N-Heterocyclic carbenes (NHCs) catalysts have been employed as effective tools in the development of various reactions, which have made notable contributions in developing diverse reaction modes and generating significant functionalized molecules. This review provides an overview of the recent advancements in the chemo- and regioselective activation of different aldehydes using NHCs, categorized into five parts based on the different activation modes. A brief conclusion and outlook is provided to stimulate the development of novel activation modes for accessing functional molecules.
Monthly Archives: March 2024
Ethylene trimerization using half‐sandwich titanium‐based catalysts supported on mesoporous silica modified with ionic liquids
The surface of mesoporous silica (MS) was modified with imidazolium-based ionic liquids (ILs). Half-sandwich titanium-based catalysts were immobilized on the surface of IL-modified MS. Catalysts immobilized on IL-modified MS showed higher 1-hexene selectivity and activity compared with the unmodified MS. C1-IL-BF4@MS catalyst showed the highest activity of 1,199 kg 1-C6 molTi−1·h−1.
Linear alpha olefins (LAOs) are produced industrially via ethylene oligomerization using catalytic methods. The cost-effective separation process has sparked significant interest in the selective oligomerization of ethylene to produce alpha-olefins, including 1-hexene (1-C6), in multi-product commercial processes. In addition, the utilization of immobilized catalysts is crucial because of their reduced environmental impact, ease of catalyst separation, recyclability, and transportability. Furthermore, the use of immobilized catalysts simplifies the purification process, making it easier to isolate pure products. In the present study, mesoporous silica (MS) was first modified with ionic liquids (ILs) consisting BF4 − and Br− counter-anions to prepare IL-BF4@MS and IL-Br@MS, respectively. Then 12 catalysts were synthesized through immobilization of the half-sandwich catalysts with different bridges on the surface of MS, IL-Br@MS, and IL-BF4@MS and characterized by BET, TGA, and SEM–EDX-Mapping analyses. UV–Visible spectroscopy showed a tetrahedral structure for the synthesized complexes. The activity and selectivity of the catalysts for the production of 1-hexene were studied under specific conditions, including an ethylene pressure of 8 bar, a temperature of 40 °C, and an Al/Ti ratio of 1:2000. The C1-IL-BF 4 @MS immobilized catalyst with cyclohexane middle bridge immobilized on MS modified with IL-BF4 revealed the highest activity (1199 kg 1-C6 molTi −1 ·h −1 ) at a catalyst concentration of 1.5 μmol. The lowest activity (138 kg 1-C molTi −1 ·h −1 ) was obtained for both C3@MS and C4@MS catalysts.
Novel thiosemicarbazone complexes as anticancer agents: Synthesis, characterization, cytotoxicity, docking, and density functional theory studies
Synthesis of new anticancer agents that are transition metal complexes based on thiosemicarbazone derivatives.
New Co (II), Ni (II), and Cu (II) complexes based on ρ-dimethylaminobenzaldehyde thiosemicarbazone (DABT) as chelating agents were synthesized. The structure of the three complexes was determined by elemental analysis and TGA with IR, UV–Vis, XRD, ESR spectra, conductivity, and magnetic measurements. It was found that the isolated DABT complexes have tetrahedral structure with a neutral nature. The antitumor activity of DABT complexes using viability assay was examined against hepatocellular carcinoma cell lines (HepG-2). The DABT complexes have moderate activity with respect to the standard drugs. DABT–Ni (II) complex has higher antitumor activity (IC50 = 12 μM) than the other DABT complexes. In addition, a molecular docking study was performed for DABT and its complexes to investigate their therapeutic features as topoisomerase inhibitors. It is clear that the most effective compound was DABT–Ni (II) complex to bind to 3QX3. Also, density functional theory calculations with the structure–activity relationship were computed for DABT and its complexes using the Gaussian program. There was confirmation between the obtained theoretical data and the experimental studies.
Tailoring the Active Species Distribution of Heterogeneous Catalyst for Regulating the Chain Structure of Nascent Polyethylene
This review systematically summarizes and highlights recent developments on the strategy of tailoring the distribution of active species in heterogeneous catalysis, which effectively enhances the controlling of molecular weight distribution, branch distribution, as well as chain entanglements. The application of different spectroscopic methods for assisting the design of heterogeneous catalysts is also introduced.
