Iron-Catalyzed Photoinduced LMCT: a 1° C-H Abstraction Enables Skeletal Rearrangements and C(sp3)-H Alkylation

Energy transfer-mediated multiphoton synergistic excitation for selective C(sp3)-H functionalization with coordination polymer

Activation and selective oxidation of inert C(sp3)-H bonds remain one of the most challenging tasks in current synthetic chemistry due to the inherent inertness of C(sp3)-H bonds. In this study, inspired by natural monooxygenases, we developed a coordination polymer with naphthalenediimide (NDI)-based ligands and binuclear iron nodes. The mixed-valence FeIIIFeII species and chlorine radicals (Cl•) are generated via ligand-to-metal charge transfer (LMCT) between FeIII and chlorine ions. These Cl• radicals abstract a hydrogen atom from the inert C(sp3)-H bond of alkanes via hydrogen atom transfer (HAT). In addition, NDI converts oxygen to 1O2 via energy transfer (EnT), which then coordinates to FeII, forming an FeIV = O intermediate for the selective oxidation of C(sp3)-H bonds. This synthetic platform, which combines photoinduced EnT, LMCT and HAT, provides a EnT-mediated parallel multiphoton excitation strategy with kinetic synergy effect for selective C(sp3)-H oxidation under mild conditions and a blueprint for designing coordination polymer-based photocatalysts for C(sp3)-H bond oxidation.

Bifunctional iron-catalyzed alkyne Z-selective hydroalkylation and tandem Z-E inversion via radical molding and flipping

The challenging synthesis of thermodynamic-unfavored cis-olefins through catalytic cross-coupling reactions requires the synergistic interaction of substrate-activating units and configuration-regulating catalysts. Successfully hitting these two birds with one stone, we herein develop a convenient photoredox access to Z-alkenes from alkynes and light alkanes with a bifunctional iron-catalyzed system possessing both C(sp3)-H activation and configuration-controlling abilities. The protocol exhibits 100% atom utilization, mild conditions, a broad substrate scope, and compatibility with multitudinous functional groups. The detailed reaction mechanism and the origin of geometry regulation are well investigated by experimental and computational studies. Progressively, a catalytic amount of diaryl disulfides is introduced for consecutive photoinduced Z-E isomerization via reversible radical addition and flipping. Big steric hindrance substituents assembled on the disulfide emerge necessity for suppressing double-bond migration. This tandem strategy paves a promising way for stereoselective alkene construction and will bring significant inspiration for the development of transition metal photocatalysis.

Building Catalytic Reactions One Electron at a Time

ConspectusClassical education in organic chemistry and catalysis, not the least my own, has centered on two-electron transformations, from nucleophilic attack to oxidative addition. The focus on two-electron chemistry is well-founded, as this brand of chemistry has enabled incredible feats of synthesis, from the development of life-saving pharmaceuticals to the production of ubiquitous commodity chemicals. With that said, this approach is in many ways complementary to the approach of nature, where enzymes frequently make use of single-electron "radical" steps to achieve challenging reactions with exceptional selectivity, including light detection and C-H hydroxylation. While the power of radical elementary steps is undeniable, the fundamental understanding of─and ability to apply─these in catalysis remains underdeveloped, constraining the palette with which chemists can make new reactions.Motivation to remedy this traditional underemphasis on radical catalysis has been intensified by the runaway success of outer-sphere photoredox catalysis, not only confirming the versatility of radicals in anthropogenic catalysis but also teaching the value of robust and well-understood catalytic cycles for reaction design. Indeed, I would argue the success of outer-sphere photoredox catalysis has been fueled by strong fundamental understanding of its underlying radical elementary steps, with consideration of single-electron transfer (SET) energetics allowing new reactions to be designed de novo with enviable confidence. However, outer-sphere photoredox catalysis is an outlier in this regard, with other mechanistic approaches remaining underexplored.Our research group is part of a growing movement to expand the vocabulary of synthetic radical catalysis beyond the traditional outer-sphere photoredox SET manifold, assembling new cycles comprised of hydrogen atom transfer (HAT), light-induced homolysis (LIH), and radical ligand transfer (RLT) steps in new combinations to achieve challenging transformations. These efforts have been made possible by the ever-growing understanding of these radical elementary steps and discovery of catalyst systems with significant mechanistic flexibility, most recently iron/thiol (Fe/S) cocatalysis.In this Account, I will focus on our efforts applying HAT and LIH steps in Fe/S cocatalysis, sharing broad guidelines we have found helpful for using these steps and demonstrating how they can be combined to make new reactions using three case studies: radical hydrogenation (HAT + HAT), decarboxylative protonation (LIH + HAT), and alkene hydrofluoroalkylation (LIH + HAT, with an intervening radical alkene addition). These efforts have highlighted the importance of several key parameters, including bond dissociation energy (BDE) and radical polarity, and I hope our findings similarly provide a valuable framework to others designing new radical catalytic reactions.

