Ullmann Coupling Reaction Research Articles

Highly dispersed palladium nanoparticles supported on graphitic carbon nitride for selective hydrogenation of nitro compounds and Ullmann coupling reaction

High catalytic activity and superior stability are important criterion for the utilization of a noble metal‐based nano‐composite catalyst. Herein, we report a facile synthesis of highly dispersed palladium nanoparticles embedded on graphitic carbon nitride (Pd@CNx) by ultrasound assisted method without aid of any external reducing agent. The Pd@CNx composite was well characterized by different techniques. This Pd@CNx shows effective catalytic activity towards easy dehydrogenation of sodium borohydride (NaBH4) and ammonia borane in aqueous medium and also can easily hydrogenate 4‐nitrophenol to 4‐aminophenol with fast kinetics. The catalyst is highly selective towards nitro groups in presence of other reducible groups with good stability. Furthermore, the catalyst exhibits superior catalytic activity towards Ullmann coupling reaction of different aryl halides with broad range of substrates scope with easy recovery of the catalyst and notable recycling property.

Silk−Fibroin‐Supported Palladium Catalyst for Suzuki‐Miyaura and Ullmann Coupling Reactions of Aryl Chlorides

AbstractRecently, we have reported the preparation of a silk fibroin‐supported Palladium catalyst (Pd/SF) and its use in the Suzuki‐Miyaura cross‐coupling of aryl iodides. Since its synthetic applicability and structural features are still far from being fully explored, a deeper investigation is reported here. Pd/SF is tested as a catalyst for the Suzuki‐Miyaura and Ullmann coupling reactions of aryl chlorides, in H2O/EtOH solvent mixture under atmospheric conditions. Starting from the analysis of the products of the Pd/SF promoted Ullmann reactions of poly‐haloarenes, we can hypothesize the existence of catalytic pockets where monoatomic palladium species can form stable complexes with SF. To shed light on this hypothesis, Pd/SF is characterized by Wide‐Angle X‐ray Scattering (WAXS) analysis, which supports the supposition of catalytic pockets. Finally, using a computational model developed by Molecular Mechanic Energy Minimization we have estimated the dimension of the catalytic pocket of Pd/SF (about 15 Å), in good agreement with the experimental results.

A covalent organic framework with electrodeposited copper nanoparticles – a desirable catalyst for the Ullmann coupling reaction

Electrochemical deposition from an electrolyte containing Cu2+ loads Cu(0) nanoparticles (3.34 wt%) into a COF, making them resemble a single-site catalyst. This Cu@COF serves as an excellent heterogeneous catalyst for multi-fold Ullmann coupling reactions.

Interfacial electric fields catalyze Ullmann coupling reactions on gold surfaces

The electric fields created at solid-liquid interfaces are important in heterogeneous catalysis. Here we describe the Ullmann coupling of aryl iodides on rough gold surfaces, which we monitor in situ using the scanning tunneling microscope-based break junction (STM-BJ) and ex situ using mass spectrometry and fluorescence spectroscopy. We find that this Ullmann coupling reaction occurs only on rough gold surfaces in polar solvents, the latter of which implicates interfacial electric fields. These experimental observations are supported by density functional theory calculations that elucidate the roles of surface roughness and local electric fields on the reaction. More broadly, this touchstone study offers a facile method to access and probe in real time an increasingly prominent yet incompletely understood mode of catalysis.

Fabrication of Metal Nanoparticle Composites by Slow Chemical Reduction of Metal-Organic Frameworks.