Abstract
Heterogeneous catalysis is the most widely used process for ethylene polymerization in industry. Active site distribution is one of the crucial issues in the heterogeneous catalysis, which strongly influences the catalytic process and the microstructure of synthesized polyethylene. In this respect, aiming at an explicit understanding of the progress, the challenge and the possible future directions in the study of heterogeneous catalysis of ethylene polymerization, the recent strategies of designing heterogeneous catalysts are reviewed from the perspective of tailoring the active site distribution, where the regulation of chain structures, including molecular weight distribution, branch distribution and entanglement state, are highlighted. The in situ and operando characterizations are also reviewed for assisting the understanding of structural information in heterogeneous catalysts.
Cooperative Covalent–Noncovalent Organocatalysis of the Knoevenagel Condensation Based on an Amine and Iodonium Salt Mixture
A synergetic cocatalytic effect provided by an iodonium salt in the base-catalyzed Knoevenagel condensation has been found. The diphenyliodonium triflate serving as the halogen bond-donating Lewis acid provides the higher cocatalytic effect than zinc(II) triflate or triflic acid serving.
Abstract
The experimentally obtained kinetic data has indicated the existence of a synergetic cocatalytic effect provided by an iodonium salt in the base-catalyzed Knoevenagel condensation. The diphenyliodonium triflate serving as the halogen bond-donating Lewis acid provides the higher cocatalytic effect than zinc(II) triflate or triflic acid serving, respectively, as the metal cation-based Lewis acid and Brønsted acid. Such a cocatalytic effect remains the same for a broad scope of carbonyl compounds covering aldehydes featuring electron-withdrawing or electron-donating substituents, as well as ketone involved in the reaction.
A C3‐Symmetric Amino Organocatalyst for Asymmetric Synthesis of Warfarin and Analogues: Mechanistic Insight from ESI‐MS Spectrometry and Computational Calculations
A C3-symmetric amino–based organocatalyst can be easily prepared starting from chiral trans-N-Boc-1,2-diaminocyclohexane. Its activity on asymmetric synthesis of warfarin and analogues has been explored allowing moderate–high yields and good stereoselectivity with very low catalyst loading. Up to three active units work in the catalytic process as confirmed by offline ESI-MS experiments. In addition, DFT calculations supported the plausible reaction mechanism.
Abstract
A novel C3-symmetric multi–amino catalyst was synthesized and evaluated in the asymmetric Michael addition to produce warfarin and its analogues. The multi–amino catalyst turned out to be efficient up to 1 % molar without the use of acidic additives providing the desired product in 82 % yield and maintaining good level of enantioselectivity. ESI-MS experiments together with data supplied by theoretical models allowed us the understand the operating mechanism of the catalyst, clarifying the role of the amine functions and confirming the catalyst's multifunctionality. The presence of three active catalytic sites makes it easy its anchoring to silica as solid support for applications in heterogeneous phase catalysis.
Polymeric Carbon Nitride Nanosheets as a Metal‐Free Heterogeneous Catalyst for Highly Efficient Methanolysis of Polycarbonates
Here, we have developed an environmentally friendly and cost-effective catalyst for the alcoholysis of C−O-linked plastics. Our catalyst is made of polymeric carbon nitride nanosheets, which are free of metals. It has shown excellent performance in the methanolysis of polycarbonate (PC), polyethylene terephthalate (PET), and polylactic acid (PLA), producing pure monomer products. Through characterization and experiments, we have determined that the amino group is the main active site of the catalyst.
Abstract
Improper end-of-life treatments of C−O-linked plastics have caused serious environmental crises and resource waste. One effective method to solve this issue is the catalytic alcoholysis of these plastics. Traditional homogeneous catalysts or metal-based heterogeneous catalysts for alcoholysis have some drawbacks, such as difficulty in complete separation, environmental threats, and economic burden. Thus, finding a green, low-cost, recyclable, and highly efficient catalyst substitute is critical. Here, we reported that polymeric carbon nitride nanosheets can serve as a metal-free heterogeneous catalyst for the methanolysis of polycarbonate, polyethylene terephthalate, and polylactic acid. The catalyst exhibits remarkable performance and could be reused at least four times. NH x was found to be the predominant active site of the catalyst by characterization and experiments. These results show promise for the design of metal-free heterogeneous catalysts to depolymerize C−O-linked plastics.