Iron-Catalyzed Electrophotochemical α-Functionalization of a Silylcyclobutanol

Four-membered ring structure is important in organic chemistry, and selective cleavage and functionalization of these strained rings are of great interest. However, direct α-functionalization of cyclobutanols is rarely reported because of the high O-H bond dissociation energy and the occurrence of β-scission of C-C bonds in these alcohols. Recently, transition-metal catalysis has facilitated alkoxy radical generation. Herein, we report a method for electrophotochemical α-functionalization of a silylcyclobutanol via visible-light-induced LMCT reactions of M-alkoxy complexes. Introduction of the silyl group into the cyclobutanol structure favored fast [1,2]-silyl transfer over ring opening, thus allowing the generation of α-functionalized products.

Iron photocatalysis via Brønsted acid-unlocked ligand-to-metal charge transfer

Reforming sustainable 3d-metal-based visible light catalytic platforms for inert bulk chemical activation is highly desirable. Herein, we demonstrate the use of a Brønsted acid to unlock robust and practical iron ligand-to-metal charge transfer (LMCT) photocatalysis for the activation of multifarious inert haloalkylcarboxylates (CnXmCOO-, X = F or Cl) to produce CnXm radicals. This process enables the fluoro-polyhaloalkylation of non-activated alkenes by combining easily available Selectfluor as a fluorine source. Valuable alkyl fluorides including potential drug molecules can be easily obtained through this protocol. Mechanistic studies indicate that the real light-harvesting species may derive from the in situ-assembly of Fe3+, CnXmCOO-, H+, and acetonitrile solvent, in which the Brønsted acid indeed increases the efficiency of LMCT between the iron center and CnXmCOO- via hydrogen-bond interactions. We anticipate that this Brønsted acid-unlocked iron LMCT platform would be an intriguing sustainable option to execute the activation of inert compounds.

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 Photoredox Alcohol α-C-H Alkylation and Tandem Intramolecular Cyclization: Facile Access to Multisubstituted 2,3-Dihydrofurans and γ-Butyrolactones

Multisubstituted furans occupy a pivotal position within the realms of synthetic chemistry and pharmacological science due to their distinctive chemical configurations and inherent properties. We herein introduce a tandem difunctionalization protocol of alcohols for the efficient synthesis of multisubstituted 2,3-dihydrofurans and γ-butyrolactones through the combination of photocatalysis and iron catalysis under mild conditions. Photoredox alcohol α-C(sp3)-H activation and Pinner-type intramolecular cyclization are two key processes. This method features significant convenience, economic benefits, and environmental friendliness.

Photogeneration of Chlorine Radical from a Self-Assembled Fluorous 4CzIPN•Chloride Complex: Application in C-H Bond Functionalization

The chlorine radical is a strong HAT (Hydrogen Atom Transfer) agent that is very useful for the functionalization of C(sp3)-H bonds. Albeit highly attractive, its generation from the poorly oxidizable chloride ion mediated by an excited photoredox catalyst is a difficult task. We now report that 8Rf8-4CzIPN, an electron-deficient fluorous derivative of the benchmark 4CzIPN photoredox catalyst belonging to the donor-acceptor carbazole-cyanoarene family, is not only a better photooxidant than 4CzIPN, but also becomes an excellent host for the chloride ion. Combining these two properties ultimately makes the self-assembled 8Rf8-4CzIPN•Cl- dual catalyst highly reactive in redox-neutral Giese-type C(sp3)-H bond alkylation reactions promoted by the chlorine radical. Additionally, because of its fluorous character, the efficient separation/recovery of 8Rf8-4CzIPN could be envisioned.

Ultrafast Events of Photoexcited Iron(III) Chloride for Activation of Benzylic C-H Bonds

The usage of rare-earth-metal catalysts in the synthesis of organic compounds is widespread in chemical industries but is limited owing to its environmental and economic costs. However, recent studies indicate that abundant-earth metals like iron(III) chloride can photocatalyze diverse organic transformations using blue-light LEDs. Still, the underlying mechanism behind such activity is debatable and controversial, especially in the absence of ultrafast spectroscopic results. To address this urgent challenge, we performed femtosecond time-resolved electronic absorption spectroscopy experiments of iron(III) chloride in selected organic solvents relevant to its photocatalytic applications. Our results show that the long-lived species [Fe(II) ← Cl•]* is primarily responsible for both oxidizing the organic substrate and reducing molecular oxygen through the diffusion process, leading to the final product and regenerating the photocatalyst rather than the most widely proposed free chloride radical (Cl•). Our study will guide the rational design of efficient earth-abundant photocatalysts.

