Diversification of aliphatic C-H bonds in small molecules and polyolefins through radical chain transfer

C(sp3)-H (N-Phenyltetrazole)thiolation as an Enabling Tool for Molecular Diversification

Methods enabling the broad diversification of C(sp3)-H bonds from a common intermediate are especially valuable in chemical synthesis. Herein, we report a site-selective (N-phenyltetrazole)thiolation of aliphatic and (hetero)benzylic C(sp3)-H bonds using a commercially available disulfide to access N-phenyltetrazole thioethers. The thioether products are readily elaborated in diverse fragment couplings for C-C, C-O, or C-N construction. The C-H functionalization proceeds via a radical-chain pathway involving hydrogen atom transfer by the electron-poor N-phenyltetrazolethiyl radical. Hexafluoroisopropanol was found to be essential to reactions involving aliphatic C(sp3)-H thiolation, with computational analysis consistent with dual hydrogen bonding of the N-phenyltetrazolethiyl radical imparting increased radical electrophilicity to facilitate the hydrogen atom transfer. Substrate is limiting reagent in all cases, and the reaction displays an exceptional functional group tolerance well suited to applications in late-stage diversification.

Light-driven C-H activation mediated by 2D transition metal dichalcogenides

C-H bond activation enables the facile synthesis of new chemicals. While C-H activation in short-chain alkanes has been widely investigated, it remains largely unexplored for long-chain organic molecules. Here, we report light-driven C-H activation in complex organic materials mediated by 2D transition metal dichalcogenides (TMDCs) and the resultant solid-state synthesis of luminescent carbon dots in a spatially-resolved fashion. We unravel the efficient H adsorption and a lowered energy barrier of C-C coupling mediated by 2D TMDCs to promote C-H activation and carbon dots synthesis. Our results shed light on 2D materials for C-H activation in organic compounds for applications in organic chemistry, environmental remediation, and photonic materials.

Cobalt-Catalyzed Cascade C-H Activation/Annulation Polymerizations toward Diversified and Multifunctional Sulfur-Containing Fused Heterocyclic Polymers

Transition-metal-catalyzed C-H activation has greatly benefited the synthesis and development of functional polymer materials, and the construction of multifunctional fused (hetero)cyclic polymers via novel C-H activation-based polyannulations has emerged as a charming but challenging area in recent years. Herein, we report the first cobalt(III)-catalyzed cascade C-H activation/annulation polymerization (CAAP) approach that can efficiently transform readily available aryl thioamides and internal diynes into multifunctional sulfur-containing fused heterocyclic (SFH) polymers. Within merely 3 h, a series of SFH polymers bearing complex and multisubstituted S,N-doped polycyclic units are facilely and efficiently produced with high molecular weights (absolute Mn up to 220400) in excellent yields (up to 99%), which are hard to achieve by traditional methods. The intermediate-terminated SFH polymer can be used as a reactive macromonomer to controllably extend or modify polymer main chains. The structural diversity can be further enriched through facile S-oxidation and N-methylation reactions of the SFH polymers. Benefiting from the unique structures, the obtained polymers exhibit excellent solution processability, high thermal and morphological stability, efficient and readily tunable aggregate-state fluorescence, stimuli-responsive properties, and high and UV-modulatable refractive indices of up to 1.8464 at 632.8 nm. These properties allow the SFH polymers to be potentially applied in diverse fields, including metal ion detection, photodynamic killing of cancer cells, fluorescent photopatterning, and gradient-index optical materials.

Deconstruction of Polymers through Olefin Metathesis

The consumption of synthetic polymers has ballooned; so has the amount of post-consumer waste generated. The current polymer economy, however, is largely linear with most of the post-consumer waste being either landfilled or incinerated. The lack of recycling, together with the sizable carbon footprint of the polymer industry, has led to major negative environmental impacts. Over the past few years, chemical recycling technologies have gained significant traction as a possible technological route to tackle these challenges. In this regard, olefin metathesis, with its versatility and ease of operation, has emerged as an attractive tool. Here, we discuss the developments in olefin-metathesis-based chemical recycling technologies, including the development of new materials and the application of olefin metathesis to the recycling of commercial materials. We delve into structure-reactivity relationships in the context of polymerization-depolymerization behavior, how experimental conditions influence deconstruction outcomes, and the reaction pathways underlying these approaches. We also look at the current hurdles in adopting these technologies and relevant future directions for the field.

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.

