Seven- and eight-membered heterocyclic biaryl synthesis through a metal-free oxidative coupling reaction

A metal-free intramolecular biaryl coupling mediated by DDQ and BF₃·OEt₂ was developed. The reaction proceeds smoothly at room temperature to give seven- and eight-membered heterocyclic biaryl compounds in up to 95% yields. Only 1 equiv of DDQ sufficed for the reaction, obviating the use of a large excess of oxidants and undesired chlorination pathway in metal-mediated process.     

Formation of bimetallic Ag-Pd nanoclusters via the reaction between Ag nanoclusters and Pd2+ ions

We have investigated systematically the mechanistic aspects of the Ag-Pd bimetallic cluster formation within sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles by using in-situ X-ray absorption spectroscopy (XAS). A two-step sequential reduction method is employed for the synthesis of Ag-Pd bimetallic clusters. The first step involves preparation of Ag nanoclusters, by mixing the Ag+ ions containing the AOT microemulsion system with a reducing agent hydrazine (N2H4) containing the AOT microemulsion system. In the second step, the addition of Pd2+ ions to Ag nanoclusters led to the formation of Ag-Pd bimetallic clusters via the reaction between Ag nanoclusters and Pd2+ ions in AOT reverse micelles. The reduction of silver ions and the formation of corresponding Ag nanoclusters are monitored as a function of the dosage of the reducing agent, hydrazine. In-situ XAS allowed probing of the reaction between Ag nanoclusters and Pd2+ ions during the formation of Ag-Pd bimetallic clusters. Analysis of Ag and Pd K-edge XAS spectra reveals that in the final stage Ag-Pd clusters, in which "Ag" atoms prefer to be surrounded by "Pd" and "Pd" atoms prefer to be surrounded by "Pd", were formed. On the basis of XAS results presented here, we are able to propose a structural model for each step so that this work provides a detailed insight into the mechanism of nucleation and growth of Ag-Pd bimetallic clusters. We also discussed the atomic distribution of Ag and Pd atoms in Ag-Pd bimetallic clusters based on the calculated XAS structural parameters.     

Highly fluorescent Ag nanoclusters: microwave-assisted green synthesis and Cr3+ sensing

Highly fluorescent Ag nanoclusters were prepared in aqueous solution via a rapid microwave-assisted green approach and used as a novel fluorescence probe for the determination of Cr(3+) ions with high sensitivity and excellent selectivity.     

Pyridine elaboration through organometallic intermediates: regiochemical control and completeness

Pyridines carrying heterosubstituents (such as carboxy, amido, amino, alkoxy or trifluoromethyl groups or solely individual halogen atoms) can be readily and site selectively metalated. Subsequent reaction with a suitable electrophile opens rational access to a wealth of new building blocks for the synthesis of biologically active compounds. This approach relies on organometallic methods, which are both efficacious and extremely flexible as far as the substitution site and the product structure are concerned (86 references).     

Enantioselective synthesis of binaphthyl polymers using chiral asymmetric phenolic coupling catalysts: oxidative coupling and tandem glaser/oxidative coupling

A series of functionalized and optically active polybinaphthyls have been synthesized from achiral substrates by asymmetric oxidative phenolic coupling using a chiral 1,5-diaza-cis-decalin copper catalyst. In most cases, a copper tetrafluoroborate catalyst was found to be superior to the copper iodide catalyst, as ortho-iodination of the substrates could be prevented. Three methods for the formation of chiral polymers are described. In the first method, two 2-naphthols linked together at C-6 are subjected to the optimized asymmetric oxidative phenolic coupling conditions to form chiral polynaphthyls. A combination of NMR and HPLC measurements secured the selectivity of the asymmetric coupling. In the second method, substrates containing only one naphthalene were utilized. By incorporating a 2-naphthol and a terminal alkyne, the chiral copper catalysts effect both Glaser-Hay coupling of the alkyne and oxidative asymmetric coupling of the 2-naphthol with remarkable chemoselectivity. The relative reaction rates of various moieties with the chiral catalysts follows the order: benzyl cyanides > aryl alkynes > electron-rich 2-naphthols > electron-deficient 2-naphthols > alkyl alkynes. Because of high chemoselectivity, this approach is useful for the organized assembly of multifunctional substrates in a single operation. In all cases, no cross-coupling is observed between the alkyne and the 2-naphthol. This approach was thus applied to a set of highly functionalized precursors. In this third case, the biaryl coupling was performed first and a Glaser-Hay coupling was performed in a separate step to generate a highly functionalized polymer. In some cases, the resultant chiral polymers exhibit very large optical rotations.     