Constructing metal nanoparticle (MNP) composites from metal-organic framework (MOF) precursors has attracted extensive attention as the MOF precursors provide an excellent porous matrix for the generation of MNP composites, which enables the direct fabrication of well-dispersed MNP composites. In this work, a novel strategy is proposed to fabricate MNP composites by slow chemical reduction (SCR) of MOF precursors at room temperature. The reduction process is skillfully slowed via using ethanol as the solvent, and the formation of MNP composites is then realized by the SCR process. Briefly, BH4- slowly diffuses into an MOF precursor and in situ reduces metal ions to well-dispersed nanoscale MNP composites. Meanwhile, this SCR process breaks the coordination bonds from MOF precursors, leading to the generation of porous structures for the resulting composites. Interestingly, the composites inherit the morphology of MOF precursors well. Besides, this SCR strategy allows construction of MNP composites from different types of MOF precursors. The resulting Cu@HK-3 composites possess well-dispersed nanoscale Cu NPs and a porous architecture, which exhibit superior catalytic performance and stability in the Ullmann coupling reaction. This strategy provides a feasible, convenient, and energy-saving route to prepare MNP composites from MOF precursors with customizable morphology and well-dispersed MNPs.

Carbon–Heteroatom Bond Formation via Coupling Reactions Performed on a Magnetic Nanoparticle Bed

Cross-coupling reactions leading to carbon–heteroatom bonds yield compounds that attract substantial interest due to their role as structural units in many synthetic protocols for bioactive and natural products. Therefore, many research works aim at the improvement of heterogeneous catalytic protocols. We have studied the use of magnetite nanoparticles and solid base compounds in organic synthetic reactions in carbon–heteroatom bond formation because they can be flocculated and dispersed, and reversibly controlled by applying a magnetic field. In this work, we have developed an efficient and simple synthetic approach for the C–O/C–N cross-coupling reaction under ligand-free conditions by using CuI as a catalyst and KF/Fe3O4 as a base. We performed the nucleophilic aromatic substitution of electron-deficient aryl halides and phenols. It was found that both the solvent nature and the base have a profound influence on the reaction process. This approach affords good to excellent yields of arylated products. KF/Fe3O4 displayed convenient magnetic properties and could be easily separated from the reaction using a magnet and recycled several times without significant loss of catalytic activity. This method has been successfully investigated for the Ullmann coupling reaction.

Green Synthesis of Cu Nanoparticles in Modulating the Reactivity of Amine-Functionalized Composite Materials towards Cross-Coupling Reactions.

Control over both dispersion and the particle size distribution of supported metal particles is of paramount importance for the catalytic activity of composite materials. We describe the synthesis of materials with Cu nanoparticles well-dispersed on different amine-functionalized supports, using the extract of Wallich Spurge as a green, reducing agent. Graphene oxide (GO), mesoporous silica (MCM-41), mesoporous zirconia, and reduced graphene oxide-mesoporous silica (RGO-MCM-41) were explored as supports. Cu nanoparticles were better stabilized on RGO-MCM-41 compared to other supports. The novel composite materials were characterized by X-ray diffraction (XRD), Raman spectra, Scanning electron microscope (SEM), Transmission electron microscopy analysis and HR-TEM. SEM and EDX techniques. High angle XRD confirmed the conversion of graphene oxide to reduced graphene oxide (RGO) with plant extract as a reducing agent. Both XRD and TEM techniques confirmed the Cu nanoparticle formation. The catalytic activity of all the prepared materials for the Ullmann coupling reactions of carbon-, oxygen-, and nitrogen-containing nucleophiles with iodobenzene was evaluated. From the results, 5 wt% Cu on amine-functionalized reduced graphene oxide/mesoporous silica nanocomposite (5 wt%Cu(0)-AAPTMS@RGO-MCM-41) exhibited excellent efficiency with 97% yield of the C-C coupling product in water at 80 °C in 5 h. The activity remained unaltered almost up to the fourth cycle. The Cu nanoparticles stabilized by organic amine group on RGO hybrid facilitated sustained activity.