Oxidative Upcycling of Polyethylene to Long Chain Diacid over Co-MCM-41 Catalyst

Plastic pollution is an emerging global threat due to lack of effective methods for transforming waste plastics into useful resources. Here, we demonstrate a direct oxidative upcycling of polyethylene into high-value and high-volume saturated dicarboxylic acids in high carbon yield of 85.9 % in which the carbon yield of long chain dicarboxylic (C10-C20) acids can reach 58.9% over cobalt-doped MCM-41 molecular sieves, in the absence of any solvent or precious metal catalyst. The distribution of the dicarboxylic acids can be controllably adjusted from short-chain (C4-C10) to long-chain ones (C10-C20) through changing cobalt loading of MCM-41 under nanoconfinement. Highly and sparsely dispersed cobalt along with confined space of mesoporous structure enables complete degradation of polyethylene and high selectivity of dicarboxylic acid in mild condition. So far, this is the first report on highly selective one-step preparation of long chain dicarboxylic acids. The approach provides an attractive solution to tackle plastic pollution and a promising alternative route to long chain diacids.

Copper-catalysed dehydrogenation or lactonization of C(sp3)-H bonds

Cytochrome P450 enzymes are known to catalyse bimodal oxidation of aliphatic acids via radical intermediates, which partition between pathways of hydroxylation and desaturation1,2. Developing analogous catalytic systems for remote C-H functionalization remains a significant challenge3-5. Here, we report the development of Cu(I)-catalysed bimodal dehydrogenation/lactonization reactions of synthetically common N-methoxyamides through radical abstractions of the γ-aliphatic C-H bonds. The feasibility of switching from dehydrogenation to lactonization is also demonstrated by altering reaction conditions. The use of a readily available amide as both radical precursor and internal oxidant allows for the development of redox-neutral C-H functionalization reactions with methanol as the sole side product. These C-H functionalization reactions using a Cu(I) catalyst with loading as low as 0.5 mol.% is applied to the diversification of a wide range of aliphatic acids including drug molecules and natural products. The exceptional compatibility of this catalytic system with a wide range of oxidatively sensitive functionality demonstrates the unique advantage of using a simple amide substrate as a mild internal oxidant.

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.

Catalytic conversion of mixed polyolefins under mild atmospheric pressure

The chemical recycling of polyolefin presents a considerable challenge, especially as upcycling methods struggle with the reality that plastic wastes typically consist of mixtures of polyethylene (PE), polystyrene (PS), and polypropylene (PP). We report a catalytic aerobic oxidative approach for polyolefins upcycling with the corresponding carboxylic acids as the product. This method encompasses three key innovations. First, it operates under atmospheric pressure and mild conditions, using O2 or air as the oxidant. Second, it is compatible with high-density polyethylene, low-density polyethylene, PS, PP, and their blends. Third, it uses an economical and recoverable metal catalyst. It has been demonstrated that this approach can efficiently degrade mixed wastes of plastic bags, bottles, masks, and foam boxes.

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

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.

Dithiophosphoric Acids for Polymer Functionalization

Dithiophosphoric acids (DTPAs) are an intriguing class of compounds that are sourced from elemental sulfur and white phosphorus and are prepared from the reaction of phosphorus pentasulfide with alcohols. The electrophilic addition of DTPAs to alkenes and unsaturated olefinic substrates is a known reaction, but has not been applied to polymer synthesis and polymer functionalization. We report on the synthesis and application of DTPAs for the functionalization of challenging poly-enes, namely polyisoprene (PI) and polynorbornene (pNB) prepared by ring-opening metathesis polymerization (ROMP). The high heteroatom content within DTPA moieties impart intriguing bulk properties to poly-ene materials after direct electrophilic addition reactions to the polymer backbone introducing DTPAs as side chain groups. The resulting materials possess both enhanced optical and flame retardant properties vs the poly-ene starting materials. Finally, we demonstrate the ability to prepare crosslinked polydiene films with di-functional DTPAs, where the crosslinking density and thermomechanical properties can be directly tuned by DTPA feed ratios.

Additive-controlled chemoselective inter-/intramolecular hydroamination via electrochemical PCET process

Electrochemically generated amidyl radical species produced distinct inter- or intramolecular hydroamination reaction products via a proton-coupled electron transfer (PCET) mechanism. Cyclic voltammetry (CV) analysis indicated that the chemoselectivity was derived from the size of the hydrogen bond complex, which consisted of the carbamate substrate and phosphate base, and could be controlled using 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as an additive. These results provide fundamental insights for the design of PCET-based redox reaction systems under electrochemical conditions.

Terminal C(sp3)-H borylation through intermolecular radical sampling

Hydrogen atom transfer (HAT) processes can overcome the strong bond dissociation energies (BDEs) of inert C(sp3)-H bonds and thereby convert feedstock alkanes into value-added fine chemicals. Nevertheless, the high reactivity of HAT reagents, coupled with the small differences among various C(sp3)-H bond strengths, renders site-selective transformations of straight-chain alkanes a great challenge. Here, we present a photocatalytic intermolecular radical sampling process for the iron-catalyzed borylation of terminal C(sp3)-H bonds in substrates with small steric hindrance, including unbranched alkanes. Mechanistic investigations have revealed that the reaction proceeds through a reversible HAT process, followed by a selective borylation of carbon radicals. A boron-sulfoxide complex may contribute to the high terminal regioselectivity observed.