Biophysical and Biochemical Characterization of Structurally Diverse Small Molecule Hits for KRAS Inhibition
Biophysical and biochemical characterization of six compounds is presented. The target biding and inhibitory profiles described here suggest that these compounds can serves as useful starting points for the development of allosteric, non-covalent inhibitors of KRAS for cancer therapy.
Abstract
We describe six compounds as early hits for the development of direct inhibitors of KRAS, an important anticancer drug target. We show that these compounds bind to KRAS with affinities in the low micromolar range and exert different effects on its interactions with binding partners. Some of the compounds exhibit selective binding to the activated form of KRAS and inhibit signal transduction through both the MAPK or the phosphatidylinositide 3-kinase PI3K-protein kinase B (AKT) pathway in cells expressing mutant KRAS. Most inhibit intrinsic and/or SOS-mediated KRAS activation while others inhibit RAS-effector interaction. We propose these compounds as starting points for the development of non-covalent allosteric KRAS inhibitors.
Frustrated Lewis Pair‐Type Reactivity of Intermolecular Rare‐Earth Aryloxide and N‐Heterocyclic Carbene/Olefin Combinations
This work reports the cooperative reactivity of rare-earth aryloxide complexes with N-heterocyclic carbene (NHC) or N-heterocyclic olefin (NHO), showcasing their synergistic effect in the activation of H2 and diverse organic substrates. Reactions of RE(OAr)3 (RE = La, Sm, and Y; Ar = 2,6-tBu2-C6H3) with unsaturated NHC ItBu (:C[N(R)CH]2, R = tBu) isolate abnormally bound RE metal NHC complexes RE/aNHC. In contrast, no metal-NHO adducts were formed when RE(OAr)3 were treated with NHO (R2C=C[N(R)C(R)]2, R = CH3). Both RE/aNHC and RE/NHO Lewis pairs enabled cooperative H2 activation. Furthermore, RE(OAr)3 were found to catalyze the hydrogenation of exocyclic C=C double bond of NHO under mild conditions. Moreover, treatment of the La/aNHC complex with benzaldehyde produced a La/C4 1,2-addition product. The La/NHO Lewis pair could react with (trimethylsilyl)diazomethane and a, b-conjugated imine, affording an isocyanotrimethylsilyl lanthanum amide complex and a La/C 1,4-addition product, respectively.
Marigold Like Structure from Methionine Mediated Growth of Positively Charged Gold Nanorod
Marigold flower like supra structure is fabricated by growth reaction of gold nanorod incubated with methionine with two binding site, which selectively interacts with intermediate Au+ and Au(111) facets during nucleation and growth stage and favors the assembly and merging of nanoparticles. This observation enlightens the role of biomolecules or small molecules behind growth of novel nano-architectures.
Abstract
During morphological evolution of gold nanoparticles, amino acids play a vital role in tuning shape, introducing chirality and inducing facet selective reactivity. Herein, we report the synthesis of unique marigold like structure (MGS) via growth reaction of methionine (Met) incubated positively charged anisotropic gold nanorod (GNR). Varying three important parameters such as growth time, concentration of Met and Au3+ reveals the combination of freshly generated small nucleated particles (fNPs) and GNR towards fabricating the unique MGS containing disk and ray floret parts. Strong interaction between Met and (111) plane of Au0 controls the orientation of (111) plane parallel to the direction of growth. This preferential interaction directs the assembly of gold nanostructures through Au (200) plane and results in merging of fNPs with concave GNR (cGNR) to fabricate the external arrangement of ray floret structure. The structural selectivity is attributed to the electron donating capacity of thioether functional group of Met(S) to Au+, generated prior to secondary nucleation. As confirmed by XPS and ζ-potential analysis, the above interaction controls the Met concentration dependent inhibition of further Au+→Au0 reduction. The growth strategy of GNR has been further validated with a Met enriched peptide to produce disk and ray florets.