Construction of a ratiometric phosgene probe by chromophore formation from auxochrome

A new fluorescent probe for ratiometric detection of phosgene is reported. This probe was constructed with classic 1,8-naphthalimide and 2-(2-aminophenyl)benzimidazole by Ullmann coupling reaction. After exposure to phosgene, yellow fluorescence weakened while blue fluorescence enhanced significantly. There was a ratiometric response between 542 nm and 490 nm. The detection limit (LOD) was 6.7 nM and the response time was within 200 s in CHCl3. Meaningfully, a new chromophore, benzo [4,5]imidazo[1,2-c]quinazolin-6(5H)-one, was formed after 2-(2-aminophenyl)benzimidazole unit reacted with phosgene, and the ratiometric response was achieved by two chromophores in which the mechanism was confirmed by 1H NMR spectra, HRMS and theoretical calculation. Furthermore, test papers and nanofibers were fabricated with the probe, which could sensitive detection of phosgene within 10 min and 1 min respectively.

Sustainable strategies of C–N bond formation via Ullmann coupling employing earth abundant copper catalyst

N-arylation reaction has become one of the key strategies in organic synthesis to access several molecules of biological, agrochemical, and medicinal importance. The century-old Ullmann reaction, for the synthesis of C–N has been realized as one of the foremost transition metal catalyzed coupling reaction. However, the limitation associated with the methodology has led to the evolution of several other synthetic strategies for the construction of C–N bonds. However, with the maturity of the copper chemistry, and the realization to look beyond palladium catalysis has led to the revival of the interest in the so-called Ullmann reaction. The need for a cheaper and sustainable catalytic system which makes use of the earth-abundant copper to catalyze the reaction has invigorated from its dormancy. Several research groups for past few years are incessantly working to develop more efficient and sustainable methodologies to address the pertinent issues of poor functional group tolerance, harsh reaction conditions prolonged reaction times associated with the Ullmann reaction and developed several sustainable strategies for the same. Therefore, this review focuses on the summary of the evolution of the Ullmann coupling reaction using earth abundant copper catalyst for the construction of C–N bond under sustainable conditions and provides an outline of the reaction strategies and mechanisms. Despite all these breakthroughs, there is still a demand for more efficient catalytic systems, more benign reaction conditions, as well as overcoming the challenges associated with the sterically and electronically demanding and substrates. This review will therefore inspire the future researchers to establish sustainable methodologies which can have applications on a large industrial scale.

Amination of ether-linked polymers via the application of Ullmann-coupling reaction: synthesis, characterization, porosity, and thermal stability evaluation

In this report, an open-chain ether-linked polymer (PE-AF) has been prepared via one-step co-condensation polymerization of 1,4-dibromo-2,5-difluorobenzene with bisphenol AF. The new polymer has shown a good solubility in non-polar solvents, good thermal stability (up to 300 °C), high char residue with a limiting oxygen index (LOI) of 33.5, surface morphology with random fluffy-like texture, and a non-porosity nature with a surface area (SA) of 35 m2g−1. Tailoring these properties has been achieved by considering post-modification on the Bromo-sites of the polymer backbone. The post-modification was attained by applying Ullmann-coupling reaction of the Bromo-sites of PE-AF and aromatic diamines in presence of a catalytic amount of copper iodide. Upon inclusion of the aromatic diamines within the polymer’s backbone, the new polymers have depicted aggregated surface morphology (e.g. spherical particles ca. 0.3–0.1 µm), pronounced porosity nature (SA up to 354 m2g−1), varied range of pore size distribution, and higher thermal stability with lower char residue contents when compared with PE-AF. The successful production of the polymers was confirmed by (C, H, O, and N) elemental analysis, solid-state 13C-NMR, and infrared (IR) spectroscopy. The application of Ullmann-coupling raises the opportunity to create cavities and pores within the polymer’s framework. Though this modification lowered the LOI of the polymers and their flame-retardant tendency, it enhanced their opportunity for gas capture and separation applications.