Upcycling Polyolefin Blends into High-Performance Materials by Exploiting Azidotriazine Chemistry Using Reactive Extrusion

The revalorization of incompatible polymer blends is a key obstacle in realizing a circular economy in the plastics industry. Polyolefin waste is particularly challenging because it is difficult to sort into its constituent components. Untreated blends of polyethylene and polypropylene typically exhibit poor mechanical properties that are suitable only for low-value applications. Herein, we disclose a simple azidotriazine-based grafting agent that enables polyolefin blends to be directly upcycled into high-performance materials by using reactive extrusion at industrially relevant processing temperatures. Based on a series of model experiments, the azidotriazine thermally decomposes to form a triplet nitrene species, which subsequently undergoes a complex mixture of grafting, oligomerization, and cross-linking reactions; strikingly, the oligomerization and cross-linking reactions proceed through the formation of nitrogen-nitrogen bonds. When applied to polyolefin blends during reactive extrusion, this combination of reactions leads to the generation of amorphous, phase-separated nanostructures that tend to exist at polymer-polymer interfaces. These nanostructures act as multivalent cross-linkers that reinforce the resulting material, leading to dramatically improved ductility compared with the untreated blends, along with high dimensional stability at high temperatures and excellent mechanical recyclability. We propose that this unique behavior is derived from the thermomechanically activated reversibility of the nitrogen-nitrogen bonds that make up the cross-linking structures. Finally, the scope of this chemistry is demonstrated by applying it to ternary polyolefin blends as well as postconsumer polyolefin feedstocks.

Copper-Catalyzed Radical Allene C(sp2 )-H Cyanation

While the hydrogen atom abstraction (HAA) from C(sp3 )-H bond has been well explored, the radical-mediated chemo- and regio-selective functionalization of allenic C(sp2 )-H bond via direct HAA from C(sp2 )-H bond of allene remains an unsolved challenge in synthetic chemistry. This is primarily due to inherent challenges with addition of radical intermediates to allenes, regioselectivity of HAA process, instability of allenyl radical toward propargyl radical et al. Herein, we report a copper catalyzed allenic C(sp2 )-H cyanation of an array of tri- and di-substituted allenes with exceptional site-selectivity, while mono-substituted allene was successfully cyanated, albeit with a low yield. In the developed strategy, steric N-fluoro-N-alkylsulfonamide, serving as precursor of hydrogen atom abstractor, plays a crucial role in achieving the desired regioselectivity and avoiding addition of N-centered radical to allene.

Light-driven C-H activation mediated by 2D transition metal dichalcogenides

C-H bond activation enables the facile synthesis of new chemicals. While C-H activation in short-chain alkanes has been widely investigated, it remains largely unexplored for long-chain organic molecules. Here, we report light-driven C-H activation in complex organic materials mediated by 2D transition metal dichalcogenides (TMDCs) and the resultant solid-state synthesis of luminescent carbon dots in a spatially-resolved fashion. We unravel the efficient H adsorption and a lowered energy barrier of C-C coupling mediated by 2D TMDCs to promote C-H activation. Our results shed light on 2D materials for C-H activation in organic compounds for applications in organic chemistry, environmental remediation, and photonic materials.

C-H Functionalization of Polyolefins to Access Reprocessable Polyolefin Thermosets

Upcycling plastic waste into reprocessable materials with performance-advantaged properties would contribute to the development of a circular plastics economy. Here, we modify branched polyolefins and postconsumer polyethylene through a versatile C-H functionalization approach using thiosulfonates as a privileged radical group transfer functionality. Cross-linking the functionalized polyolefins with polytopic amines provided dynamically cross-linked polyolefin networks enabled by associative bond exchange of diketoenamine functionality. A combination of resonant soft X-ray scattering and grazing incidence X-ray scattering revealed hierarchical phase morphology in which diketoenamine-rich microdomains phase-separate within amorphous regions between polyolefin crystallites. The combination of dynamic covalent cross-links and microphase separation results in useful and improved mechanical properties, including a ∼4.5-fold increase in toughness, a reduction in creep deformation at temperatures relevant to use, and high-temperature structural stability compared to the parent polyolefin. The dynamic nature of diketoenamine cross-links provides stress relaxation at elevated temperatures, which enabled iterative reprocessing of the dynamic covalent polymer network with little cycle-to-cycle property fade. The ability to convert polyolefin waste into a reprocessable thermoformable material with attractive thermomechanical properties provides additional optionality for upcycling to enable future circularity.