Iodine-catalyzed tandem oxidative coupling reaction: A one-pot strategy for the synthesis of new coumarin-fused pyrroles

The simple and facile strategy for the synthesis of 2,3-disubstituted-chromeno[4,3-b]pyrrole-4(1H)-ones has been established. This method describes the Kornblum oxidation reaction of acetophenones, followed by the Knoevenagle treatment of the resulted (het)arylglyoxals with active methylene compounds and consequently iodine-activated Michael type reaction with 4-amino coumarin in a one-pot manner to afford disubstituted chromeno[4,3-b]pyrrole-4(1H)-one derivatives.     

Palladium-catalyzed oxidative Heck coupling reaction for direct synthesis of 4-arylcoumarins using coumarins and arylboronic acids

An efficient protocol for the direct synthesis of 4-arylcoumarins via palladium-catalyzed oxidative Heck coupling reaction of coumarins and arylboronic acids was developed. 4-Arylcoumarins were obtained in moderate to excellent yields, and the reaction also showed tolerance toward functional groups such as hydro, methoxy, diethylamino, nitro, and chloro groups.     

Intramolecular oxidative C-H coupling for medium-ring synthesis

An oxidative C-H coupling is described for medium-ring synthesis.     

Single-molecule resolution of an organometallic intermediate in a surface-supported Ullmann coupling reaction

We have studied the organometallic intermediate of a surface-supported Ullmann coupling reaction from 4, 4″-dibromo-p-terphenyl to poly(para-phenylene) by scanning tunneling microscopy/spectroscopy and density functional theory calculations. Our study reveals at a single-molecular level that the intermediate consists of biradical terphenyl (ph)(3) units that are connected by single Cu atoms through C-Cu-C bridges. Upon further increasing the temperature, the neighboring biradical (ph)(3) units are coupled by C-C bonds forming poly(para-phenylene) oligomers while the Cu atoms are released.     

Facile synthesis of palladium nanoclusters and their catalytic activity in Sonogashira coupling reactions

This work reports a facile synthesis of palladium nanoclusters (PdNCs) in MeCN/MeOH mixture without any stabilizer. The PdNCs were found to be effective catalysts for copper-free, amine-free and ligand-free Songashira coupling reactions under ambient conditions.     

Visible light-induced oxidative coupling reaction: easy access to Mannich-type products

An efficient methodology for the functionalization of sp(3) C-H bond adjacent to nitrogen has been developed utilizing visible light-induced photoredox catalysis. Through optimization of solvent and light source, the reaction can be rapidly achieved to provide the desired product under mild reaction conditions.     

Electrochemically time-dependent oxidative coupling/coupling-cyclization reaction between heterocycles: tunable synthesis of polycyclic indole derivatives with fluorescence properties

Science China Chemistry - A mild and practical protocol for selectively time-dependent dehydrogenative C-C coupling, as well as tandem coupling-cyclization reaction between indoles or/and other...     

Preparation on oligostilbenes of isorhapontigenin by oxidative coupling reaction

Four new compounds 1-4 were obtained from an oxidative coupling reaction of (E)-isorhapontigenin using FeCl(3) as oxidant. Their structures and stereochemistry were determined on the basis of spectroscopic evidence [UV, IR, MS, (1)H-, (13)C-NMR, NOE and 2D NMR], and their possible formation mechanisms were also discussed, respectively.     

Room-temperature oxidative Suzuki coupling reaction of 1,2,3-triazole N-oxides

This study develops a new efficient pathway for synthesis of 2,4-disubstituted 1,2,3-triazoles through regioselective direct arylation between 2-aryl-1,2,3-triazole N-oxides and Ar-B(OH)₂. The reaction proceeds smoothly at room temperature and exhibits good yield and high C5 position selectivity. The possible pathway of oxidative Suzuki coupling is also discussed. This simple methodology can be used to construct 2,4-disubstituted 1,2,3-triazole moiety.     

X-ray and UV photoelectron spectroscopy of Ag nanoclusters

The main purpose of the present work is to analyze a series of Ag nanoparticles (NPs) with different size or ligand functionalization by using X-ray photoelectron spectroscopy (XPS) and to identify the differences in the band-shape and energy peak position of photoemission spectra due to the particle dimension. A transmission electron microscopy characterization was performed, to verify the consistency of the results. Three types of samples were prepared starting from AgNO₃ water solution and adding different capping agents. In the first two cases, the formation of NPs was promoted by the reduction of silver ions Ag⁺¹ to metallic Ag⁰ through the addition of sodium borohydride, whereas in the last case, it was triggered by the exposure to UV light. Depending on the size of the NPs, a different physical behavior can be recognized. NPs with diameter of about 5 nm are characterized by the phenomenon of localized surface plasmon resonance (LSPR). The other type of samples having a diameter of about 1.5 nm presents discrete energy levels instead of electronic bands, and in this case, a typical fluorescence phenomenon can be observed. In the latter case, we can refer to such systems as nanoclusters. The XPS analyses were focused on the Ag 3D spectra looking for the possible shifts of the Ag doublet as a function of the particles size. The ultraviolet photoelectron spectroscopy with He II source was used for the investigation of possible changes in the valence band.     