Synthesis, characterization, molecular structure, and computational studies on 4(1H)-pyran-4-one and its derivatives

4(1H)-Pyridone-based compounds have shown promise as potent bioactive inhibitors against a broad range of diseases, particularly malaria. Our interest in 4(1H)-pyridones initiated the design and synthesis of a series of 4(1H)-pyridone derivatives, with the hope of finding new antimalarial leads. The synthesis of 4(1H)-pyran-4-one analogues (4(1H)-pyridone intermediates was successfully achieved using palladium catalyzed Suzuki-Miyaura coupling reactions. The synthesized 4(1H)-pyran-4-ones were unambiguously confirmed by TGA, 13C, 1H NMR, and IR spectroscopic analysis. Further comprehensive insights to rationalize the outcome of several reactions were deduced by means of single crystal X-ray diffraction, which was used to determine the single crystal structures of compounds 4 and 5. Compounds 5, 9 and 9a-e were successfully synthesized in moderate to excellent yields, apart from compound 9e which gave a very low yield. NMR and XRD analysis of the by-product of compound 9e showed that the boronic acid substituents reacted with each other in a side-reaction reminiscent to aryl boronic acid homo coupling or Ullmann coupling reactions. Complimentary DFT analysis of the pyranone and pyridone compounds provided insight into the substituent effect of the pyranone compounds with regards to the stability and lack of reactivity to form the corresponding pyridone compounds.

An Efficient and Short Total Synthesis of (−)‐Heliannuol A by Intramolecular Ullmann C−O Coupling for the Construction of an Eight‐Membered Ether Ring

AbstractHerein, we report a method for the synthesis of eight‐membered cyclic ethers by a direct C−O bond connection using an intramolecular Ullmann coupling reaction based on our previously published methodology. We found that allelopathically active heliannuol A could be rapidly synthesised using this method. The use of a strongly coordinating 4‐pyrrolidinopyridine ligand accelerates the reaction rate and leads to the C−O coupling reaction of a typically inaccessible sterically bulky tertiary alcohol. Furthermore, this method circumvents the protection and deprotection steps, presenting a new strategy for the syntheses of benzannulated eight‐membered ring structures.

Post-functionalization of Ether-linked Polymer via the Application of Ullmann-coupling Reaction: Synthesis, Characterization and Thermal Degradation Kinetics

A new ether-linked polymer (PE-A) was synthesized via the polycondensation of 1,4-dibromo-2,5-difluorobenzene with biphenol (A). The new polymer has shown a good solubility in non-polar solvents, as well as moderate thermal stability (up to 300 °C). The parent PE-A was subjected to post-functionalization modification applying the Ullmann-coupling reaction on the C-Br bonds of PE-A, where the bromo-sites have been replaced by aniline-linked moieties. The resulting polymer (PE-Ani-A) shows slight solubility in several solvents. Its thermal stability was enhanced by 36% when compared with PE-A. To ensure the significant impact of the inclusion of the aniline group on thermal stability, a thermal degradation kinetics study was performed and the Coats-Redfren and Broido theoretical models were applied to explore the degradation process. The calculated activation energy for PE-Ani-A degradation was thereby found to be higher than that of PE-A, which indicates the higher thermal stability of PE-Ani-A. For further insights into the thermal stability of the polymers, the limited oxygen index (LOI), which represents the flame-retardant property of the polymers, was calculated. The results indicate that PE-A is a promising candidate as a flame-retardant polymer (LOI = 46.7), since it has a high bromo-content. On the other hand, PE-Ani-A has a lower LOI although it has a higher thermal stability.

Preparation and characterization of Cu based on 5,5'-bistetrazole as a recyclable metal-organic framework and application in synthesis of diaryl ether by the Ullmann coupling reaction

Cu based on 5,5′-bistetrazole ([Cu2BT · 2H2O] n ) as a recyclable metal-organic framework (MOF) heterogeneous catalyst was characterized by FT-IR, 13C NMR, XRD, SEM, EDX, BET, BJH, and ICP-AES analysis. The catalytic activity of the catalyst was probed through the Ullmann reaction for synthesis of diaryl ether derivatives from two component reactions of aromatic arylhalides and phenol derivatives in DMSO. Simple procedure, high yields, short reaction time, and environmentally benign methods are advantages of this protocol. The catalyst was readily separated by simple filtration and reusable without significant loss of its catalytic efficiency.