Copper-Catalyzed Benzylic C-H Cross-Coupling Enabled by Redox Buffers: Expanding Synthetic Access to Three-Dimensional Chemical Space

ConspectusCross-coupling methods are the most widely used synthetic methods in medicinal chemistry. Existing reactions are dominated by methods such as amide coupling and arylation reactions that form bonds to sp2-hybridized carbon atoms and contribute to the formation of "flat" molecules. Evidence that three-dimensional structures often have improved physicochemical properties for pharmaceutical applications has contributed to growing demand for cross-coupling methods with sp3-hybridized reaction partners. Substituents attached to sp3 carbon atoms are intrinsically displayed in three dimensions. These considerations have led to efforts to establish reactions with sp3 cross-coupling partners, including alkyl halides, amines, alcohols, and carboxylic acids. As C(sp3)-H bonds are much more abundant that these more conventional coupling partners, we have been pursuing C(sp3)-H cross-coupling reactions that achieve site-selectivity, synthetic utility, and scope competitive with conventional coupling reactions.In this Account, we outline Cu-catalyzed oxidative cross-coupling reactions of benzylic C(sp3)-H bonds with diverse nucleophilic partners. These reactions commonly use N-fluorobenzenesulfonimide (NFSI) as the oxidant. The scope of reactivity is greatly improved by using a "redox buffer" that ensures that the Cu catalyst is available in the proper redox state to promote the reaction. Early precedents of catalytic Cu/NFSI oxidative coupling reactions, including C-H cyanation and arylation, did not require a redox buffer, but reactions with other nucleophiles, such as alcohols and azoles, were much less effective under similar conditions. Mechanistic studies show that some nucleophiles, such as cyanide and arylboronic acids, promote in situ reduction of CuII to CuI, contributing to successful catalytic turnover. Poor reactivity was observed with nucleophiles, such as alcohols, that do not promote CuII reduction in the same manner. This insight led to the identification of sacrificial reductants, termed "redox buffers", that support controlled generation of CuI during the reactions and enable successful benzylic C(sp3)-H cross-coupling with diverse nucleophiles. Successful reactions include those that feature direct coupling of (hetero)benzylic C-H substrates with coupling partners (alcohols, azoles) and sequential C(sp3)-H functionalization/coupling reactions. The latter methods feature generation of a synthetic linchpin that can undergo subsequent reaction with a broad array of nucleophiles. For example, halogenation/substitution cascades afford benzylic amines, (thio)ethers, and heterodiarylmethane derivatives, and an isocyanation/amine-addition sequence generates diverse benzylic ureas.Collectively, these Cu-catalyzed (hetero)benzylic C(sp3)-H cross-coupling reactions rapidly access diverse molecules. Analysis of their physicochemical and topological properties highlights the "drug-likeness" and enhanced three-dimensionality of these products relative to existing bioactive molecules. This consideration, together with the high benzylic C-H site-selectivity and the broad scope of reactivity enabled by the redox buffering strategy, makes these C(sp3)-H cross-coupling methods ideally suited for implementation in high-throughput experimentation platforms to explore novel chemical space for drug discovery and related applications.

General Synthesis of N-Alkylindoles from N,N-Dialkylanilines via [4 + 1] Annulative Double C-H Functionalization

Strategies enabling the construction of indoles and novel polycyclic heterocycles from simple building blocks streamline syntheses in synthetic and medicinal chemistry. Herein, we report a C-H functionalization approach to N-alkylindoles proceeding via a double, site-selective C(sp3)-H/C(sp2)-H [4 + 1] annulation of readily accessed N,N-dialkylanilines. This protocol features a site-selective hydrogen atom transfer by a tuned N-tBu amidyl radical and addition of a sulfonyl diazo coupling partner, which promotes highly site-selective homolytic aromatic substitution of the (hetero)aromatic core. Mild decarboxylation of the annulation product enables the overall introduction of a carbyne equivalent into the N,N-dialkylaniline scaffold. Furthermore, the site-selectivity and mild conditions of the indolization facilitate direct access to N-alkyl indole scaffolds in late-stage functionalization (LSF) settings.

Introducing covalent warheads on spirocyclic sp2-sp3 fragments by innate C-H functionalization

Sp2-sp3 fragments play a vital role in fragment-based drug design (FBDD). Strategies to chemically modify them and efficiently access libraries of these compounds have been goals of the highest priority in the last decades. In this work, a series of sp2-sp3 fragments was synthesized and validated for that purpose, based on their measured physical-chemical properties. Selective C-H cyanation and allylation of these fragments was demonstrated by simple heating in presence of an appropriate hydrogen-atom transfer reagent and a radical acceptor. These conditions enabled a streamlined access to covalent fragments in a single step, by direct introduction of the desired covalent binder. Preliminary results on vinylation, as well as late-stage functionalization of a drug analogue were disclosed.

