Pd Metal Catalysts for Cross-Couplings and Related Reactions in the 21st Century: A Critical Review

Electrifying Redox-Neutral Palladium-Catalyzed Carbonylations: Multielectron Transfer as a Catalyst Driving Force

Palladium-catalyzed bond-forming reactions such as carbonylations offer an efficient and versatile avenue to access products from often feedstock reagents. However, the use of catalysts also comes with a cost, as their need to be regenerated after each product-forming cycle requires balancing thermal operations. The latter can lead to high barriers even with catalysts as well as restrict their application to many products. We introduce herein an alternative approach to palladium catalyst design, where instead electrochemical potential can drive catalysis by continual two-electron cycling of the metal oxidation state. The power behind these redox steps offers a route to carry out carbonylation reactions, including the catalytic synthesis of high-energy aroyl halide electrophiles, at unprecedentedly mild ambient temperature and pressure. More generally, analysis suggests this catalyst functions by a distinct multi-electron exchange pathway, where two-electron reduction enables oxidative addition and two-electron oxidation drives product elimination. The combination creates a unique platform where both these reverse operations are favored in the same system and with electrochemical potential energy as the only added energy source.

Recent advances in electrochemical utilization of NHPI esters

Electrochemical methods employing N-hydroxyphthalimide (NHPI) esters have emerged as powerful tools for sustainable organic synthesis. Derived from abundant and stable alkyl carboxylic acids, NHPI esters enable the generation of alkyl radicals through single-electron transfer (SET) and decarboxylation, facilitating carbon-carbon (C-C) and carbon-heteroatom (C-X) bond formation. These reactions are vital for synthesizing complex molecules used in pharmaceuticals, agrochemicals, and advanced materials, yet traditional approaches often depend on harsh conditions, toxic reagents, or costly metals. This review explores recent progress in electrochemical applications of NHPI esters, highlighting their role in both metal-catalyzed (e.g., Ni, Cr) and metal-free systems.

Cu-β-CD-catalyzed Csp-P coupling of alkynes with dialkylphosphites and phosphine oxides

In this study, an easily accessible Cu-β-CD complex has been introduced for C-P cross-coupling reactions. The Cu-β-CD complex was used to catalyze the C-P coupling of alkynes with both dialkylphosphites and phosphinoxides for the synthesis of alkynyl-phosphonates and -phosphinoxides. The results showed that the coupling reactions could be carried out with appropriate yields for both aryl and alkyl alkynes. The applicability of Cu-β-CD for the oxidative decarboxylative coupling of phenylpropiolic acid with dialkyl phosphites and diphenylphosphine oxide was also studied. XPS and cyclic voltammetry analysis of the catalyst confirmed the presence of both Cu(I) and Cu(II) in the complex structure. The reaction proceeded via the oxidative addition of dialkyl phosphites and alkynes to the catalyst, and the final C-P cross-coupling product was obtained by a reductive elimination process. The presented catalytic method allows the easier and more cost-efficient cross-coupling of alkynes with dialkylphosphites and phosphine oxides under mild and base-free conditions.

Metal complexes featuring organotellurium ligands: synthesis, coordination behavior, and applications

Organotellurium ligand-facilitated metal structures are known for their catalytic activity, anti-cancer activities, and nanomaterial applications. A wide range of organotellurium ligands, including telluroethers, pincer-based frameworks, and tellurolates, have been extensively explored for their coordination with diverse metal centers. These ligands have been successfully complexed with base metals, platinum group metals, rare earth elements, and representative metals, providing a unified resource of literature on metal complexes of organotellurium ligands. The resulting metal-organotellurium complexes exhibit intriguing structural diversities and potential applications in catalysis, materials science, and coordination chemistry. A comprehensive review on various applications of organotellurium ligands has not been available since 2005. Moreover, within the broader field of chalcogen chemistry, organotellurium ligands remain relatively less explored. Hence, this review focuses on the synthetic strategies and various applications of metal complexes containing organotellurium ligands, addressing a significant and critical gap existing in the literature. This review provides a comprehensive analysis of organotellurium ligands, focusing on their synthesis, structural diversity, and coordination chemistry. Beyond their fundamental significance, these ligands play a vital role in life sciences, nanochemistry, and materials science. Their catalytic proficiency is evident in essential organic reactions, including the Suzuki-Miyaura and Mizoroki-Heck couplings, alcohol oxidation, C-N bond formation, aldehyde activation, and nitrophenol reduction.

Persistent acyclic Cp*Ir(III) complexes and their reactivities in cross-coupling reactions

Iridium(III) complexes play a prominent role in organometallic chemistry, with significant research efforts directed toward Cp*Ir(III) species, broadly categorized into cyclic and acyclic types. Although studies on these two classes began roughly simultaneously, the development of acyclic Cp*Ir(III) complexes has lagged significantly behind their cyclic counterparts. Herein, we report a general and efficient strategy for synthesizing various persistent aryl Cp*Ir(III)(CO)Cl complexes directly from aryl aldehydes, with in situ generated CO as a stabilizing ligand. These acyclic Cp*Ir(III) complexes showcase exceptional reactivity, undergoing reactions with up to eight classes of nucleophiles to generate diverse diorganoiridium(III) species with remarkable stability. Electrochemical analysis of these complexes provides insights into their reductive elimination processes. Guided by these findings, Cp*Ir(III)-mediated decarbonylative C-C and C-O cross-couplings of aryl aldehydes are successfully developed. This study establishes a robust platform for the exploration of acyclic Cp*Ir(III) complexes, paving the way for further advancements in iridium(III) chemistry.

A unified approach to meta-selective methylation, mono-, di- and trifluoromethylation of arenes

Matched molecular series (MMS) are series of molecules that differ only by a single modification at a specific site. The synthesis of MMS is a desirable strategy in drug discovery campaigns. Small aliphatic motifs, notably methyl, mono-, di- and trifluoromethyl substituents (C1 units), are known to have profound effects on the physiochemical properties and/or potency of drug candidates. In this context, we herein report a unique strategy for achieving direct meta-selective methylation, mono-, di-, and trifluoromethylation from the same parent compound. This approach takes advantage of a highly meta-selective ruthenium(ii)-catalyzed alkylation, followed by a subsequent photocatalyzed protodecarboxylation or silver-mediated fluorodecarboxylation to reveal the (fluoro)methyl moiety. This method enables the late-stage access to MMS in small molecules bearing a variety of orienting groups as well as bio-relevant molecules containing complex functionalities, bypassing the need for de novo synthesis to access individual compounds in a series. Moreover, key physiochemical properties of drug candidates were successfully modulated, highlighting opportunities to accelerate medicinal chemistry programs in a sustainable fashion.

