Controllable C-H Alkylation of Polyethers via Iron Photocatalysis

Stereoselective Nitration of Olefins via Visible-Light-Induced Iron-Complex β-Homolysis

Herein, we develop a green and efficient method for the stereoselective synthesis of nitroolefins with ferric nitrate as the nitration reagent. With a visible-light-induced iron-complex β-homolysis pathway and I- as a crucial media for electron transfer and dehydrogenation, a broad scope of nitroolefins including some valuable biorelevant derivatives are achieved in high E-selectivity. The reaction is conveniently controlled by nitrate equivalences, and scale-up experiments in batch and flow indicate excellent practicability for this protocol.

Organophotoredox-Catalyzed Postfunctionalization of Poly(methacrylate) Derivatives via Radical-Polar Crossover Phosphonylation

Photoredox-catalyzed postfunctionalization of poly(meth)acrylate-based copolymers bearing phthalimide moieties can produce novel polymer architectures that are inaccessible using polymerization approaches; however, this method is restricted to radical reactions. To expand the scope of reactions applicable to photoredox-catalyzed postfunctionalization, this study focused on the radical-polar crossover (RPC) process, in which a carbocation equivalent is generated to react with nucleophiles. The organophotoredox-catalyzed reaction of a random copolymer of N-(methacryloxy)phthalimide and methyl acrylate using trialkyl phosphite nucleophiles afforded the corresponding phosphonate-containing polymers. Although simultaneous hydrogenation was inevitable, a novel polymer comprising dialkyl isopropenylphosphonate, propylene, and methyl acrylate units was obtained, which is of interest because phosphonate-containing polymers are potentially applicable as thermoresponsive materials, flame-retardant materials, and additives in Li-ion batteries. A series of copolymers with different components were successfully applied to the postfunctionalization method. The organophotoredox catalysis-based postfunctionalization strategy developed in this study provides versatile functional polymers in a simple, green, and sustainable manner.

Light-Triggered Cross-Linking of Linear Poly(ethylene glycol) Hydrogels for Enhanced Antibacterial Delivery

Due to the limited functional groups on poly(ethylene glycol) (PEG) terminals, hydrogels are usually prepared by PEG chains with multiple arms. It is challenging for the preparation of hydrogels using linear PEG. Herein, we report the light-triggered synthesis of hydrogels composed of linear PEG with bis-maleimide groups. In the presence of a photoinduced catalyst, the α-C-H bonds on the PEG chain are cleaved, allowing the maleimide groups to be incorporated into the main chain, forming a cross-linked structure. The cleavage of the unstable C-O bonds could result in degradation of hydrogels. The release of encapsulated tobramycin from degradable hydrogels shows better antibacterial properties compared to undegradable hydrogels, which results in highly effective antimicrobials at the wound site. This work presents a method for the preparation of degradable hydrogels using linear PEG as a component for biological applications.

Electrochemical Commodity Polymer Up- and Re-Cycling: Toward Sustainable and Circular Plastic Treatment

The demand for commodity plastics reaches unprecedented dimensions. In contrast to the well-developed plethora of methods for polymer synthesis, sustainable strategies for the end-of-life management of plastics continue to be scarce. While mechanical re-cycling often results in downgraded materials, chemical re-cycling or up-cycling offers tremendous potential for an efficient and green approach, thereby addressing the precarious treatment of post-use plastics within a circular carbon economy. Recently, electrochemistry surfaced as a uniquely powerful tool for polymer up-cycling via polymer functionalization or degradation obtaining either novel polymers with valorized properties or high-value recycled small molecules, respectively. While discussing recent progress in that domain, future perspectives of electrochemical polymer modifications until January 2025 are outlined herein.