Chemical Modification of Oxidized Polyethylene Enables Access to Functional Polyethylenes with Greater Reuse

Polyethylene is a commodity material that is widely used because of its low cost and valuable properties. However, the lack of functional groups in polyethylene limits its use in applications that include adhesives, gas barriers, and plastic blends. The inertness of polyethylene makes it difficult to install groups that would enhance its properties and enable programmed chemical decomposition. To overcome these deficiencies, the installation of pendent functional groups that imbue polyethylene with enhanced properties is an attractive strategy to overcome its inherent limitations. Here, we describe strategies to derivatize oxidized polyethylene that contains both ketones and alcohols to monofunctional variants with bulk properties superior to those of unmodified polyethylene. Iridium-catalyzed transfer dehydrogenation with acetone furnished polyethylenes with only ketones, and ruthenium-catalyzed hydrogenation with hydrogen furnished polyethylenes with only alcohols. We demonstrate that the ratio of these functional groups can be controlled by reduction with stoichiometric hydride-containing reagents. The ketones and alcohols serve as sites to introduce esters and oximes onto the polymer, thereby improving surface and bulk properties over those of polyethylene. These esters and oximes were removed by hydrolysis to regenerate the original oxygenated polyethylenes, showing how functionalization can lead to materials with circularity. Waste polyethylenes were equally amenable to oxidative functionalization and derivatization of the oxidized material, showing that this low- or negative-value feedstock can be used to prepare materials of higher value. Finally, the derivatized polymers with distinct solubilities were separated from mechanically mixed plastic blends by selective dissolution, demonstrating that functionalization can lead to novel approaches for distinguishing and separating polymers from a mixture.

Diverse functional polyethylenes by catalytic amination

Functional polyethylenes possess valuable bulk and surface properties, but the limits of current synthetic methods narrow the range of accessible materials and prevent many envisioned applications. Instead, these materials are often used in composite films that are challenging to recycle. We report a Cu-catalyzed amination of polyethylenes to form mono- and bifunctional materials containing a series of polar groups and substituents. Designed catalysts with hydrophobic moieties enable the amination of linear and branched polyethylenes without chain scission or cross-linking, leading to polyethylenes with otherwise inaccessible combinations of functional groups and architectures. The resulting materials possess tunable bulk and surface properties, including toughness, adhesion to metal, paintability, and water solubility, which could unlock applications for functional polyethylenes and reduce the need for complex composites.

Methyl Ketones as Alkyl Halide Surrogates: A Deacylative Halogenation Approach for Strategic Functional Group Conversions

Alkyl halides are versatile precursors to access diverse functional groups (FGs). Due to their lability, the development of surrogates for alkyl halides is strategically important for complex molecule synthesis. Given the stability and ease of derivatization inherent in common alkyl ketones, here we report a deacylative halogenation approach to convert various methyl ketones to the corresponding alkyl chlorides, bromides, and iodides. The reaction is driven by forming an aromatic byproduct, i.e., 1,2,4-triazole, in which N'-methylpicolinohydrazonamide (MPHA) is employed to form a prearomatic intermediate and halogen atom-transfer (XAT) reagents are used to quench the alkyl radical intermediate. The reaction is efficient in yielding primary and secondary alkyl halides from a wide range of methyl ketones with broad FG tolerance. It also works for complex natural-product-derived and fluoro-containing substrates. In addition, one-pot conversions of methyl ketones to various other FGs and annulations with alkenes and alkynes through deacylative halogenation are realized. Moreover, an unusual iterative homologation of alkyl iodides is also demonstrated. Finally, mechanistic studies reveal an intriguing double XAT process for the deacylative iodination reaction, which could have implications beyond this work.

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.

Base-Modulated 1,3-Regio- and Stereoselective Carboboration of Cyclohexenes

While chain-walking stimulates wide interest in both polymerization and organic synthesis, site- and stereoselective control of chain-walking on rings is still a challenging task in the realm of organometallic catalysis. Inspired by a controllable chain-walking on cyclohexane rings in olefin polymerization, we have developed a set of chain-walking carboborations of cyclohexenes based on nickel catalysis. Different from the 1,4-trans-selectivity disclosed in polymer science, a high level of 1,3-regio- and cis-stereoselectivity is obtained in our reactions. Mechanistically, we discovery that the base affects the reduction ability of B2 pin2 and different bases lead to different catalytic cycles and different regioselective products (1,2- Vs 1,3-addition). This study provides a concise and modular method for the synthesis of 1,3-disubstituted cyclohexylboron compounds. The incorporation of a readily modifiable boronate group greatly enhances the value of this method, the synthetic potential of which was highlighted by the synthesis of a series of high-valued commercial chemicals and pharmaceutically interesting molecules.

A General Photocatalytic Strategy for Nucleophilic Amination of Primary and Secondary Benzylic C-H Bonds

We report a visible-light photoredox-catalyzed method that enables nucleophilic amination of primary and secondary benzylic C(sp3)-H bonds. A novel amidyl radical precursor and organic photocatalyst operate in tandem to transform primary and secondary benzylic C(sp3)-H bonds into carbocations via sequential hydrogen atom transfer (HAT) and oxidative radical-polar crossover. The resulting carbocation can be intercepted by a variety of N-centered nucleophiles, including nitriles (Ritter reaction), amides, carbamates, sulfonamides, and azoles, for the construction of pharmaceutically relevant C(sp3)-N bonds under unified reaction conditions. Mechanistic studies indicate that HAT is amidyl radical-mediated and that the photocatalyst operates via a reductive quenching pathway. These findings establish a mild, metal-free, and modular protocol for the rapid diversification of C(sp3)-H bonds to a library of aminated products.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.