Palladium(II) and platinum(II) complexes of disubstituted imidazo[1,5-a]pyridine and imidazolylpyridine: coordination chemistry, versatile catalysis, and biophysical study

Pincer-type mono- and poly-nuclear Pd(II) and Pt(II) complexes bearing imidazo[1,5-a]pyridine and imidazolylpyridine moieties were synthesized and characterized using several spectroscopic methods. Determination of molecular structures of these complexes using single crystal X-ray diffraction studies revealed a distorted square planar geometry around the bivalent palladium and platinum in all the complexes. These Pd(II) complexes displayed high catalytic activity in various reactions, such as the Suzuki-Miyaura cross-coupling reaction, transfer hydrogenation reaction, and alkyne homocoupling. The experimental results matched well with the theoretical data of all catalysts. Substantial deviations in the catalytic activity were observed by changing the co-ligand, binding mode of the ligand and the number of metal centres. Under optimal conditions, the Suzuki cross-coupling and transfer hydrogenation reactions were successfully accomplished with a wide range of functional groups by taking only 0.1 mol% of tetranuclear Pd(II) complex (5) as the catalyst. Intermediates in the Suzuki coupling reaction were also detected using mass spectroscopy. Among the studied complexes, the tetranuclear palladium complex exhibited the highest catalytic activity. Further, Pd(II) complexes were tested in a model reaction of the homocoupling of phenylacetylene, and diphenylbutadiyne was produced in excellent yield. Additionally, the interactions of all the complexes with calf thymus DNA (CT-DNA) and bovine serum albumin (BSA) were investigated using electronic spectroscopy. Absorption study showed minor groove binding of DNA with these complexes, while intercalative binding through displacement of ethidium bromide (EB) in EB-DNA was observed in all the complexes, quenching the fluorescence intensity. The complexes also displayed high binding affinity toward BSA, as confirmed by emission, synchronous fluorescence, and steady-state fluorescence anisotropy measurements. Moreover, the pharmacokinetic properties of two bioactive compounds (3s and 3t) obtained from the Suzuki coupling reaction were calculated, and to evaluate their activity as leukotriene A4 hydrolase (LTA4H) inhibitor, these molecules were docked with human LTA4H enzyme.

Aryl halide cross-coupling via formate-mediated transfer hydrogenation

Transfer hydrogenation is widely practised across all segments of chemical industry, yet its application to aryl halide reductive cross-coupling is undeveloped because of competing hydrogenolysis. Here, exploiting the distinct reactivity of PdI species, an efficient catalytic system for the reductive cross-coupling of activated aryl bromides with aryl iodides via formate-mediated hydrogen transfer is described. These processes display orthogonality with respect to Suzuki and Buchwald-Hartwig couplings, as pinacol boronates and anilines are tolerated and, owing to the intervention of chelated intermediates, are effective for challenging 2-pyridyl systems. Experimental and computational studies corroborate a unique catalytic cycle for reductive cross-coupling where the PdI precatalyst, [Pd(I)(PtBu3)]2, is converted to the dianionic species, [Pd2I4][NBu4]2, from which aryl halide oxidative addition is more facile. Rapid, reversible Pd-to-Pd transmetallation delivers mixtures of iodide-bridged homo- and hetero-diarylpalladium dimers. The hetero-diarylpalladium dimers are more stable and have lower barriers to reductive elimination, promoting high levels of cross-selectivity.

Mechanistic Insights into Molecular Copper Hydride Catalysis: the Kinetic Stability of CuH Monomers toward Aggregation is a Critical Parameter for Catalyst Performance

The activity of molecular copper hydride (CuH) complexes toward the selective insertion of unsaturated hydrocarbons under mild conditions has contributed significantly to versatile methodologies for upgrading these feedstocks. However, these catalysts are particularly susceptible to deleterious aggregation, leading to the depletion of active CuH species. Little is known about the mechanisms of CuH aggregation, how it influences overall catalyst performance, and how it can be controlled. We address these challenges with mechanistic studies on a model reaction of unactivated alkene hydroboration catalyzed by (IPr*CPh3)CuH (LCuH). We report a comprehensive mechanistic investigation of this system, identifying an aggregation pathway that continuously depletes catalytically active LCuH to form inactive CuH clusters during turnover. Deactivation of LCuH is controlled primarily by the competition between the kinetics of the initial LCuH dimerization step and that of alkene insertion into LCuH. We therefore propose that a comprehensive understanding of CuH catalyst performance must account for the kinetics of the initial LCuH dimerization step, revising a previously explored thermodynamic understanding of CuH aggregation, where the concentration of active species is controlled by equilibria established between CuH clusters and monomers. With a series of (NHC)CuH congeners (NHC = N-heterocyclic carbene), we demonstrate that ostensibly minor structural modifications to the ligand peripheries can drastically affect the LCuH dimerization kinetics, while maintaining reactivity toward on-cycle alkene insertion. We employed a computational approach based on molecular dynamics simulations to provide an in-depth understanding of how specific structural ligand modifications can substantially increase the kinetic stability of monomeric CuH catalysts. Our combined experimental and computational studies suggest strategies for rational ligand design that can be broadly applied to molecular catalyst systems that are susceptible to deactivation via aggregation pathways.

Magnesium Nickelate Complexes and Their Implications in Ni-Catalyzed Cross-Couplings of Aryl Fluorides and Aryl Ethers with Grignard Reagents

The nickel-catalyzed Kumada-Tamao-Corriu cross-coupling reaction is widely used to form C-C bonds and receives continued interest due to the unique ability of nickel to activate challenging organic electrophiles containing C-F and C-O bonds. Recent studies on the nickel-catalyzed cross-coupling of Ar-F and Ar-OMe electrophiles with organolithium nucleophiles have unveiled the key involvement of highly reactive anionic nickelates, in which Li and Ni cooperatively promote the activation of these substrates. However, the possible formation of related heterobimetallic intermediates when employing organomagnesium nucleophiles as cross-coupling partners still remains widely underexplored. Filling this gap in the knowledge, we use air-stable Ni-tris(olefin), Ni(4-Me-stb)3 (where stb = stilbene), as a Ni(0) precursor to systematically investigate its reactivity toward several organomagnesium and organolithium reagents, which has allowed for the structural and spectroscopic characterization of a new family of nickelate complexes. Their possible implications and the influence of their constitution and speciation on the outcome of Kumada-Tamao-Corriu cross-couplings are also investigated through a series of stoichiometric and catalytic reactions, which have uncovered a dramatic solvent effect, hinting at the formation of contacted ion pair species as key to maximizing Mg (or Li)/Ni(0) cooperativity.

Bimetallic AuPd alloy nanoparticles on TiO2nanotube arrays: a highly efficient photocatalyst for hydrogen generation

Decoration of TiO2nanotube (TNT) arrays by AuPd nanoparticles (NPs) produces a dramatic enhancement in the rate of hydrogen generation through photocatalytic water-splitting under solar illumination. XRD and TEM confirmed alloy formation in bimetallic AuPd NPs while XPS ruled out a core-shell architecture in the AuPd NPs. Well-dispersed, size-controlled AuPd NPs were formed by sequential physical vapor deposition of Au and Pd on TNTs followed by spontaneous thermal dewetting (TNT-AuPd). TNT-AuPd samples were characterized by small tensile microstrains. For comparison purposes and to derive physical insights, an identical method was used to form TNT-Au and TNT-Pd samples wherein TNTs were decorated by monometallic Au and Pd NPs respectively. In every case, an accumulation-type heterointerface between TiO2and the metallic/bimetallic NPs was indicated by binding energy shifts in the Ti2p high-resolution x-ray photoelectron spectra (HR-XPS). Initial and final state effects in the Au4f HR-XPS pointed to a large number of Au atoms in low coordinate sites such as edges, kinks and corners as well as a slower excited atom relaxation in the alloy. A similar preponderance of Pd atoms at low coordinate sites was found along with the presence of a small amount of palladium oxide. The alloying of Au with a low Pd content on TNT yields significant enhancement in hydrogen production under UV-visible light in aqueous triethanolamine solutions. TNT-AuPd demonstrated the highest photocatalytic H2production rate of 2920µmol g-1h-1, which is 8.9 times higher than that of TNTs, 2.1 times that of TNT-Au, and 1.69 times that of TNT-Pd under solar illumination. We studied H2generation under UV-filtered solar illumination with TNT-AuPd outperforming monometallic Au- and Pd-NP decorated TNTs, which is attributed to the enhancement of the catalytic activity of Pd in an Au environment, the presence of Pd and Au atoms at low coordinate sites, and photoinduced electron transfer between TNTs and AuPd alloy NPs, where AuPd acts as an efficient electron sink, in turn reducing carrier recombination losses. AuPd bimetallic nanoparticles on TNTs, prepared via a simple anodization and vapor deposition method, exhibit excellent stability across multiple cycles and offer valuable insights for the development of efficient photocatalysts with promising potential for emerging energy applications.