Iron-Catalyzed Oxidative Rearrangement of Cyclopropanone Hemiaminals: General Access to Pyrroloindolones from Indoles

A concise synthetic approach to medicinally relevant pyrroloindolones and related fused heterocycles is reported via the diastereoselective N-addition of unprotected indoles to readily accessible cyclopropanone equivalents. The resulting stable hemiaminals are shown to smoothly rearrange to pyrroloindolones in mild conditions using Fe(III) catalysis in the presence of inexpensive ammonium persulfate as a stoichiometric oxidant. Experimental evidence points toward the formation of a β-carboxylic radical intermediate prone to cyclization and oxidative rearomatization as the operative mechanistic pathway.

Photosynthesis of C-1-Deuterated Aldehydes via Chlorine Radical-Mediated Selective Deuteration of the Formyl C-H Bond

C-1-deuterated aldehydes are essential building blocks in the synthesis of deuterated chemicals and pharmaceuticals. This has led chemists to devise mild methodologies for their efficient production. Ideally, hydrogen-deuterium exchange (HDE) is the most effective approach. However, the traditional HDE for creating C-1-deuterated aldehydes often requires a complex system involving multiple catalysts and/or ligands. In this study, we present a mild photocatalytic HDE of the formyl C-H bond with D2O. This process is facilitated by chlorine radicals that are generated in situ from low-cost FeCl3. This strategy demonstrated a broad reaction scope and high functional group tolerance, affording good yields and ≤99% D incorporation. To bridge the gap between research and industrial applications, we designed a new flow photoreactor equipped with a high-intensity light-emitting diode bucket, enabling the synthesis of C-1-deuterated aldehydes on a scale of 85 g. Finally, we successfully produced several important deuterated aldehydes that are integral to the synthesis of deuterated pharmaceuticals.

Expedient radical phosphonylations via ligand to metal charge transfer on bismuth

Bismuth, in spite of its low cost and low toxicity, has found limited application in organic synthesis. Although the photoactivity of Bi(iii) salts has been well studied, this has not been effectively exploited in photocatalysis. To date, only a single report exists for the Bi-based photocatalysis, wherein carbon centered radicals were generated using ligand to metal charge transfer (LMCT) on bismuth. In this regard, expanding the horizon of bismuth LMCT catalysis for the generation of heteroatom centered radicals, we hereby report an efficient radical phosphonylation using BiCl3 as the LMCT catalyst. Phosphonyl radicals generated via visible-light induced LMCT of BiCl3 were subjected to a variety of transformations like alkylation, amination, alkynylation and cascade cyclizations. The catalytic system tolerated a wide range of substrate classes, delivering excellent yields of the scaffolds. The reactions were scalable and required low catalytic loading of bismuth. Detailed mechanistic studies were carried out to probe the reaction mechanism. Diverse radical phosphonylations leading to the formation of sp3-C-P, sp2-C-P, sp-C-P, and P-N bonds in the current work present the candidacy of bismuth as a versatile photocatalyst for small molecule activation.

Iron-Catalyzed Photochemical Synthesis of Sulfinamides from Aliphatic Hydrocarbons and Sulfinylamines

An iron-catalyzed photochemical sulfinamidation of hydrocarbons with N-sulfinylamines has been developed. The merger of ligand-to-metal charge transfer (LMCT) of FeCl3 with hydrogen atom transfer (HAT) process is the key for the generation of alkyl radicals from hydrocarbons, and the resultant alkyl radicals were readily trapped by N-sulfinylamines to produce structurally diverse sulfinamides. Contrary to traditional methods that inevitably use sensitive organometallic reagents and prefunctionalized substrates, our approach features simple operation and the wide availability of starting materials. Gratifyingly, the reaction is scalable, and the obtained sulfinamides can be conveniently converted to highly functionalized sulfur(VI) derivatives.

Multi-stimuli-responsive polymer degradation by polyoxometalate photocatalysis and chloride ions

Photocatalytic polymer degradation based on harnessing the abundant light energy present in the environment is one of the promising approaches to address the issue of plastic waste. In this study, we developed a multi-stimuli-responsive photocatalytic polymer degradation system facilitated by the photocatalysis of a polyoxometalate [γ-PV2W10O40]5- in conjunction with chloride ions (Cl-) as harmless and abundant stimuli. The degradation of various polymers was significantly accelerated in the presence of Cl-, which was attributed to the oxidation of Cl- by the polyoxometalate photocatalysis into a highly reactive chlorine radical that can efficiently generate a carbon-centered radical for subsequent polymer degradation. Although organic and organometallic photocatalysts decomposed under the conditions for photocatalytic polymer degradation in the presence of Cl-, [γ-PV2W10O40]5- retained its structure even under these highly oxidative conditions.