Hydroformylation of pyrolysis oils to aldehydes and alcohols from polyolefin waste

Waste plastics are an abundant feedstock for the production of renewable chemicals. Pyrolysis of waste plastics produces pyrolysis oils with high concentrations of olefins (>50 weight %). The traditional petrochemical industry uses several energy-intensive steps to produce olefins from fossil feedstocks such as naphtha, natural gas, and crude oil. In this work, we demonstrate that pyrolysis oil can be used to produce aldehydes through hydroformylation, taking advantage of the olefin functionality. These aldehydes can then be reduced to mono- and dialcohols, oxidized to mono- and dicarboxylic acids, or aminated to mono- and diamines by using homogeneous and heterogeneous catalysis. This route produces high-value oxygenated chemicals from low-value postconsumer recycled polyethylene. We project that the chemicals produced by this route could lower greenhouse gas emissions ~60% compared with their production through petroleum feedstocks.

Functionalization of polyethylene with hydrolytically-stable ester groups

Low-density (LD) and high-density polyethylene (HDPE), recycled or not, incorporates up to 7 wt% of ester groups after reacting either with ethyl diazoacetate (EDA) under catalytic and solvent free-reaction conditions, or with maleic anhydride (MA) and acrylates (AC) under catalytic radical conditions. The resulting upcycled polyethylene esters are hydrolytically stable at extreme pH (0-14) and can be further transformed into carboxylic acids, carboxylates, other esters and amides.

Polymer up-cycling by mangana-electrocatalytic C(sp3)-H azidation without directing groups

The chemical up-cycling of polymers into value-added materials offers a unique opportunity to place plastic waste in a new value chain towards a circular economy. Herein, we report the selective up-cycling of polystyrenes and polyolefins to C(sp3)-H azidated materials under electrocatalytic conditions. The functionalized polymers were obtained with high retention of mass average molecular mass and high functionalization through chemo-selective mangana-electrocatalysis. Our strategy proved to be broadly applicable to a variety of homo- and copolymers. Polyethylene, polypropylene as well as post-consumer polystyrene materials were functionalized by this approach, thereby avoiding the use of hypervalent-iodine reagents in stoichiometric quantities by means of electrocatalysis. This study, hence, represents a chemical oxidant-free polymer functionalization by electro-oxidation. The electrocatalysis proved to be scalable, which highlights its unique feature for a green hydrogen economy by means of the hydrogen evolution reaction (HER).

Radical approaches to C-S bonds

Organosulfur functionalities are ubiquitous in nature, pharmaceuticals, agrochemicals, materials and flavourants. Historically, these moieties were introduced almost exclusively using ionic chemistry; however, radical-based methods for the installation of sulfur-based functional groups have recently come to the fore. These radical methods have enabled their late-stage introduction into complex molecules, avoiding the need to preserve labile organosulfur moieties through multistep synthetic sequences. Here, we discuss homolytic C-S bond-forming processes, with a particular emphasis on radical substitution approaches to sulfide, disulfide and sulfinyl products, and the use of sulfur dioxide and its surrogates to build sulfonyl products. We also highlight the mechanistic considerations that we hope will guide further development of radical-based strategies compatible with the various organosulfur moieties that feature in modern chemistry.

Catalytic Amounts of an Antibacterial Monomer Enable the Upcycling of Poly(Ethylene Terephthalate) Waste

Poly(ethylene terephthalate) (PET) is an important polymer with an annual output second only to polyethylene. The development of PET recycling technologies is therefore necessary to not only eliminate the harm associated with white pollution and microplastics, but also to reduce carbon emissions. Antibacterial PET, one of the most high-value advanced materials, has improved the ability to treat bacterial infections. However, current methods of manufacturing commercial antibacterial PET require blending with an excess of metal-based antibacterial agents, which leads to biotoxicity and a nonpersistent antibacterial activity. In addition, high-efficiency organic antibacterial agents have yet to be employed in antibacterial PET due to their poor thermal stabilities. Herein, a solid-state reaction for the upcycling of PET waste using a novel hyperthermostable antibacterial monomer is described. This reaction is catalyzed by the residual catalyst present in the PET waste. It is found that a catalytic amount of the antibacterial monomer enabled the low-cost upcycling of PET waste to produce high-value recycled PET with a strong and persistent antibacterial activity, as well as similar thermal properties to the virgin PET. This work provides a feasible and economic strategy for the large-scale upcycling of PET waste and exhibits potential for application in the polymer industry.