A general copper catalytic system for cross-coupling of aryl iodides with chlorosilanes under reductive conditions

Directly forged linkages between commercially available electrophiles are powerful synthetic tools for chemical bond construction. This strategy could eliminate the pre-synthesis of reactive organometallic reagents in couplings with electrophiles, thus providing efficient, easily-handled and step-economical routes in organic synthesis. Reported approaches are mainly utilized in carbon-carbon bond formations, whereas carbon-silicon bond construction employing halosilanes with carbon electrophiles is still underexplored. Copper-catalysis has made significant achievements in the coupling reactions of carbon halides in the past decades, yet silyl electrophiles are seldom involved in these systems. Herein, we establish a practical, efficient, and economical copper system catalyzing the construction of Csp2-Si bonds by directly using aryl/vinyl iodides with various chlorosilanes under ligand-free and reductive conditions, thus providing a general platform for organosilane synthesis with broad scope, high functionality tolerance, scalability and operational simplicity. An unprecedented mechanistic motif was obtained to suggest that the copper catalyst was likely to lower the energy barrier in the reaction of the in situ generated arylzinc with halosilanes, rather than proceed via the traditional metal-aryl species.

Developing Heterogeneous Porous 3D-Printed SiO2-Pd-K2SiO3 Monolithic Catalyst via Surface MOF Growth and Pyrolysis for the Synthesis of Antitumoral Isatins

Background/Objectives: The isatin nucleus is a privileged scaffold in drug discovery, particularly due to its proven relevance in anticancer research. Developing reusable heterogeneous 3D catalysts for drug synthesis represents a critical challenge in both industrial and academic contexts. This multi and interdisciplinary work aimed to design and synthesize a novel 3D-printed silica-based porous catalyst functionalized with palladium, evaluate its catalytic performance in isatin drug synthesis, and assess the antiproliferative activity of the resulting compounds against tumor cell lines such as HeLa, MCF-7, and MDA-MB231. Methods: The novel multifaceted approach to synthesizing this heterogeneous catalyst involved the surface growth of a metal-organic framework (ZIF-8) on 3D-printed silica support, followed by potassium silicate coating and pyrolysis. Results: After detailed physicochemical characterization, the catalyst was tested in challenging "double" palladium-catalyzed cross-coupling reactions (Suzuki, Stille, and Heck), demonstrating robustness, reusability, and high efficiency in producing bis-1,5-aryl, alkynyl, and alkenyl-isatin derivatives. Importantly, no leaching of palladium species was detected during the catalytic cycles, further underscoring the stability of the system. These isatin-based compounds exhibited remarkable cytotoxicity, with selective molecules achieving nanomolar potency against MCF-7 cells, surpassing reference drugs such as doxorubicin and sunitinib. Conclusions: This study not only introduces a novel strategy for fabricating porous heterogeneous catalysts from sintered surfaces but also highlights new biomolecules with promising applications in cancer research.

Palladium nanoparticles from β-cyclodextrin and cellulose methyl carboxylate as an effective catalyst for Sonogashira coupling and the reduction of alkynes

This research presents an effective approach for synthesizing palladium nanoparticle (PdNP) catalysts, employing β-cyclodextrin (β-CD) as a reducing agent and cellulose methyl carboxylate (CMC) as a stabilizer. PdNP-based nanocomposites were prepared by varying the mass ratios of CMC to β-CD from 1 : 1 to 3 : 2. Their structural and physicochemical properties were thoroughly analyzed using multiple characterization techniques, including ultraviolet-visible spectroscopy (UV-vis), FTIR, DLS, Zeta potential, XRD, TGA, and TEM. The resulting PdNPs exhibited a crystalline structure with particle dimensions spanning from 2 to 10 nm, with the majority falling between 4 and 6 nm. These nanoparticles demonstrated outstanding catalytic activity in Sonogashira coupling reactions, operating without additional catalysts and efficiently converting alkynes into (Z)-alkenes utilizing KOH/DMF as a hydrogen donor. The high yield and selectivity observed highlight the potential of CMC/β-CD-stabilized PdNPs as promising catalysts for organic synthesis applications.

Synthesis of Trisubstituted Alkenes Bearing a Quaternary Carbon by Lewis-Acid Catalyzed Regioselective Reaction of Internal Alkynols with 2-Pyrrolylanilines

Substituted alkenes, crucial in organic chemistry, are typically accessed from internal alkynes by using transition-metal catalysis. Herein, we report a novel approach to the Lewis acid-catalyzed synthesis of trisubstituted alkenes bearing a quaternary carbon from internal alkynols and 2-pyrrolyl arylamines. This method involves allene formation followed by intramolecular [5 + 1] annulation to furnish a diverse range of trisubstituted alkenes with excellent regioselectivity and poor diastereoselectivity. Further, we demonstrated the gram-scale synthesis and synthetic transformations and proposed the reaction mechanism based on the isolation of intermediates.

Defying the oxidative-addition prerequisite in cross-coupling through artful single-atom catalysts

Heterogeneous single-atom catalysts (SACs) have gained significant attention for their maximized atom utilization and well-defined active sites, but they often struggle with multi-stage organic cross-coupling reactions due to limited coordination space and reactivity. Here, we report an "anchoring-borrowing" strategy combined facet engineering to develop artful single-atom catalysts (ASACs) through anchoring foreign single atoms onto specific facets of the non-innocent reducible carriers. ASACs exhibit adaptive coordination, effectively bypassing the oxidative-addition prerequisite for bivalent elevation at a single metal site in both homogenous and heterogeneous cross-couplings. For example, Pd1-CeO2(110) ASAC exhibits unparalleled activity in coupling with more accessible aryl chlorides, and challenging heterocycles, outperforming traditional catalysts with a remarkable turnover number of 45,327,037. Mechanistic studies reveal that ASACs leverage dynamic structural changes, with reducible carriers acting as electron reservoirs, significantly lowering reaction barriers. Furthermore, ASACs enable efficient synthesis of biologically significant compounds, drug intermediates, and active pharmaceutical ingredients (APIs) through a scalable high-speed circulated flow synthesis, underscoring great potential for sustainable fine chemical manufacturing.

High-throughput hit identification with acoustic ejection mass spectrometry

This mini-review provides an overview of recent developments in AEMS supporting hit identification in drug discovery, emphasizing its potential to enhance the quality and efficiency of label-free HTS. Future advancements that may further expand the role of AEMS in the drug discovery process will also be discussed.