Photogenerated chlorine radicals activate C(sp3)-H bonds of alkylbenzenes to access quinazolinones

An Fe-catalyzed visible-light induced condensation of alkylbenzenes with anthranilamides has been developed. Upon irradiation, the trivalent iron complex could generate chlorine radicals, which successfully abstracted the hydrogen of benzylic C-H bonds to form benzyl radicals. And these benzyl radicals were converted into oxygenated products under air conditions, which subsequently reacted with anthranilamides for the synthesis of quinazolinones.

Unraveling the Prominent Existence of Trace Metals in Photocatalysis: Exploring Iron Impurity Effects

Metal impurities can complicate the identification of active catalyst species in transition metal catalysis and electrocatalysis, potentially leading to misleading findings. This study investigates the influence of metal impurities on photocatalysis. Specifically, the photocatalytic reaction of inert alkanes using chlorides without the use of an external photocatalyst was studied, achieving successful C(sp3)-H functionalization. The observations reveal that Fe and Cu impurities are challenging to avoid in a typical laboratory environment and are prominently present in normal reaction systems, and iron impurities play a dominant role in the aforementioned apparent 'metal-free' reaction. Additionally, iron exhibits significantly higher catalytic activity compared to Cu, Ce, and Ni at low metal concentrations in the photocatalytic C(sp3)-H functionalization using chlorides. Considering the widespread presence of Fe and Cu impurities in typical laboratory environments, this study serves as a reminder of their involvement in reaction processes.

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.

Photo-induced versatile aliphatic C-H functionalization via electron donor-acceptor complex

The ability to selectively introduce diverse functionality onto hydrocarbons is of substantial value in the synthesis of both small molecules and pharmaceuticals. In this endeavour, as a photocatalyst- and metal-free process, the electron donor-acceptor (EDA) strategy has not been well explored. Here we report an approach to aliphatic carbon-hydrogen bond diversification through an EDA complex constituted by HCl and SIV=O groups. As an efficient hydrogen atom transfer (HAT) reagent, chlorine radical can be produced via a proton-coupled electron transfer process in this system. Based on this unusual path, a photo-promoted versatile aliphatic C-H functionalization is developed without photo- and metal-catalysts, including thiolation, arylation, alkynylation, and allylation. This conversion has concise and ambient reaction conditions, good functional group tolerance, and substrate diversity, and provides an alternative solution for the high value-added utilization of bulk light alkanes.

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.

Visible light-induced hydrogen atom transfer trifluoromethylthiolation of aldehydes with bismuth catalyst

By using a combination of BiCl3 and TBACl as a ligand-to-metal charge transfer (LMCT) photocatalyst, hydrogen atom transfer trifluoromethylthiolation of aldehydes was achieved under visible light irradiation. The present method provides economical and operationally simple access to trifluoromethylthioesters using low toxicity and cost-effective bismuth catalysts under mild reaction conditions. Based on the radical trapping experiments, the direct conversion of aldehydes to acyl radicals via chlorine radicals as HAT reagents was proposed as the reaction mechanism.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Synthesis of α-Branched Enones via Chloroacylation of Terminal Alkenes

Here, we show the conversion of unactivated alkenes into α-branched enones via regioselective chloroacylation with acyl chlorides. The method relies upon the initial in situ generation of chlorine radicals directly from the acyl chloride precursor under cooperative nickel/photoredox catalysis. Subsequent HCl elimination provides enones and α,β-unsaturated esters that are not accessible via the conventional acylation approaches that provide the other, linear constitutional isomer.

Diastereoselective Ring Expansion of Cyclic Ketones Enabled by HAT-Initiated Radical Cascade

Herein we disclose an iron-catalyzed method for stereoselective synthesis of multisubstituted cyclic ketones containing a synthetically challenging quaternary carbon from readily accessible β-vinyl keto esters in good yields. This cascade reaction is initiated by a hydrogen atom transfer (HAT) process, after which a Dowd-Beckwith-type ring-expansion reaction occurs. This strategic transformation offers access to synthetically valuable cyclic ketones bearing two contiguous stereocenters, including quaternary stereocenters, which hold paramount significance within the realm of synthetic chemistry.