Repeatedly Recyclable 3D Printing Catalyst-Free Dynamic Thermosetting Photopolymers

Photo-curing 3D printing technology has promoted the advanced manufacturing in various fields, but has exacerbated the environmental crisis by the demand for the chemically cross-linked thermosetting photopolymers. Here, the authors report a generic strategy to develop catalyst-free dynamic thermosetting photopolymers, based on photopolymerization and transesterification, that can enable users to realize repeatable 3D printing, providing a practical solution to the environmental challenges. That the β-carbonyl group adjacent to the ester group greatly accelerates the rate of transesterification is demonstrated. The generated resins from the immobilization of the catalyst-free reversible bonds into the photopolymers leads to a dynamic covalently crosslinked network structure upon UV based 3D printing, which exhibit controllable mechanical properties with elastomeric behaviors to thermadapt shape memory polymers. Furthermore, the resulting network can be reverted into an acrylate-functioned photopolymer that is suitable for 3D printing again, presenting an on-demand, repeatedly recyclable thermosetting photopolymer platform for sustainable 3D printing.

Direct B-H Functionalization of Icosahedral Carboranes via Hydrogen Atom Transfer

The efficient and selective functionalization of icosahedral carboranes (C₂B₁₀H₁₂) at the boron vertexes is a long-standing challenge owing to the presence of 10 inert B-H bonds in a similar chemical environment. Herein, we report a new reaction paradigm for direct B-H functionalization of icosahedral carboranes via B-H homolysis enabled by a nitrogen-centered radical-mediated hydrogen atom transfer (HAT) strategy. Both the HAT process of the carborane B-H bond and the resulting boron-centered carboranyl radical intermediate have been confirmed experimentally. The reaction occurs at the most electron-rich boron vertex with the lowest B-H bond dissociation energy (BDE). Using this strategy, diverse carborane derivatization, including thiolation, selenation, alkynylation, alkenylation, cyanation, and halogenation, have been achieved in satisfactory yields under a photoinitiated condition in a metal-free and redox-neutral fashion. Moreover, the synthetic utility of the current protocol was also demonstrated by both the scale-up reaction and the construction of carborane-based functional molecules. Therefore, this methodology opens a radical pathway to carborane functionalization, which is distinct from the B-H heterolytic mechanism in the traditional strategies.

Photoinduced Disulfide-Catalyzed Intramolecular Anti-Markovnikov Hydroamination through in Situ N-S Species

The photoinduced anti-Markovnikov hydroamination of olefins typically required photocatalysts with a high oxidative ability to initiate the single-electron process. Herein, we alternatively utilize bis(2,4,6-triisopropylphenyl) disulfide, an inexpensive reagent with relatively low oxidative ability, as a photo and hydrogen atom transfer catalyst to achieve intramolecular hydroamination. The mechanistic studies as well as the DFT calculations are consistent with a novel process involving N-centered radical generation through the homolysis of the in situ formed N-S species and subsequent cyclization. An array of diverse nitrogen-containing cycles could be obtained.

Circular Upcycling of Bottlebrush Thermosets

The inability to re-process thermosets hinders their utility and sustainability. An ideal material should combine closed-loop recycling and upcycling capabilities. This trait is realized in polydimethylsiloxane bottlebrush networks using thermoreversible Diels-Alder cycloadditions to enable both reversible disassembly into a polymer melt and on-demand reconfiguration to an elastomer of either lower or higher stiffness. The crosslink density was tuned by loading the functionalized networks with a controlled fraction of dormant crosslinkers and crosslinker scavengers, such as furan-capped bis-maleimide and anthracene, respectively. The resulting modulus variations precisely followed the stoichiometry of activated furan and maleimide moieties, demonstrating the lack of side reactions during reprocessing. The presented circularity concept is independent from the backbone or side chain chemistry, making it potentially applicable to a wide range of brush-like polymers.

Chemodivergent C(sp3)-H and C(sp2)-H Cyanomethylation Using Engineered Carbene Transferases

The ubiquity of C-H bonds presents an attractive opportunity to elaborate and build complexity in organic molecules. Methods for selective functionalization, however, often must differentiate among multiple chemically similar and, in some cases indistinguishable, C-H bonds. An advantage of enzymes is that they can be finely tuned using directed evolution to achieve control over divergent C-H functionalization pathways. Here, we demonstrate engineered enzymes that effect a new-to-nature C-H alkylation with unparalleled selectivity: two complementary carbene C-H transferases derived from a cytochrome P450 from Bacillus megaterium deliver an α-cyanocarbene into the α-amino C(sp3)-H bonds or the ortho-arene C(sp2)-H bonds of N-substituted arenes. These two transformations proceed via different mechanisms, yet only minimal changes to the protein scaffold (nine mutations, less than 2% of the sequence) were needed to adjust the enzyme's control over the site-selectivity of cyanomethylation. The X-ray crystal structure of the selective C(sp3)-H alkylase, P411-PFA, reveals an unprecedented helical disruption which alters the shape and electrostatics in the enzyme active site. Overall, this work demonstrates the advantages of enzymes as C-H functionalization catalysts for divergent molecular derivatization.