Liquid palladium for high-turnover carbon-carbon bond formation

Carbon-carbon (C─C) bond formation is a key step in diverse chemical processes and requires high-performance catalysts to enable energy-efficient technologies. Here, we present liquid Pd catalysts, formed by dissolving Pd in liquid Ga, for high-turnover C─C coupling reactions. The liquid Pd catalyst achieved a turnover frequency of 2.5 × 108 hour-1 for a model coupling reaction at 70°C, surpassing all reported Pd catalysts by 1000-fold. Our results show that Pd atoms in the Ga matrix are liquid-like, exhibiting unique electronic and interfacial properties that substantially lower the energy barrier and enhance reaction kinetics. The system retained full activity over five cycles and showed no Pd leaching, highlighting the transformative potential of liquid-phase metals to advance high-throughput and sustainable C─C bond-forming strategies.

An [Ag3(dppy)2(NO3)3] n cluster polymer with narrowing fluorescence

Low-dimensional nanomaterials with lattice confinement, including those of nanoclusters (NCs), offer benefits for fluorescence narrowing. Compared to quantum dots of metal NCs, however, one-dimensional structures of such NCs challenge the single-crystal synthesis. Here, we report the synthesis of a novel [Ag3(dppy)2(NO3)3] n cluster polymer through the reduction of AgNO3 with NaBH4 in a dark environment. This cluster polymer incorporates the coordination and passivation of both diminished nitro groups (NO3) and diphenyl-2-pyridylphosphine (dppy) ligands. The weak Ag-Ag metallic bonds within this cluster polymer are governed by argentophilic interactions, with each Ag3 unit connected by a NO3 group. This cluster polymer exhibits photoluminescence with three emission bands at 308, 352, and 620 nm, aligned with the purple (308/352 nm) and red (620 nm) regions, respectively. We synthesized microfibers of this cluster polymer using reprecipitation, resulting in a fluorescence bandwidth reduction to approximately one-tenth in the microfiber samples relative to the diluted solution.

Fluorine-Based Oxidant Enables Room-Temperature Pd-Catalyzed C-H Arylation with Boronic Acids

The biaryl motif is important in many fields within chemistry and the life sciences. Thus, better strategies to forge aryl-aryl bonds are valuable. Herein, we report conditions permitting the room temperature arylation of N-aryl amide substrates, using ubiquitous aryl boronic acid reagents. Critical to the success of this method is the use of Selectfluor as a sustainable alternative oxidant to silver-based additives and the deployment of a bulky QuinOx ligand supporting the palladium catalyst.

Palladium Membrane Applications in Hydrogen Energy and Hydrogen-Related Processes

In recent years, increased attention has been paid to environmental issues and, in connection with this, to the development of hydrogen energy. In turn, this requires the large-scale production of ultra pure hydrogen. Currently, most hydrogen is obtained by converting natural gas and coal. In this regard, the issue of the deep purification of hydrogen for use in fuel cells is very relevant. The deep purification of hydrogen is also necessary for some other areas, including microelectronics. Only palladium membranes can provide the required degree of purification. In addition, the use of membrane catalysis is very relevant for the widely demanded processes of hydrogenation and dehydrogenation, for which reactors with palladium membranes are used. This process is also successfully used for the single-stage production of high-purity hydrogen. Polymeric palladium-containing membranes are also used to purify hydrogen and to remove various pollutants from water, including organochlorine products, nitrates, and a number of other substances.

Catalytic Activity of Water-Soluble Palladium Nanoparticles with Anionic and Cationic Capping Ligands for Reduction, Oxidation, and C-C Coupling Reactions in Water

The availability of water-soluble nanoparticles allows catalytic reactions to occur in highly desirable green environments. The catalytic activity and selectivity of water-soluble palladium nanoparticles capped with 6-(carboxylate)hexanethiolate (C6-PdNP) and 5-(trimethylammonio)pentanethiolate (C5-PdNP) were investigated for the reduction of 4-nitrophenol, the oxidation of α,β-conjugated aldehydes, and the C-C coupling of phenylboronic acid. The study showed that between the two PdNPs, C6-PdNP exhibits better catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride and the selective oxidation of conjugated aldehydes to conjugated carboxylic acids. For the latter reaction, molecular hydrogen (H2) and H2O act as oxidants for the surface palladium atoms on PdNPs and conjugated aldehyde substrates, respectively. The results indicated that the competing addition activities of Pd-H and H2O toward the π-bond of different unsaturated substrates promote either reduction or oxidation reactions under mild conditions in organic solvent-free environments. In comparison, C5-PdNP exhibited higher catalytic activity for the C-C coupling of phenylboronic acid. Gas chromatography-mass spectrometry (GC-MS) was mainly used as an analytical technique to examine the products of catalytic reactions.

Ring strain governs transmetalation behaviour at a tucked-in iron complex

Studies that independently investigate [M]-C transmetalation reactions using two different metals are uncommon and yet understanding this reactivity is important to unlocking new synthetic approaches and product classes. Here, we show that the strained [Fe]-C complex, [(η6-C5Me4-CH2)Fe(diphosphine)] undergoes transmetalation with rhodium(I) and iridium(I) diolefin salts, leading to rapid Fe-C(sp3) bond cleavage and M-C(sp3) (M = Rh or Ir) bond generation.

Ni-catalysed acceptorless dehydrogenative aromatisation of cyclohexanones enabled by concerted catalysis specific to supported nanoparticles

The dehydrogenative aromatisation of cyclohexanone derivatives has had a transformative influence on the synthesis of aromatic compounds because functional groups can be easily introduced at desired positions via classic organic reactions without being limited by ortho-, meta- or para-orientations. However, research is still limited on acceptorless dehydrogenative aromatisation, especially with regard to nonprecious-metal catalysts. Ni is a promising candidate catalyst as a congener of Pd, but thermally Ni-catalysed dehydrogenative aromatisation has not been reported even in an oxidative manner because of the difficulty of β-hydride elimination and the fast re-insertion of Ni-H species. Here, we report a CeO2-supported Ni(0) nanoparticle catalyst for acceptorless dehydrogenative aromatisation of cyclohexanone derivatives. This catalyst is widely applicable to various compounds such as cyclohexanols, cyclohexylamines, N-heterocycles, enamines and β-heteroatom-substituted ketones. Through various experiments, we demonstrate that the present reaction was achieved by the concerted catalysis utilizing metal ensembles unique to supported metal nanoparticle catalysts.