Photo-Triggered, Copper(II) Chloride-Catalyzed Radical Hydroalkylation and Hydrosilylation of Vinylboronic Esters To Access Alkylboronic Esters

Alkyl boronic acids and their derivatives constitute vital building blocks in organic synthesis and are important motifs identified in medicinal chemistry. Herein, we present a phototriggered, CuCl2-catalyzed radical hydroalkylation and hydrosilylation of vinylboronic esters to alkylboronic esters. This approach exhibits mild reaction conditions, utilization of easily accessible reagents, and scalability up to a gram scale. Further synthetic transformations of the hydrosilylation products and mechanistic studies are also demonstrated.

Visible light-triggered selective C(sp2)-H/C(sp3)-H coupling of benzenes with aliphatic hydrocarbons

The direct and selective coupling of benzenes with aliphatic hydrocarbons is a promising strategy for C(sp2)-C(sp3) bond formation using readily available starting materials, yet it remains a significant challenge. In this study, we have developed a simplified photochemical system that incorporates catalytic amounts of iron(III) halides as multifunctional reagents and air as a green oxidant to address this synthetic problem. Under mild conditions, the reaction between a strong C(sp2)-H bond and a robust C(sp3)-H bond has been achieved, affording a broad range of cross-coupling products with high yields and commendable chemo-, site-selectivity. The iron halide acts as a multifunctional reagent that responds to visible light, initiates C-centered radicals, induces single-electron oxidation to carbocations, and participates in a subsequent Friedel-Crafts-type process. The gradual release of radical species and carbocation intermediates appears to be critical for achieving desirable reactivity and selectivity. This eco-friendly, cost-efficient approach offers access to various building blocks from abundant hydrocarbon feedstocks, and demonstrates the potential of iron halides in sustainable synthesis.

Visible-Light-Enabled Lanthanum-Mediated Intramolecular Epoxy-Ring Opening/Dehydrogenative Lactonization

Catalytic C(sp3)-H functionalization has afforded great opportunities to prepare organic substances, facilitating the derivatization of complex drugs and natural molecules. This letter describes an efficient and practical protocol for lanthanum-catalyzed continuous epoxy-ring opening and oxidative dehydrogenative lactonization under visible-light irradiation. Notably, the lanthanum catalyst also acts as a photocatalyst while acting as a Lewis acid in this reaction; therefore, no additional photocatalyst is required. We can conveniently prepare a series of diverse isochromanones with oxygen-containing spirocyclic structural units under a balloon-oxygen atmosphere at room temperature. Mechanistic studies and control experiments reveal that the in situ-generated lanthanum bromide should be crucial in the reaction.

Photoinduced iron-catalyzed C-H alkylation of polyolefins

Chemically introducing diverse polar groups into polyolefins via carbon-hydrogen bond alkylation with polar olefins is of substantial value in the synthesis of next-generation lightweight thermoplastics, which is still underdeveloped. In this work, we report a new approach for efficient carbon-hydrogen bond alkylation in commodity polyolefins using photoinduced iron catalysis. Various polyolefins could be functionalized with broad scope. Polar groups could be incorporated in a single step. The controllable synthesis of multi-polar functional polyolefins could be achieved by a designed module-assembled process. Remarkably, even low levels of functionalization could upcycle the polyolefin materials to exhibit unusual physical properties, such as enhancement of the transparencies, strains, stresses at break of the materials, and hydrophilicity.

Radical Phosphorylation of Aliphatic C-H Bonds via Iron Photocatalysis

The synthesis of tertiary phosphines(III) has been a long-standing challenge in synthetic chemistry because of inevitable issues including harsh conditions, sensitive organometallic reagents, and prefunctionalized substrates in traditional synthesis. Herein, we report a strategically novel C(sp3)-H bond phosphorylation that enables the assembly of structurally diverse tertiary phosphines(III) from industrial phosphine(III) sources under mild photocatalytic conditions. The merger of ligand-to-metal charge transfer (LMCT) of FeCl3 with the hydrogen atom-transfer (HAT) process is the key for the generation of alkyl radicals from hydrocarbons. Strikingly, this catalytic system can be successfully applied for the polymerization of electron-deficient alkenes.

Divergent Synthesis of β-Fluoroamides via Silver-Catalyzed Oxidative Deconstruction of Cyclopropanone Hemiaminals

An expedient approach for the synthesis of challenging β-fluoroamides from readily accessible cyclopropanone equivalents is reported. Following the addition of pyrazole used here as a transient leaving group, silver-catalyzed regiospecific ring-opening fluorination of the resulting hemiaminal leads to a β-fluorinated N-acylpyrazole intermediate reactive to substitution with amines, ultimately affording β-fluoroamides. The process could also be extended to the synthesis of β-fluoroesters and γ-fluoroalcohols via the addition of alcohols or hydrides as terminal nucleophiles, respectively.