Visualizing and Understanding Ordered Surface Phases during the Ullmann Coupling Reaction

The more than century old copper metal-catalyzed Ullmann coupling reaction of aryl halides has seen renewed interest from the nanoscience community as a means to perform on-surface C–C coupling bas...

Ullmann Coupling Reaction of Bicyclic Amidines DBU/DBN with Aryl Halides: A Pathway to the Synthesis of ϵ‐Caprolactam Derivatives

AbstractThis paper demonstrates a selective, mild approach to Ullmann amination of aryl halides to synthesizeN‐alkylated derivatives of ϵ‐caprolactam. The synthetic route involves anin‐situring‐opening of 1,8‐diazabicyclo[5.4.0]undec‐8‐ene (DBU) followed by concurrent arylation with aryl halides in the presence of copper iodide as a catalyst under ligand‐free conditions. This method provides a new entry to a wide variety of ϵ‐caprolactam derivatives in good to excellent yields in a single synthetic sequence. Similarly, other bicyclic amidines such as 1, 5‐diazabicyclo‐[4.3.0]non‐5‐ene (DBN), and 1,5,7 triazabicyclo[4.4.0] dec‐5‐ene (TBD) also showed good to very high reactivity.Azepan‐2‐one, or Caprolactam, is an important synthon in polymer chemistry and has a global demand as it is employed to make Nylon 6 filament, fiber, and plastics.[1]The global caprolactam market size is expected to expand for the growing textile industry with rising demand for plastics in the construction of automotive, electrical, and electronic sectors. Additionally, technological advancements aimed at improving the cost‐effective manufacturing process of caprolactam, to minimize the release of hazardous waste into the environment is of vital necessity. Derivatives of ϵ‐caprolactam are of interest for the production of modified nylons[2]and nanogels.[3]Several Azepinones and their analogs play an important role in medicinal chemistry,[4]used in the synthesis of pharmaceutical drugs including Benazepril,[5]Telcagepant,[6]and Ivabradine.[7]

Synthesis and Characterization of a Carborane-containing Precursor for B<sub>4</sub>C Ceramics

B₄C ceramic is one of widely used ceramics with properties of low density, high hardness, high thermal-stability and high neutron absorption cross-section. However, the high melting point and high hardness of B₄C make it difficult to be prepared as materials with complex shape. Precursor derived method is an effective approach to prepare special-shaped ceramics. In the early research of B₄C ceramic precursors, highly toxic and unstable decaborane was generally used as the boron source material, and precursors were also unstable and difficult to synthesize. In this work, a new B₄C precursor is designed and synthesized, and m-carborane, which has stable chemical properties and high thermal stability, is used as the boron source. Firstly, m-carborane is converted into lithium salt after removing active hydrogen by n-butyllithium. Then, catalyzed by Cu(I), the lithium salt reacts with m-chloroiodobenzene so that monomer, bis(3-chlorophenyl)-m-carborane, is prepared. The monomer is polymerized through Ullmann Coupling reaction at the present of Ni(0) catalyst and Zinc powder, and precursor poly[bis(3-chlorophenyl)-m-carborane] is finally synthesized. The precursor is stable at room temperature in air and soluble in Tetrahydrofuran, with Mw of 1013g/mol, and ceramic yield of 62.20% at 900°C in N₂. The precursor pyrolytic product is composed of B₄C and graphite after heat treated at 1600°C in inert atmosphere.

Transformation of a graphene nanoribbon into a hybrid 1D nanoobject with alternating double chains and polycyclic regions.