π-Lewis Base Activation of Carbonyls and Hexafluorobenzene

We report hitherto elusive side-on η2-bonded palladium(0) carbonyl (anthraquinone, benzaldehyde) and arene (benzene, hexafluorobenzene) palladium(0) complexes and present the catalytic hydrodefluorination of hexafluorobenzene by cyclohexene. The comparison with respective cyclohexene, pyridine and tetrahydrofuran complexes reveals that the experimental ligand binding strengths follow the order THF Collapse

Key Words

• Catalysis
• Chemical Bonding
• Intermediates
• Organometallic Chemistry
• Spectroscopy

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MESH Headings Collapse

Grants

• 948185 H2020 European Research Council
• 256/506-1 Deutsche Forschungsgemeinschaft
• 256/582-1 Deutsche Forschungsgemeinschaft
• 440719683 Deutsche Forschungsgemeinschaft
• 2008-390540038 Deutsche Forschungsgemeinschaft

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Affiliation(s)

Aditesh Mondal Coordination Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany
Kevin Breitwieser Coordination Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany
Sergi Danés Departament de Química, Institut de Química Computacional I Catàlisi, Universitat de Girona, c/m. Aurelia Capmany 69, 17003, Girona, Spain
Annette Grünwald Coordination Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany Inorganic and General Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 1, D-91058, Erlangen, Germany
Frank W Heinemann Inorganic and General Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 1, D-91058, Erlangen, Germany
Bernd Morgenstern Solid State Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany
Frank Müller Experimental Physics and Center for Biophysics, Saarland University, Campus E2.9, D-66123, Saarbrücken, Germany
Michael Haumann Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195, Berlin, Germany
Maximilian Schütze Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489, Berlin, Germany
Dustin Kass Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489, Berlin, Germany
Kallol Ray Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489, Berlin, Germany
Dominik Munz Coordination Chemistry, Saarland University, Campus C4.1, D-66123, Saarbrücken, Germany

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Collaborators Collapse

Pd(0)/Pd(II) Electromerism Triggered by Lewis Base Coordination to a Redox-Active Silicon Z-Type Ligand

Electromerism (aka. valence tautomerism) corresponds to the switching of electronic distributions between redox-active ligands and central elements. While this phenomenon is well established for several transition metals, the Pd(0)/Pd(II) couple could not yet be involved due to the high energy of the Pd(0) state. In this study, we present Pd(0)/Pd(II) electromerism by using a redox-active bis(phosphanyl-amidophenolato)silane. Strong silicon-based Z-type interaction stabilizes the Pd(0) state with an oxidized dicationic open-shell singlet ligand while binding of a Lewis base at the Lewis acidic silicon results in the loss of Z-type interaction and the intramolecular rearrangement to the Pd(II)/reduced ligand electromer. It introduces a novel concept for spin state switching by combining ligand ambiphilicity and redox activity.

Activation and C-C Coupling of Aryl Iodides via Bismuth Photocatalysis

Within the emerging field of bismuth redox catalysis, the catalytic formation of C-C bonds using aryl halides would be highly desirable; yet such a process remains a synthetic challenge. Herein, we present a chemoselective bismuth-photocatalyzed activation and subsequent coupling of (hetero)aryl iodides with pyrrole derivatives to access C(sp2)-C(sp2) linkages through C-H functionalization. This unique reactivity is the result of the bismuth complex featuring two redox state-dependent interactions with light, which 1) activates the Bi(I) complex for oxidative addition via MLCT, and 2) promotes the homolytic cleavage of aryl Bi(III) intermediates through a LLCT process.

Bimetallic Nickel-Palladium Nanoparticles Supported on Multiwalled Carbon Nanotubes for Suzuki Cross-Coupling Reactions in Continuous Flow

An efficient Suzuki cross-coupling reaction under continuous flow conditions was developed utilizing an immobilized solid supported catalyst consisting of bimetallic nickel-palladium nanoparticles (Ni-Pd/MWCNTs). In this process, the reactants can be continuously pumped into a catalyst bed at a high flow rate of 0.6 mL/min and the temperature of 130 °C while the Suzuki products are recovered in high steady-state yields for prolonged continuous processing. The catalyst was prepared by mechanical shaking of the appropriate nickel and palladium salts using ball-mill energy without the requirement of any solvent or reducing agent. This straightforward, facile, and simple method allows for bulk production of Ni-Pd/MWCNTs nanoparticles with a small particle size ideal for application in continuous flow cross-coupling catalysis. The as-prepared catalyst mostly contains nickel (7.9%) with a very small amount of palladium (0.81%) according to ICP-OES analysis. This remarkable immobilized catalyst can be used several times for different Suzuki reactions with a minimum loss of reactivity and no detectable leaching of the metal nanoparticles. Notably, by modifying the groups on both aryl halides and phenylboronic acids, the method provides access to a diverse array of the Suzuki products in flow with high steady-state yield, making it suitable for applications in industrial and pharmaceutical scales. Moreover, several spectroscopic techniques were employed to identify the structure and composition of the as-prepared Ni-Pd/MWCNTs nanoparticles before and after the reaction in flow such as transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), BET surface area (physisorption), and FTIR spectroscopy.

Increasing the Donor Strength of Alkenylphosphines by Twisting the C=C Double Bond

Electron-rich phosphines play a crucial role in transition metal-based homogeneous catalysis. While alkyl groups have traditionally been employed to increase the phosphine donor strength, recent studies have shown that zwitterionic functional groups such as phosphorus ylides can result in a further enhancement. Herein we report the concept of twisting a C=C double bond to introduce a zwitterionic substituent by the synthesis and application of N-heterocyclic olefin phosphines with a sulfonyl substituent (sNHOP). This sulfonyl group enables the twisting of the olefin moiety due to steric and electronic stabilization of the carbanionic center. The resulting zwitterionic structure leads to a significant increase of the donor strength of the sNHOP ligands compared to conventional NHOP systems with a planar N-heterocyclic olefin moiety. The potential of this new ligand platform for catalysis is demonstrated by its application in the gold-catalyzed hydroamination and cyclo-isomerization of alkynes. Here, the ligands outperform the original NHOP ligands suggesting favorable properties for future catalysis applications.

Recent advances in electrochemical copper catalysis for modern organic synthesis

In recent decades, organic electrosynthesis has emerged as a practical, sustainable, and efficient approach that facilitates valuable transformations in synthetic chemistry. Combining electrochemistry with transition-metal catalysis is a promising and rapidly growing methodology for effectively forming challenging C-C and C-heteroatom bonds in complex molecules in a sustainable manner. In this review, we summarize the recent advances in the combination of electrochemistry and copper catalysis for various organic transformations.

NMR and MD Simulations of Non-Ionic Surfactants

Non-ionic surfactants are an important solvent in the field of green chemistry with tremendous application potential. Understanding their phase properties in bulk or in confined environments is of high commercial value. In recent years, the combination of molecular dynamics (MD) simulations with multinuclear solid-state NMR spectroscopy and calorimetric techniques has evolved into the most powerful tool for their investigation. Showing recent examples from our groups, the present review demonstrates the power and versatility of this approach, which can handle both small model-surfactants like octanol and large technical surfactants like technical polyethylene glycol (PEG) mixtures and reveals otherwise unobtainable knowledge about their phase behavior and the underlying molecular arrangements.

Unlocking the Nucleophilic Functionalization Potential of a Natural Asphalt: Grafting a Pd(0)-Diethanolamine Complex as a Recyclable Catalyst for Upgrading Biaryl Synthesis

This study introduces a novel method for functionalizing natural asphalt, presenting new opportunities for upgrading asphaltenes from road to a catalyst. The process utilizes a metal-free sonobromination technique in acetic acid to incorporate carbon-halogen substituents onto natural asphalt. These sites are then targeted by nucleophilic substitution with diethanolamine, followed by complexation with Pd(0) to create a unique palladium complex grafted onto natural asphalt. This stabilized complex serves as a heterogeneous and recoverable catalyst in the Suzuki reaction. This complex facilitates the reaction between aryl boronic acids and various ortho-, meta-, and para-substituted aryl halides under mild conditions using polyethylene glycol-400 as the green solvent. The reaction conversion rate is significantly influenced by the leaving group ability of the halides and the electronic and steric effects of the substituents on both reactants. This environmentally friendly process offers a broad substrate scope (24 examples) and achieves excellent yields of biphenyl derivatives. Notably, it employs a naturally derived catalytic support, underscoring its sustainability. This research potentially unlocks the bonding of nucleophiles to the natural asphalt for developing novel functional materials from this renewable resource.