Photochemical iron-catalyzed decarboxylative azidation via the merger of ligand-to-metal charge transfer and radical ligand transfer catalysis

Ligand-to-metal charge transfer (LMCT) using stoichiometric copper salts has recently been shown to permit decarboxylative C-N bond formation via an LMCT/radical polar crossover (RPC) mechanism; however, this method is unable to function catalytically and cannot successfully engage unactivated alkyl carboxylic acids, presenting challenges to the general applicability of this approach. Leveraging the concepts of ligand-to-metal charge transfer (LMCT) and radical-ligand-transfer (RLT), we herein report the first photochemical, iron-catalyzed direct decarboxylative azidation. Simply irradiating an inexpensive iron nitrate catalyst in the presence of azidotrimethylsilane allows for a diverse array of carboxylic acids to be converted to corresponding organic azides directly with broad functional group tolerance and mild conditions. Intriguingly, no additional external oxidant is required for this reaction to proceed, simplifying the reaction protocol. Finally, mechanistic studies are consistent with a radical mechanism and suggest that the nitrate counteranion serves as an internal oxidant for turnover of the iron catalyst.

Iron Photoredox Catalysis-Past, Present, and Future

Photoredox catalysis of organic reactions driven by iron has attracted substantial attention throughout recent years, due to potential environmental and economic benefits. In this Perspective, three major strategies were identified that have been employed to date to achieve reactivities comparable to the successful noble metal photoredox catalysis: (1) Direct replacement of a noble metal center by iron in archetypal polypyridyl complexes, resulting in a metal-centered photofunctional state. (2) In situ generation of photoactive complexes by substrate coordination where the reactions are driven via intramolecular electron transfer involving charge-transfer states, for example, through visible-light-induced homolysis. (3) Improving the excited-state lifetimes and redox potentials of the charge-transfer states of iron complexes through new ligand design. We seek to give an overview and evaluation of recent developments in this rapidly growing field and, at the same time, provide an outlook on the future of iron-based photoredox catalysis.

Iron-Catalyzed C(Sp3)-H Borylation, Thiolation, and Sulfinylation Enabled by Photoinduced Ligand-to-Metal Charge Transfer

Catalytic C(sp³)-H functionalization has provided enormous opportunities to construct organic molecules, facilitating the derivatization of complex pharmaceutical compounds. Within this framework, direct hydrogen atom transfer (HAT) photocatalysis becomes an appealing approach to this goal. However, the viable substrates utilized in these protocols are limited, and the site selectivity shows preference to activated and thermodynamically favored C(sp³)-H bonds. Herein, we describe the development of undirected iron-catalyzed C(sp³)-H borylation, thiolation, and sulfinylation reactions enabled by the photoinduced ligand-to-metal charge transfer (LMCT) process. These reactions exhibit remarkably broad substrate scope (>150 examples in total), and most importantly, all of these three reactions show unconventional regioselectivity, with the occurrence of C(sp³)-H borylation, thiolation, and sulfinylation preferentially at the distal methyl position. The procedures are operationally simple and readily scalable and provide access to high-value products from simple hydrocarbons in one step. Mechanistic studies and control experiments indicate that the afforded site selectivity is not only relevant to the HAT species but also largely affected by the use of boron- and sulfone-based radical acceptors.

Visible-light-induced dehydrogenative amidation of aldehydes enabled by iron salts

A direct dehydrogenative amidation reaction of aldehydes and amines under a visible light mediated ligand-to-metal charge transfer (LMCT) process was described. In this protocol, aldehyde substrates were activated by photoinduced hydrogen atom abstraction (HAA), generating acyl chloride intermediates followed by nucleophilic addition of amines. The synthetic method furnishes good functional group tolerance and broad substrate scope toward both aliphatic and aromatic components.

Chemoselective Decarboxylative Protonation Enabled by Cooperative Earth-Abundant Element Catalysis

Decarboxylative protonation is a general deletion tactic to replace polar carboxylic acid groups with hydrogen or its isotope. Current methods rely on the pre-activation of acids, non-sustainable hydrogen sources, and/or expensive/highly oxidizing photocatalysts, presenting challenges to their wide adoption. Here we show that a cooperative iron/thiol catalyst system can readily achieve this transformation, hydrodecarboxylating a wide range of activated and unactivated carboxylic acids and overcoming scope limitations in previous direct methods. The reaction is readily scaled in batch configuration and can be directly performed in deuterated solvent to afford high yields of d-incorporated products with excellent isotope incorporation efficiency; characteristics not attainable in previous photocatalyzed approaches. Preliminary mechanistic studies indicate a radical mechanism and kinetic results of unactivated acids (KIE=1) are consistent with a light-limited reaction.