Zirconium-Catalyzed C-H Alumination of Polyolefins, Paraffins, and Methane

C-H/Et-Al exchange in zirconium-catalyzed reactions of saturated hydrocarbons and AlEt₃ affords versatile organoaluminum compounds and ethane. The grafting of commercially available Zr(OtBu)₄ on silica/alumina gives monopodal ≡SiO-Zr(OtBu)₃ surface pre-catalyst sites that are activated in situ by ligand exchange with AlEt₃. The catalytic C-H alumination of dodecane at 150 °C followed by quenching in air affords n-dodecanol as the major product, revealing selectivity for methyl group activation. Shorter hydrocarbon or alcohol products were not detected under these conditions. Catalytic reactions of cyclooctane and AlEt₃, however, afford ring-opened products, indicating that C-C bond cleavage occurs readily in methyl group-free reactants. This selectivity for methyl group alumination enables the C-H alumination of polyethylenes, polypropylene, polystyrene, and poly-α-olefin oils without significant chain deconstruction. In addition, the smallest hydrocarbon, methane, undergoes selective mono-alumination under solvent-free catalytic conditions, providing a direct route to Al-Me species.

Regiodivergent and Enantioselective Hydroxylation of C-H bonds by Synergistic Use of Protein Engineering and Exogenous Dual-Functional Small Molecules

It is a great challenge to optionally access diverse hydroxylation products from a given substrate bearing multiple reaction sites of sp³ and sp² C-H bonds. Herein, we report the highly selective divergent hydroxylation of alkylbenzenes by an engineered P450 peroxygenase driven by a dual-functional small molecule (DFSM). Using combinations of various P450BM3 variants with DFSMs enabled access to more than half of all possible hydroxylated products from each substrate with excellent regioselectivity (up to >99 %), enantioselectivity (up to >99 % ee), and high total turnover numbers (up to 80963). Crystal structure analysis, molecular dynamic simulations, and theoretical calculations revealed that synergistic effects between exogenous DFSMs and the protein environment controlled regio- and enantioselectivity. This work has implications for exogenous-molecule-modulated enzymatic regiodivergent and enantioselective hydroxylation with potential applications in synthetic chemistry.

Digital light 3D printing of a polymer composite featuring robustness, self-healing, recyclability and tailorable mechanical properties

Producing lightweight structures with high weight-specific strength and stiffness, self-healing abilities, and recyclability, is highly attractive for engineering applications such as aerospace, biomedical devices, and smart robots. Most self-healing polymer systems used to date for mechanical components lack 3D printability and satisfactory load-bearing capacity. Here, we report a new self-healable polymer composite for Digital Light Processing 3D Printing, by combining two monomers with distinct mechanical characteristics. It shows a desirable and superior combination of properties among 3D printable self-healing polymers, with tensile strength and elastic modulus up to 49 MPa and 810 MPa, respectively. Benefiting from dual dynamic bonds between the linear chains, a healing efficiency of above 80% is achieved after heating at a mild temperature of 60 °C without additional solvents. Printed objects are also endowed with multi-materials assembly and recycling capabilities, allowing robotic components to be easily reassembled or recycled after failure. Mechanical properties and deformation behaviour of printed composites and lattices can be tuned significantly to suit various practical applications by altering formulation. Lattice structures with three different architectures were printed and tested in compression: honeycomb, re-entrant, and chiral. They can regain their structural integrity and stiffness after damage, which is of great value for robotic applications. This study extends the performance space of composites, providing a pathway to design printable architected materials with simultaneous mechanical robustness/healability, efficient recoverability, and recyclability.

Copper-catalyzed [1,3]-nitrogen rearrangement of O-aryl ketoximes via oxidative addition of N–O bond in inverse electron flow†

The [1,3]-nitrogen rearrangement reactions of O-aryl ketoximes were promoted by N-heterocyclic carbene (NHC)-copper catalysts and BF₃·OEt₂ as an additive, affording ortho-aminophenol derivatives in good yields. The reaction of substrates with electron-withdrawing substituents on the phenol moiety are accelerated by adding silver salt and modifying the substituent at the nitrogen atom. Density functional theory calculations suggest that the rate-determining step of this reaction is the oxidative addition of the N–O bond of the substrate to the copper catalyst. The negative ρ values of the substituent at both the oxime carbon and phenoxy group indicate that the donation of electrons by the oxygen and nitrogen atoms accelerates the oxidative addition.

[1,3]-Nitrogen rearrangement reactions of O-aryl ketoximes was catalytically promoted by IPrCuBr and BF₃·OEt₂. The oxidative addition of the N–O bond to the Cu catalyst is accelerated by donation of electrons from both nitrogen and oxygen atoms.