as halogen sources to afford haloalkanes in an efficient manner. The sustainability of this protocol is highlighted by an important waste recovery protocol as well as by atom economy and carbon efficiency parameters.

Decarboxylative sulfoximination of benzoic acids enabled by photoinduced ligand-to-copper charge transfer

Sulfoximines are synthetically important scaffolds and serve important roles in drug discovery. Currently, there is no solution to decarboxylative sulfoximination of benzoic acids; although thoroughly investigated, limited substrate scope and harsh reaction conditions still hold back traditional thermal aromatic decarboxylative functionalization. Herein, we realize the first decarboxylative sulfoximination of benzoic acids via photo-induced ligand to copper charge transfer (copper-LMCT)-enabled decarboxylative carbometalation. The transformation proceeds under mild reaction conditions, has a broad substrate scope, and can be applied to late-stage functionalization of complex small molecules.