Photocatalytic Decarboxylative Heck-Type Postpolymerization Modification of Polyacrylates

Postpolymerization modification of commodity polymers has emerged as a powerful strategy for synthesizing functional polymers that are challenging to obtain via direct polymerization. Moreover, it offers opportunities to upcycle existing polymers and enhance their material properties. In this work, we introduce a photocatalytic decarboxylative Heck-type approach to upcycle polyacrylates. By employing an acridine-based photocatalyst in combination with a cobaloxime catalyst, hydrolyzed polyacrylates with varying carboxylic acid contents were efficiently converted into polymers bearing olefin functionalities. The degree of olefin incorporation notably influenced the thermal properties of the resulting materials. Importantly, this protocol is applicable to a broad range of vinyl comonomers, yielding 1,3-diene copolymers with exclusive 1,2-insertion selectivity. Furthermore, a variety of terminal alkenes can be utilized to functionalize the acrylate copolymers, and subsequent thiol-ene click reactions further expand the versatility of the accessible polymers.

Iron-Catalyzed Aerobic Carbonylation of Methane via Ligand-to-Metal Charge Transfer Excitation

The integration of ligand-to-metal charge transfer (LMCT) catalytic paradigms with radical intermediates has transformed the selective functionalization of inert C-H bonds, facilitating the use of nonprecious metal catalysts in demanding transformations. Notably, aerobic C-H carbonylation of methane to acetic acid remains formidable due to the rapid oxidation of methyl radicals, producing undesired C1 oxygenates. We present an iron terpyridine catalyst utilizing LMCT to achieve exceptional C2/C1 selectivity through synergistic photoexcitation, methyl radical generation, and carbonylation. Mechanistic studies highlight the critical roles of Fe(II) and Fe-carbonyl complexes in bypassing methyl radical oxidation via a radical rebound-like pathway, unlocking unprecedented efficiency in methane aerobic carbonylation.

Photochemical upcycling of polymers via visible light-driven C-H bond activation

The excessive use and improper disposal of plastics have placed a significant burden on the environment. To mitigate this impact, prioritizing the chemical upcycling of plastics is crucial. Unlike traditional thermochemical upcycling, which requires harsh conditions such as high temperatures and pressures, photochemical upcycling is viewed as a more environmentally friendly and cost-effective alternative. This includes using light to promote C-H bond activation to achieve the oxidative degradation of plastics, generating various valuable small molecules, or employing light-induced C-H bond activation for post-polymerization modification of post-consumer plastics to obtain polymers with enhanced properties. These methods are highly attractive approaches within the realm of chemical upcycling. This mini-review highlights the scientific breakthroughs in upcycling polymers through oxidative degradation and post-polymerization modification via visible light-driven C-H bond activation. In addition, the reaction mechanism compatibility as well as practical application have been emphatically discussed.

C-H Functionalization of Poly(ethylene oxide) - Embracing Functionality, Degradability, and Molecular Delivery

This study presents an organocatalytic C-H functionalization approach for postpolymerization modification (PPM) of poly(ethylene oxide) (PEO). Most of PEO PPM is previously processed at the end hydroxy group, but recent advances in C-H functionalization open a way to modify the backbone position. Structurally diverse carboxylic acids are attached to PEO through a cascade process of radical generation by peroxide and oxidation to oxocarbenium by tertiary butylammonium iodide. Attaching carboxylic acids yields a series of functionalize PEO with acetal units (2-5 mol%) in a backbone, which is not accessible via conventional copolymerization of epoxides. The optimized conditions minimizes the uncontrolled degradation or crosslinking from the highly reactive radical and oxocarbenium intermediate. The newly introduced acetal units bring degradability of PEO as well as delivery of carboxylic acid molecules. Hydrolysis studies with high molecular weight functionalization PEO (Mn = 13.0 kg mol-1) confirm the steady release of fragmented PEO (Mn ∼ 2.0 kg mol-1) and carboxylic acid over days and the process rate is not sensitive to pH variation between pH 5 and 9. The presented method offers a versatile and efficient way to modify PEO with potential energy and medical applications.