Leveraging natural language processing to curate the tmCAT, tmPHOTO, tmBIO, and tmSCO datasets of functional transition metal complexes

The breadth of transition metal chemical space covered by databases such as the Cambridge Structural Database and the derived computational database tmQM is not conducive to application-specific modeling and the development of structure-property relationships. Here, we employ both supervised and unsupervised natural language processing (NLP) techniques to link experimentally synthesized compounds in the tmQM database to their respective applications. Leveraging NLP models, we curate four distinct datasets: tmCAT for catalysis, tmPHOTO for photophysical activity, tmBIO for biological relevance, and tmSCO for magnetism. Analyzing the chemical substructures within each dataset reveals common chemical motifs in each of the designated applications. We then use these common chemical structures to augment our initial datasets for each application, yielding a total of 21 631 compounds in tmCAT, 4599 in tmPHOTO, 2782 in tmBIO, and 983 in tmSCO. These datasets are expected to accelerate the more targeted computational screening and development of refined structure-property relationships with machine learning.

Migratory Aryl Cross-Coupling

A fundamental property of cross-coupling reactions is regiospecificity, meaning that the site of bond formation is determined by the leaving group's location on the electrophile. Typically, achieving a different substitution pattern requires the synthesis of a new, corresponding starting-material isomer. As an alternative, we proposed the development of cross-coupling variants that would afford access to multiple structural isomers from the same coupling partners. Here, we first demonstrate that a bulky palladium catalyst can facilitate the efficient, reversible transposition of aryl halides by temporarily forming metal aryne species. Despite the nearly thermoneutral equilibrium governing this process, combining it with the gradual addition of a suitable nucleophile results in dynamic kinetic resolution of the isomeric intermediates and high yields of unconventional product isomers. The method accommodates a range of oxygen- and nitrogen-centered nucleophiles and tolerates numerous common functional groups. A Curtin-Hammett kinetic scheme is supported by computational and experimental data, providing a general mechanistic framework for extending this migratory cross-coupling concept.

Effective Alkyl-Alkyl Cross-Coupling with an Iron-Xantphos Catalyst: Mechanistic and Structural Insights

While iron-catalyzed C(sp2)-C(sp3) cross-couplings have been widely studied and developed in the last decade, alkyl-alkyl cross-coupling systems with iron remain underdeveloped despite the importance of C(sp3)-C(sp3) bonds in organic synthesis. A major challenge to the development of these reactions is the current lack of fundamental insight into ligand effects and organoiron intermediates that enable effective alkyl-alkyl couplings. The current study addresses this longstanding limitation using a combination of 57Fe Mössbauer spectroscopy, SC-XRD (single-crystal X-ray diffraction) and reactivity studies of alkyl-alkyl coupling with iron-Xantphos to define the in situ formed iron-Xantphos intermediates in catalysis. Combined with detailed reactivity studies, the nature of the key mechanistic pathways in catalysis and ligands effects to achieve effective alkyl-alkyl cross-coupling over competing β-H elimination pathways are probed. Overall, these foundational studies provide a platform for future bespoke ligand and pre-catalyst design for alkyl-alkyl cross-coupling methods development with sustainable iron catalysis.

Synthesis of β-lactam-zidovudine pronucleosides as potential selective narrow-spectrum antibacterial agents

Since the discovery of penicillin, the forerunner of the most widely used class of antibiotics (i.e. β-lactams), natural compounds and their derivatives represented a major source of antibacterial therapeutic products whose availability enabled modern medical practices (invasive surgery, organ transplant, etc.). However, the relentless emergence of resistant bacteria is challenging the long-term efficacy of antibiotics, also decreasing their economic attractiveness for big pharma, leading to a significant decay in antibacterial development in the 21st century and an increased use of last-resort drugs such as carbapenems or colistin. Indeed, bacteria evolved an arsenal of resistance mechanisms, leading to the emergence of totally-drug resistant isolates, already sporadically isolated among Gram-negative bacterial species. To face this deadly peril, it is fundamental to explore new ground-breaking approaches. In view of the significance of both β-lactam antibiotics and the production of one or more β-lactamases as the major resistance mechanism (especially in Gram-negative bacteria), we implemented an original approach to selectively deliver antibacterial zidovudine (AZT) exploiting the β-lactamase-mediated hydrolysis of a β-lactam-conjugate prodrug. The synthesis of the targeted pronucleosides was performed in 5-7 steps and based on an original Pd-catalyzed cross-coupling reaction. Enzymatic and microbiological evaluations were performed to evaluate the synthesized pronucleosides, yielding new insights into molecular recognition of β-lactamase enzymes. This approach would potentially allow a targeted and selective eradication of antibiotic-resistant β-lactamase-producing (opportunistic) pathogens, as the inactive prodrug is unable to harm the commensal microbial flora.

Demonstrating Synergistic Activity of Magnetic Iron Oxide Nano Photocatalyst for C-H Activation in Heterogeneous Phase

This report describes a dual catalytic approach for the versatile C-H arylation of arenes under photo-excitation at room temperature. The cooperative catalysis utilizes iron oxide magnetic nanoparticles (which mostly contain Fe3O4 along with some γ-Fe2O3) as the potential photocatalyst, which merges with the Pd-catalyzed C-H activation cycle for the reductive generation of aryl radical from aryl diazonium salt, revealing its photocatalytic activities. The method is applicable to a wide range of aryl coupling partners and different directing groups, demonstrating excellent productivity, nice co-operativity and recyclability. Adequate control experiments and mechanistic studies assisted in establishing the radical-based reaction mechanism for the C-H arylation occurring in the heterogeneous phase.

Microwave-Assisted One-Step Synthesis of Palladium-Encapsulated Covalent Organic Frameworks for Heterogeneous Catalysis

Metal-encapsulated covalent organic frameworks (metal/COFs) represent an emerging paradigm in heterogeneous catalysis. However, the time-intensive (usually 4 or more days) and tedious multi-step synthesis of metal/COFs remains a significant stumbling block for their broad application. To address this challenge, we introduce a facile microwave-assisted in situ metal encapsulation strategy to cooperatively combine COF formation and in situ palladium(II) encapsulation in one step. With this unprecedented approach, we synthesize a diverse range of palladium(II)-encapsulated COFs (termed Mw-Pd/COF) in the air within just an hour. Notably, this strategy is scalable for large-scale production (~0.5 g). Leveraging the high crystallinity, porosity, and structural stability, one representative Mw-Pd/COF exhibits remarkable activity, functional group tolerance, and recyclability for the Suzuki-Miyaura coupling reaction at room temperature, surpassing most previously reported Pd(II)/COF catalysts with respect to catalytic performance, preparation time, and synthetic ease. This microwave-assisted in situ metal encapsulation strategy opens a facile and rapid avenue to construct metal/COF hybrids, which hold enormous potential in a multitude of applications including heterogeneous catalysis, sensing, and energy storage.