Identification of Alkoxy Radicals as Hydrogen Atom Transfer Agents in Ce-Catalyzed C-H Functionalization

The intermediacy of alkoxy radicals in cerium-catalyzed C-H functionalization via H-atom abstraction has been unambiguously confirmed. Catalytically relevant Ce(IV)-alkoxide complexes have been synthesized and characterized by X-ray diffraction. Operando electron paramagnetic resonance and transient absorption spectroscopy experiments on isolated pentachloro Ce(IV) alkoxides identified alkoxy radicals as the sole heteroatom-centered radical species generated via ligand-to-metal charge transfer (LMCT) excitation. Alkoxy-radical-mediated hydrogen atom transfer (HAT) has been verified via kinetic analysis, density functional theory (DFT) calculations, and reactions under strictly chloride-free conditions. These experimental findings unambiguously establish the critical role of alkoxy radicals in Ce-LMCT catalysis and definitively preclude the involvement of chlorine radical. This study has also reinforced the necessity of a high relative ratio of alcohol vs Ce for the selective alkoxy-radical-mediated HAT, as seemingly trivial changes in the relative ratio of alcohol vs Ce can lead to drastically different mechanistic pathways. Importantly, the previously proposed chlorine radical-alcohol complex, postulated to explain alkoxy-radical-enabled selectivities in this system, has been examined under scrutiny and ruled out by regioselectivity studies, transient absorption experiments, and high-level calculations. Moreover, the peculiar selectivity of alkoxy radical generation in the LMCT homolysis of Ce(IV) heteroleptic complexes has been analyzed and back-electron transfer (BET) may have regulated the efficiency and selectivity for the formation of ligand-centered radicals.

Photochemical diazidation of alkenes enabled by ligand-to-metal charge transfer and radical ligand transfer

Vicinal diamines are privileged synthetic motifs in chemistry due to their prevalence and powerful applications in bioactive molecules, pharmaceuticals, and ligand design for transition metals. With organic diazides being regarded as modular precursors to vicinal diamines, enormous efforts have been devoted to developing efficient strategies to access organic diazide generated from olefins, themselves common feedstock chemicals. However, state-of-the-art methods for alkene diazidation rely on the usage of corrosive and expensive oxidants or complicated electrochemical setups, significantly limiting the substrate tolerance and practicality of these methods on large scale. Toward overcoming these limitations, here we show a photochemical diazidation of alkenes via iron-mediated ligand-to-metal charge transfer (LMCT) and radical ligand transfer (RLT). Leveraging the merger of these two reaction manifolds, we utilize a stable, earth abundant, and inexpensive iron salt to function as both radical initiator and terminator. Mild conditions, broad alkene scope and amenability to continuous-flow chemistry rendering the transformation photocatalytic were demonstrated. Preliminary mechanistic studies support the radical nature of the cooperative process in the photochemical diazidation, revealing this approach to be a powerful means of olefin difunctionalization.

Iron-mediated ligand-to-metal charge transfer enables 1,2-diazidation of alkenes

Given the widespread significance of vicinal diamine units in organic synthesis, pharmaceuticals and functional materials, as well as in privileged molecular catalysts, an efficient and practical strategy that avoids the use of stoichiometric strong oxidants is highly desirable. We herein report the application of ligand-to-metal charge transfer (LMCT) excitation to 1,2-diazidation reactions from alkenes and TMSN3 via a coordination-LMCT-homolysis process with more abundant and greener iron salt as the catalyst. Such a LMCT-homolysis mode allows the generation of electrophilic azidyl radical intermediate from Fe-N3 complexes poised for subsequent radical addition into carbon-carbon double bond. The generated carbon radical intermediate is further captured by iron-mediated azidyl radical transfer, enabling dual carbon-nitrogen bond formation. This protocol provides a versatile platform to access structurally diverse diazides with high functional group compatibility from readily available alkenes without the need of chemical oxidants.

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.

Visible Light-Induced Unactivated δ-C(sp3 )-H Amination of Alcohols Catalyzed by Iron

An iron-catalyzed remote C(sp³ )-H amination of alcohols through 1,5-hydrogen atom transfer is developed. This protocol provides a method to generate δ-C(sp³ )-N bonds from primary, secondary, and tertiary alcohols under mild conditions. A wide substrate scope and a good functional group tolerance are presented. Mechanistic studies show that a LMCT course of an Fe-OR species and a chlorine radical-induced hydrogen abstraction of an alcohol are possible to generate the alkoxy radical intermediate.