Unlocking the Potential of Deep Eutectic Solvents and Ligand-to-Metal Charge Transfer Processes: A Reusable Iron-and-UV-Based System for Sustainable C-C Bond Formation

Catalytic C-H functionalization has provided new opportunities to access novel organic molecules more sustainably and efficiently. However, these procedures typically rely on precious metals or complex organic catalysts as well as on hazardous solvents or reaction conditions. Herein, a pioneering methodology for direct C-C bond formation enabled by Ligand-to-Metal Charge Transfer (LMCT) and mediated by UV irradiation has been developed using Deep Eutectic Solvents (DESs) as sustainable reaction media. This direct C-H bond functionalization via a radical addition to electrophiles was successfully confirmed over a broad scope of substrates. More importantly, this is the first example of photocatalytic C-C bond formation in DESs. An inexpensive and abundant iron catalyst (FeCl3) was used under air and mild conditions. Different functional groups were well tolerated obtaining promising results that were comparable to those reported in the literature. Additionally, the reaction medium along with the catalyst could be reused for up to 5 consecutive cycles without a significant loss in the reaction outcome. Several green metrics were calculated and compared to those of conventional procedures, revealing the advantages of using DESs.

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.

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.

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.

In-situ noncovalent interaction of ammonium ion enabled C-H bond functionalization of polyethylene glycols

The noncovalent interactions of ammonium ion with multidentate oxygen-based host has never been reported as a reacting center in catalytic reactions. In this work, we report a reactivity enhancement process enabled by non-covalent interaction of ammonium ion, achieving the C-H functionalization of polyethylene glycols with acrylates by utilizing photoinduced co-catalysis of iridium and quinuclidine. A broad scope of alkenes can be tolerated without observing significant degradation. Moreover, this cyano-free condition respectively allows the incorporation of bioactive molecules and the PEGylation of dithiothreitol-treated bovine serum albumin, showing great potentials in drug delivery and protein modification. DFT calculations disclose that the formed α-carbon radical adjacent to oxygen-atom is reduced directly by iridium before acrylate addition. And preliminary mechanistic experiments reveal that the noncovalent interaction of PEG chain with the formed quinuclidinium species plays a unique role as a catalytic site by facilitating the proton transfer and ultimately enabling the transformation efficiently.

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.

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.

Visible-Light Activation of Diorganyl Bis(pyridylimino) Isoindolide Aluminum(III) Complexes and Their Organometallic Radical Reactivity

We report on the synthesis and characterization of a series of (mostly) air-stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible-light excitation. The redox non-innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon-centered radicals. The photo-induced homolytic cleavage of the Al-C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin-free Giese-type reactions and carbonyl alkylations under ambient conditions, which both indicated radical-polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench-stable aluminum organometallics.

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.

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.

Macromolecular Photoediting Using Single-Electron Logic

Accessing the chemistry of reactive intermediates under mild conditions has significantly expanded the available chemical space for molecular transformations. Nowhere is this more apparent than in the context of photoredox catalysis. Despite abundant literature precedents for using this powerful methodology to build complex targets, there are comparatively few reports that leverage photoredox catalysis for macromolecular editing. Here, we report a mild photoredox approach that enables both the functionalization and degradation of polyalkenamers to valuable feedstocks. Irradiation with visible light (including natural sunlight) in the presence of a pyrillium photoredox catalyst promoted facile chain scission in a variety of substrates. This metal-free approach transformed high molar mass materials (>300 kDa) to low molar mass species (<15 kDa) within 10 min. Moreover, we could completely degrade macromolecules into a range of useful targets (C16-C29 species) within 96 h. Mechanistic and kinetic experiments were carried out to understand this reactivity, which could be coupled with hydrofunctionalizations to create tailored products.

Radical chemistry in polymer science: an overview and recent advances

Radical chemistry is one of the most important methods used in modern polymer science and industry. Over the past century, new knowledge on radical chemistry has both promoted and been generated from the emergence of polymer synthesis and modification techniques. In this review, we discuss radical chemistry in polymer science from four interconnected aspects. We begin with radical polymerization, the most employed technique for industrial production of polymeric materials, and other polymer synthesis involving a radical process. Post-polymerization modification, including polymer crosslinking and polymer surface modification, is the key process that introduces functionality and practicality to polymeric materials. Radical depolymerization, an efficient approach to destroy polymers, finds applications in two distinct fields, semiconductor industry and environmental protection. Polymer chemistry has largely diverged from organic chemistry with the fine division of modern science but polymer chemists constantly acquire new inspirations from organic chemists. Dialogues on radical chemistry between the two communities will deepen the understanding of the two fields and benefit the humanity.

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.