Characterization of Bimetallic Pd-Fe Nanoparticles Synthesized in Escherichia coli

Biologically mediated nanoparticle (NP) synthesis offers a reliable and sustainable alternative route for metal NP production. Compared with conventional chemical and physical production methods that require hazardous materials and considerable energy expenditure, some microorganisms can reduce metal ions into NPs during standard metabolic processes. However, to be considered a feasible commercial option, the properties and inherent activity of bio-NPs still need to be significantly improved. In this work, we present an Escherichia coli-mediated synthesis method for catalytically active Pd-Fe NPs. The produced biogenic Pd-Fe NPs with varying Fe content were characterized using complementary analytical techniques to assess their size, composition, and structural properties. In addition, their catalytic performance was assessed by using standardized chemical reactions. We demonstrate that the combination of Pd with Fe leads to synergistic effects that enhance the catalytic performance of Pd NPs and make biogenic Pd-Fe NPs excellent potential substitutes for currently used catalysts. Briefly, the apparent rates for the model reaction of 4-nitrophenol reduction to 4-aminophenol catalyzed by Pd-based nanoparticles were as high as 0.1312 min-1 using bimetallic Pd-Fe NPs, which is far superior to the rates of monometallic Pd NPs counterparts. This study provides a feasible strategy for the synthesis of multimetallic Pd-based NPs using common microbial processes. It emphasizes the potential of biogenic Pd-Fe NPs as efficient and sustainable catalysts for hydrogenation reactions, offering an environmentally friendly synthesis for various applications, including wastewater treatment and the production of fine chemicals.

A computational mechanistic study on the formation of aryl sulfonyl fluorides via Bi(III) redox-neutral catalysis and further rational design

Sulfonyl fluorides hold significant importance as highly valued intermediates in chemical biology due to their optimal balance of biocompatibility with both aqueous stability and protein reactivity. The Cornella group introduced a one-pot strategy for synthesizing aryl sulfonyl fluorides via Bi(III) redox-neutral catalysis, which facilitates the transmetallation and direct insertion of SO2 into the BiC(sp2) bond giving the aryl sulfonyl fluorides. We report herein a comprehensive computational investigation of the redox-neutral Bi(III) catalytic mechanism, disclose the critical role of the Bi(III) catalyst and base (i.e., K3PO4), and uncover the origin of SO2 insertion into the Bi(III)C(sp2) bond. The entire catalysis can be characterized via three stages: (i) transmetallation generating the Bi(III)-phenyl intermediate IM3 facilitated by K3PO4. (ii) SO2 insertion into IM3 leading to the formation of Bi(III)-OSOAr intermediate IM5. (iii) IM5 undergoes S(IV)-oxidation yielding the aryl sulfonyl fluoride product 4 and liberating the Bi(III) catalyst for the next catalytic cycle. Each stage is kinetically and thermodynamically feasible. Moreover, we explored other some small molecules (NO2, CO2, H2O, N2O, etc.) insertion reactions mediated by the Bi(III)-complex, and found that NO2 insertions could be easily achieved due to the low insertion barriers (i.e., 17.5 kcal/mol). Based on the detailed mechanistic study, we further rationally designed additional Bi(III) and Sb(III) catalysts, and found that some of which exhibit promising potential for experimental realization due to their low barriers (<16.4 kcal/mol). In this regard, our study contributes significantly to enhancing current Bi(III)-catalytic systems and paving the way for novel Bi(III)-catalyzed aryl sulfonyl fluoride formation reactions.

Nonheme iron catalyst mimics heme-dependent haloperoxidase for efficient bromination and oxidation

The [Fe]/H2O2 oxidation system has found wide applications in chemistry and biology. Halogenation with this [Fe]/H2O2 oxidation protocol and halide (X-) in the biological system is well established with the identification of heme-iron-dependent haloperoxidases. However, mimicking such halogenation process is rarely explored for practical use in organic synthesis. Here, we report the development of a nonheme iron catalyst that mimics the heme-iron-dependent haloperoxidases to catalyze the generation of HOBr from H2O2/Br- with high efficiency. We discovered that a tridentate terpyridine (TPY) ligand designed for Fenton chemistry was optimal for FeBr3 to form a stable nonheme iron catalyst [Fe(TPY)Br3], which catalyzed arene bromination, Hunsdiecker-type decarboxylative bromination, bromolactonization, and oxidation of sulfides and thiols. Mechanistic studies revealed that Fenton chemistry ([Fe]/H2O2) might operate to generate hydroxyl radical (HO•), which oxidize bromide ion [Br-] into reactive HOBr. This nonheme iron catalyst represents a biomimetic model for heme-iron-dependent haloperoxidases with potential applications in organic synthesis, drug discovery, and biology.

Ni-Catalyzed Electro-Reductive Cross-Electrophile Couplings of Alkyl Amine-Derived Radical Precursors with Aryl Iodides

In recent years, the "Escape-from-Flatland" trend has prompted the synthetic community to develop a set of cross-coupling strategies to introduce sp3-carbon-based fragments in organic compounds. This study presents a novel nickel-catalyzed electrochemical methodology for reductive cross-electrophile coupling. The method enables C(sp2)-C(sp3) linkages using inexpensive amine-derived radical precursors and aryl iodides. The use of electrochemistry as a power source reduces waste and avoids chemical reductants, making this approach a more sustainable alternative to traditional cross-coupling methods.

Late-Stage Halogenation of Complex Substrates with Readily Available Halogenating Reagents

ConspectusLate-stage halogenation, targeting specific positions in complex substrates, has gained significant attention due to its potential for diversifying and functionalizing complex molecules such as natural products and pharmaceutical intermediates. Utilizing readily available halogenating reagents, such as hydrogen halides (HX), N-halosuccinimides (NXS), and dichloroethane (DCE) reagents for late-stage halogenation shows great promise for expanding the toolbox of synthetic chemists. However, the reactivity of haleniums (X+, X = Cl, Br, I) can be significantly hindered by the presence of various functional groups such as hydroxyl, amine, amide, or carboxylic acid groups. The developed methods of late-stage halogenation often rely on specialized activating reagents and conditions. Recently, our group (among others) has put great efforts into addressing these challenges and unlocking the potential of these readily available HX, NXS, and DCE reagents in complex molecule halogenation. Developing new methodologies, catalyst systems, and reaction conditions further enhanced their utility, enabling the efficient and selective halogenation of intricate substrates.With the long-term goal of achieving selective halogenation of complex molecules, we summarize herein three complementary research topics in our group: (1) Efficient oxidative halogenations: Taking inspiration from naturally occurring enzyme-catalyzed oxidative halogenation reactions, we focused on developing cost-effective oxidative halogenation reactions. We found the combination of dimethyl sulfoxide (DMSO) and HX (X = Cl, Br, I) efficient for the oxidative halogenation of aromatic compounds and alkenes. Additionally, we developed electrochemical oxidative halogenation using DCE as a practical chlorinating reagent for chlorination of (hetero)arenes. (2) Halenium reagent activation: Direct electrophilic halogenation using halenium reagents is a reliable method for obtaining organohalides. However, compared to highly reactive reagents, the common and readily available NXS and dihalodimethylhydantoin (DXDMH) demonstrate relatively lower reactivity. Therefore, we focused on developing oxygen-centered Lewis base catalysts such as DMSO, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and nitromethane to activate NXS or DXDMH, enabling selective halogenation of bioactive substrates. (3) Halogenation of inert substrates: Some substrates, such as electron-poor arenes and pyridines, are inert toward electrophilic functionalization reactions. We devised several strategies to enhance the reactivity of these molecules. These strategies, characterized by mild reaction conditions, the ready availability and stability of catalysts and reagents, and excellent tolerance for various functional groups, have emerged as versatile protocols for the late-stage aromatic halogenation of drugs, natural products, and peptides. By harnessing the versatility and selectivity of these catalysts and methodologies, synthetic chemists can unlock new possibilities in the synthesis of halogenated compounds, paving the way for the development of novel functional materials and biologically active molecules.