One-pot synthesis of carbazoles by palladium-catalyzed N-arylation and oxidative coupling

One-pot N-arylation and oxidative coupling can be promoted by a common palladium catalyst in the presence of appropriate additives: palladium-catalyzed N-arylation of anilines with aryl triflates under the standard conditions followed by addition of acetic acid under oxygen or air atmosphere afforded various types of functionalized carbazoles in good to excellent yields.     

New atomically precise M1Ag21 (M = Au/Ag) nanoclusters as excellent oxygen reduction reaction catalysts

By introducing 1,1′-bis-(diphenylphosphino)ferrocene (dppf) as an activating ligand, two novel nanoclusters, M1Ag21 (M = Au/Ag), have been controllably synthesized and structurally characterized. The atomically precise structures of the M1Ag21 nanoclusters were determined by SCXC and further confirmed by ESI-TOF-MS, TGA, XPS, DPV, and FT-IR measurements. The M1Ag21 nanoclusters supported on activated carbon (C) are exploited as efficient oxygen reduction reaction (ORR) catalysts in alkaline solutions. Density functional theory (DFT) calculations verify that the catalytic activities of the two cluster-based systems originate from the significant ensemble synergy effect between the M₁₃ kernel and dppf ligand in M1Ag21. This work sheds lights on the preparation of cluster-based electrocatalysts and other catalysts that are activated and modified by peripheral ligands.

The presence of 1,1′-bis-(diphenylphosphino)ferrocene ligands and ensemble effects in novel nanoclusters M₁Ag₂₁(dppf)₃(SAdm)₁₂ (M = Au/Ag) provide excellent ORR performances.     

Exploiting the oxidative coupling reaction of MBTH for indapamide determination

The oxidative coupling reaction between aromatic amines and 3-methylbenzothiazolin-2-one hydrazone (MBTH) in the presence of cerium(IV) has been extensively used with quantitative analytical purposes. Nevertheless, a literature survey reveals that different wavelengths (visible range) can be used to monitor the reaction products when using MBTH and Ce(IV) as colour developing reagents.In the present work, the oxidative coupling reaction of indapamide (an oral antihypertensive diuretic drug) with MBTH in the presence of cerium(IV) was evaluated using distinct reaction approaches and was implemented in an automated multipumping flow system. The developed methodology was applied in the spectrophotometric control of the drug in pharmaceutical formulations. The optimization of the proposed multipumping flow system was performed by using an experimental design approach, namely by exploiting a Plackett-Burman factorial design and a central cubic faces design.This work revealed the formation of products with different reaction kinetics, chemical stabilities and absorbance spectra, depending on the sequence of addition of the reagents. By exploiting a specific sequence in the addition of reagents, the proposed automatic system allowed the rapid quantification of indapamide in pharmaceutical formulations, with a determination rate of about 25 h⁻¹, and a relative deviation under 2.1% when comparing with the reference procedure. Detection limit was approximately 1 mg L⁻¹.     

Lowering reaction temperature: Electrochemical ammonia synthesis by coupling various electrolytes and catalysts

Ammonia is a vital emerging energy carrier and storage medium in the future hydrogen economy, even presenting relevant advantages compared with methanol due to the higher hydrogen content(17.6 wt% for ammonia versus 12.5 wt% for methanol). The rapidly growing demand for ammonia is still dependent on the conventional high-temperature and high-pressure Haber–Bosch process, which can deliver a conversion rate of about 10%–15%. However, the overall process requires a large amount of fossil fuels,resulting in serious environmental problems. Alternatively, electrochemical routes show the potential to greatly reduce the energy consumption, including sustainable energy sources and simplify the reactor design. Electrolytes perform as indispensable reaction medium during electrochemical processes, which can be further classified into solid oxide electrolytes, molten salt electrolytes, polymer electrolytes, and liquid electrolytes. In this review, recent developments and advances of the electrocatalytic ammonia synthesis catalyzed by a series of functional materials on the basis of aforementioned electrolytes have been summarized and discussed, along with the presentation and evaluation of catalyst preparation, reaction parameters and equipment.