Molecular dynamics simulations show that a graphene nanoribbon with alternating regions which are one and three hexagons wide can transform into a hybrid 1D nanoobject with alternating double chains and polycyclic regions under electron irradiation in HRTEM. A scheme of synthesis of such a nanoribbon using Ullmann coupling and dehydrogenation reactions is proposed. The reactive REBO-1990EVC potential is adapted for simulations of carbon-hydrogen systems and is used in combination with the CompuTEM algorithm for modeling of electron irradiation effects. The atomistic mechanism of formation of the new hybrid 1D nanoobject is found to be the following. Firstly hydrogen is removed by electron impacts. Then spontaneous breaking of bonds between carbon atoms leads to the decomposition of narrow regions of the graphene nanoribbon into double chains. Simultaneously, thermally activated growth of polycyclic regions occurs. Density functional theory calculations give barriers along the growth path of polycyclic regions consistent with this mechanism. The electronic properties of the new 1D nanoobject are shown to be strongly affected by the edge magnetism and make this nanostructure promising for nanoelectronic and spintronic applications. The synthesis of the 1D nanoobject proposed here can be considered as an example of the general three-stage strategy of production of nanoobjects and macromolecules: (1) precursors are synthesized using a traditional chemical method, (2) precursors are placed in HRTEM with the electron energy that is sufficient only to remove hydrogen atoms, and (3) as a result of hydrogen removal, the precursors become unstable or metastable and transform into new nanoobjects or macromolecules.

Manifold dynamic non-covalent interactions for steering molecular assembly and cyclization.

Deciphering rich non-covalent interactions that govern many chemical and biological processes is crucial for the design of drugs and controlling molecular assemblies and their chemical transformations. However, real-space characterization of these weak interactions in complex molecular architectures at the single bond level has been a longstanding challenge. Here, we employed bond-resolved scanning probe microscopy combined with an exhaustive structural search algorithm and quantum chemistry calculations to elucidate multiple non-covalent interactions that control the cohesive molecular clustering of well-designed precursor molecules and their chemical reactions. The presence of two flexible bromo-triphenyl moieties in the precursor leads to the assembly of distinct non-planar dimer and trimer clusters by manifold non-covalent interactions, including hydrogen bonding, halogen bonding, C–H⋯π and lone pair⋯π interactions. The dynamic nature of weak interactions allows for transforming dimers into energetically more favourable trimers as molecular density increases. The formation of trimers also facilitates thermally-triggered intermolecular Ullmann coupling reactions, while the disassembly of dimers favours intramolecular cyclization, as evidenced by bond-resolved imaging of metalorganic intermediates and final products. The richness of manifold non-covalent interactions offers unprecedented opportunities for controlling the assembly of complex molecular architectures and steering on-surface synthesis of quantum nanostructures.

Triskelion‐Shaped π‐Luminophores Bearing Coumarin: Syntheses, Structures, and Luminescence Properties

AbstractTriskelion‐shaped π‐luminophores bearing three coumarin units 1 a and 1 b were successfully synthesized by the intramolecular Ullmann coupling reaction of triphenyl benzene‐1,3,5‐tricarboxylate derivatives. X‐ray crystal structure analysis revealed that compound 1 adopts a unique curved and three‐blade propeller‐shaped structure. These crystals are a racemic mixture consisting of PPP‐ and MMM‐1 due to the helicity resulting from the steric hindrance. A computational study suggests that this helical inversion process between PPP‐ and MMM‐1 a occurs with comparative ease. The emission spectra of 1 a and 1 b in THF solution showed sharp emission bands at 454 nm and 492 nm, respectively. The Stokes shifts were relatively large at 7385 cm−1 and 6556 cm−1 due to their degenerate frontier molecular orbitals. A solution of 1 b in different organic solvents demonstrated solvatofluorochromism due to intramolecular charge transfer of the coumarin unit in the excited states. Interestingly, both triskelion‐shaped molecules formed aggregates in a mixture of THF‐H2O and exhibited aggregation‐induced emission enhancement characteristics.