Bio-inspired fabrication of chitosan/PEO/Ti3C2Tx 2D MXene nanosheets supported palladium composite nanofiber catalysts via electrospinning

In this study, novel chitosan/polyethylene oxide/Ti3C2Tx 2D MXene nanosheets (CS/PEO/Ti3C2Tx) nanofibers were successfully prepared by a continuous electrospinning process. During the electrospinning process, induced by the syringe tip capillary effects and electric field force, the Ti3C2Tx nanosheets were aligned along the direction of the nanofiber formation to occur a highly oriented structure. This well-ordered arrangement of the inorganic Ti3C2Tx nanosheets within the organic polymer matrix nanofiber was similar with nacre-like 'brick-and-motar' structure to some extent, resulting in a marked increase in thermal stability and mechanical properties of the resultant CS/PEO/Ti3C2Tx nanofiber. As 4 wt% of Ti3C2Tx nanosheets loaded, the highest tensile strength of the CS/PEO/Ti3C2Tx nanofiber mats was achieved as 31.7 MPa, about two times that of neat CS/PEO nanofibers. Uniformly dispersed Pd nanoparticles in size of about 1.6 nm have been successfully immobilized on the composite nanofiber with a solution impregnation process. With a loading as low as 0.2 mol% of Pd, the resultant Pd@CS/PEO/Ti3C2Tx composite nanofiber catalysts were highly active for both Heck and Sonogashira coupling reactions with broad reactants application scope, and could be recycled 15 runs without significant loss of activities.

Cross-Coupling of NHC/CAAC-Based Carbodicarbene: Synthesis of Electron-Deficient Diradicaloids

Herein, we report nickel(0)-catalyzed cross-coupling reactions of NHC/CAAC-based carbodicarbene (NHC = N-heterocyclic carbene and CAAC = cyclic(alkyl)(amino)carbene) with different aryl chlorides, bromides, and iodides. The resulting aryl-substituted cationic carbodicarbene derivatives are prone to one-electron oxidation yielding radical-dications, which, depending on the aryl motif employed, follow different modes of radical-radical dimerization and constitute an entry point to carbon/nitrogen- and nitrogen/nitrogen-centered diradicaloids. Subsequently, this coupling strategy was strategically applied to the synthesis of p-phenylene- and p,p'-biphenylene-bridged carbon/carbon-centered electron-deficient diradicaloids. The employed π-conjugated spacer plays a crucial role in determining the triplet population at room temperature by modulation of the singlet-triplet gap: EPR inactive for p-phenylene vs EPR active for p,p'-biphenylene. Nearly two decades after the disclosure of carbodicarbenes as donor-stabilized atomic carbon equivalents by Tonner and Frenking in 2007, we demonstrate their cross-couplings with a series of aryl halides/dihalides and, based on this, developed a modular methodology for the systematic synthesis of various electron-deficient diradicaloids.

Strategies for arene dissociation from transition metal η6-arene complexes

Transition metal η6-arene complexes have unique properties that facilitate a variety of arene substitution reactions, rendering π-activation a powerful approach for arene functionalization. For decades, these complexes have been studied in the context of coordination chemistry and synthetic methodology via stoichiometric reactivity; one central challenge in expanding the utility of arene functionalization via transition-metal-π-activation is the dissociation of the arene product that remains bound to the transition metal. In this perspective, we highlight representative strategies and methods for the removal and/or exchange of arenes from such complexes. Recent studies that implement these strategies toward catalytic processes are discussed, along with remaining challenges in this area.

Accessing the synthesis of natural products and their analogues enabled by the Barbier reaction: a review

The Barbier reaction is significantly referred to as one of the efficient carbon-carbon bond forming reactions which involves the treatment of haloalkanes and carbonyl compounds by utilizing the catalytic role of a diverse range of metals and metalloids. The Barbier reaction is tolerant to a variety of functional groups, allowing a broad substrate scope with the employment of lanthanides, transition metals, amphoteric elements or alkaline earth metals. This reaction is also water-resistant, thereby overcoming the challenges posed by moisture sensitive organometallic species involving C-C bond formation reactions. The Barbier reaction has significantly found its applicability towards the synthesis of intricate and naturally occurring organic compounds. Our review provides an outlook on the synthetic applications of the Barbier reaction and its variants to accomplish the preparation of several natural products, reported since 2020.

Magnesium-promoted nickel-catalysed chlorination of aryl halides and triflates under mild conditions

In this study, we present a ligand-free nickel(II)-catalyzed halogen exchange of aromatic halides with magnesium chloride. This method effectively facilitates the retro-Finkelstein reaction for a wide range of aryl bromides, iodides and triflates, demonstrating excellent functional group tolerance. Mechanistic studies reveal that magnesium plays a crucial role in the challenging reductive elimination from Ni(II) intermediates.

A universal strategy for the synthesis of transition metal single atom catalysts toward electrochemical CO2 reduction

Herein, a pyrolysis-induced precursor transformation strategy has been proposed. Using pre-synthesized PDA-M as a precursor, the production of transition metal single atom catalysts (SACs) has been achieved, with compositional flexibility at high metal loadings. In particular, the Ni SAC sample has shown promising CO selectivity when evaluated for the electrochemical CO2 reduction reaction, reaching 29.8 mA cm-2 CO partial current density and 90.3% CO faradaic efficiency at -1.05 V vs. RHE.

Photoinduced Nickel-Catalyzed Homolytic C(sp3)-N Bond Activation of Isonitriles for Selective Carbo- and Hydro-Cyanation of Alkynes

The exploration of strong chemical bonds as synthetic handles offers new disconnection strategies for the synthesis of functionalized molecules via transition metal catalysis. However, the slow oxidative addition rate of these covalent bonds to a transition metal center hampers their synthetic utility. Here, we report a C(sp3)-N bond activation strategy that bypasses thermodynamically challenging 2e- or 1e- oxidative addition via a distinct pathway in nickel catalysis. This strategy leverages a previously unknown activation pathway of photoinduced inner-sphere charge transfer of low-valent nickel(isonitriles), triggering a C(sp3)-N bond cleavage distal to the metal-ligand interaction to deliver nickel(cyanide) and versatile alkyl radicals. Utilizing this catalytic strategy, the selective intermolecular 1,2-carbocyanation reaction of alkynes with alkyl isonitriles as both alkylating and cyanating agents can be achieved, delivering a wide array of trisubstituted alkenyl nitriles with excellent atom-economy, regio-, and stereoselectivity under mild conditions. Furthermore, Markovnikov-selective hydrocyanation of aliphatic alkynes can be accomplished through the synergistic action of a photocatalyst utilizing isonitriles as the cyanation agents. Mechanistic investigations support the photogeneration of low-valent Ni(isonitrile) complexes that undergo photochemical homolysis of the C(sp3)-N bond to engage catalytic cyanation with alkynes.