Photoinduced B-Cl Bond Fission in Aldehyde-BCl3 Complexes as a Mechanistic Scenario for C-H Bond Activation

In concert with carbonyl compounds, Lewis acids have been identified as a versatile class of photocatalysts. Thus far, research has focused on activation of the substrate, either by changing its photophysical properties or by modifying its photochemistry. In this work, we expand the established mode of action by demonstrating that UV photoexcitation of a Lewis acid-base complex can lead to homolytic cleavage of a covalent bond in the Lewis acid. In a study on the complex of benzaldehyde and the Lewis acid BCl₃, we found evidence for homolytic B-Cl bond cleavage leading to formation of a borylated ketyl radical and a free chlorine atom only hundreds of femtoseconds after excitation. Both time-dependent density functional theory and transient absorption experiments identify a benzaldehyde-BCl₂ cation as the dominant species formed on the nanosecond time scale. The experimentally validated B-Cl bond homolysis was synthetically exploited for a BCl₃-mediated hydroalkylation reaction of aromatic aldehydes (19 examples, 42-76% yield). It was found that hydrocarbons undergo addition to the C═O double bond via a radical pathway. The photogenerated chlorine radical abstracts a hydrogen atom from the alkane, and the resulting carbon-centered radical either recombines with the borylated ketyl radical or adds to the ground-state aldehyde-BCl₃ complex, releasing a chlorine atom. The existence of a radical chain was corroborated by quantum yield measurements and by theory. The photolytic mechanism described here is based on electron transfer between a bound chlorine and an aromatic π-system on the substrate. Thereby, it avoids the use of redox-active transition metals.

Iron/Photosensitizer Hybrid System Enables the Synthesis of Polyaryl-Substituted Azafluoranthenes

Photosensitization of organometallics is a privileged strategy that enables challenging transformations in transition-metal catalysis. However, the usefulness of such photocatalyst-induced energy transfer has remained opaque in iron-catalyzed reactions despite the intriguing prospects of iron catalysis in synthetic chemistry. Herein, we demonstrate the use of iron/photosensitizer-cocatalyzed cycloaddition to synthesize polyarylpyridines and azafluoranthenes, which have been scarcely accessible using the established iron-catalyzed protocols. Mechanistic studies indicate that triplet energy transfer from the photocatalyst to a ferracyclic intermediate facilitates the thermally demanding nitrile insertion and accounts for the distinct reactivity of the hybrid system. This study thus provides the first demonstration of the role of photosensitization in overcoming the limitations of iron catalysis.

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

An efficient and inexpensive protocol for the direct decarboxylative fluorination of aliphatic carboxylic acids catalyzed with iron salts under visible light is presented. This new method allows the facile fluorination of a diverse array of carboxylic acids even on gram scale using a Schlenk flask without loss of efficiency. Mechanistic studies suggest that the photoinduced ligand-to-metal charge transfer process enables the generation of the key step to generate the carboxyl radical intermediates.

Visible-Light-Promoted Fe(III)-Catalyzed N-H Alkylation of Amides and N-Heterocycles

The combination of the radical chemistry of ligand-to-metal charge transfer with metal catalysis by a single iron salt helps to realize the visible-light-promoted N-H alkylation of amides and N-heterocycles. A wide variety of amides and nitrogen-containing heterocycles were tolerated in our protocol to give N-alkylated products. The applicability of this protocol was further demonstrated by late-stage alkylation of N-H-containing pharmaceuticals. Moreover, N-H-alkylated α-amino tetrahydrofurans could be transformed into versatile ring-opened amino alcohols under reducing conditions. A mechanistic study revealed that hydrogen atom transfer by a tert-butoxyl radical and a chlorine radical was responsible for the activation of C(sp³)-H precursors.

Silver Ions Promoted Palladium-Catalyzed Inactive β-C(sp3)-H Bond Arylation in Batch and Continuous-Flow Conditions

A palladium(II)-catalyzed protocol for inactive β-C(sp³)-H bond functionalization has been first accomplished. The reaction proceeds through five-membered carbocycles for the formation of C-C bonds via the Pd(II)/Pd(IV) cycle. This reaction was carried out with various aryl iodides and benzothiazoles/benzoxazoles/benzimidazoles, which were well-tolerated in this reaction and successfully generated β-C(sp³)-H arylated products. Further implementation of this batch protocol to continuous flow by utilizing a PTFE (polytetrafluoroethylene) capillary reactor enhanced the reaction efficiency and decreased the reaction time (18.4 min) as compared to batch conditions (8 h). Even on the gram scale, the process produced excellent yield with negligible diarylations. Functional group tolerance, a continuous-flow approach, and easy-to-handle reaction conditions make this inactive β-C(sp³)-H bond functionalization protocol very attractive.