BippyPhos: A Highly Versatile Ligand for Pd-Catalyzed C−N,C−O and C−C Couplings

Numerous ligands have been designed for the Buchwald-Hartwig Amination (BHA). Among the ligands developed is BippyPhos. This ligand was originally designed to enable a coupling of primary amines with aryl halides. Further studies showed that the ligand has fairly broad utility for Pd-catalyzed C−N, C−O and C−C couplings. This review describes the various Pd-catalyzed applications involving BippyPhos as a supporting ligand. While BippyPhos may not often be the most optimal ligand for various Pd-catalyzed couplings, it typically will provide adequate results as a starting point prior to screening for optimization.     

A Simple, Inexpensive, and Reliable Method for Measuring Brønsted-Acid Site Densities in Solid Acids

A method based on amine decomposition has been developed for measuring the Brønsted-acid site densities using a standard, tubular reactor with an on-line gas chromatograph (GC). After exposing a sample to n-propylamine vapor and flushing it in flowing He at 473 K, the sample was ramped in flowing He to 773 K while trapping the products in the GC sample loop using liquid nitrogen. Site densities were determined from the quantity of propene formed by the decomposition of the n-propylammonium ions via the Hofmann-elimination reaction. The use of small amounts of sample was shown to be important for elimination of secondary products. The method was tested using H-ZSM-5, H-FER, H-MOR and H-Y zeolites, as well as a -Al₂O₃ sample, and shown to give essentially exact agreement with temperature-programmed desorption and thermogravimetric analysis measurements of n-propylamine performed in vacuum.     

Catalysts Based Upon Organoclay with Tunable Polarity and Dispersion Behavior: New Catalysts for Hydrogenation, C–C Coupling Reactions and Fluorous Biphase Catalysis

Spherical Pd nanoparticles (2–8 nm) were immobilized on the nanometer-scaled platelets of montmorillonite (MMT) by means of cation exchange of Pd²⁺ for Na⁺ followed by chemical reduction. The resulting Pd catalysts were readily dispersible in water and polar solvents. Polarity design and thus, variation of the dispersion behavior was achieved by subsequent organophilic MMT modification involving cation exchange with N-alkyl and N-perfluoroalkylethyl pyridinium cations. This allowed for easy catalyst dispersion in a wide range of organic solvents. According to characterization by TEM, EDXS, AAS and WAXS the nanoparticle formation did not destroy the superstructure of unmodified and organophilic MMT. Catalysts were applied to hydrogenation, Suzuki–Miyaura C–C coupling and fluorous biphase catalysis. The tunable dispersion behavior of MMT catalysts was successfully employed to guarantee easy catalyst recovery and recycling.     

Oxidation behaviour of ceramic fibres from the Si–C–N–O system and related sub-systems

The oxidation of Si–C–N–O fibres has been investigated. The oxidation rates and the activation energies for the Si–C–O system are similar to those for crystalline SiC. The oxygen and the free carbon concentrations in the ceramics have a limited influence on the oxidation behaviour. As long as the formed silica scale is protective, oxidation kinetics are essentially controlled by the diffusion of oxygen through SiO₂. The parabolic rates in the Si–C–N–O and Si–N–O systems are lower and their activation energies higher than those for SiC. Their values strongly depend on the ratios of C and N bonds to Si and continuously vary from those for SiC (E_a=110−140kJ mol⁻¹) to Si₃N₄ (E_a=330–490 kJ mol⁻¹). The oxidation mechanism might be related to a complex diffusion/reaction regime via the formation of an intermediate silicon-oxynitride (like for Si₃N₄) or silicon-oxycarbonitride layer. The oxidation behaviour of such complex systems is not significantly influenced by the oxygen nor the free carbon contents. It might be governed by the C/Si and N/Si ratios, limiting the nitrogen concentration gradient of the silicon-oxy(carbo)nitride sub-layer and therefore affecting the diffusion/reaction rates.     

New Trends and Future Opportunities in the Enzymatic Formation of C−C,C−N,and C−O bonds

Organic chemistry provides society with fundamental products we use daily. Concerns about the impact that the chemical industry has over the environment is propelling major changes in the way we manufacture chemicals. Biocatalysis offers an alternative to other synthetic approaches as it employs enzymes, Nature's catalysts, to carry out chemical transformations. Enzymes are biodegradable, come from renewable sources, operate under mild reaction conditions, and display high selectivities in the processes they catalyse. As a highly multidisciplinary field, biocatalysis benefits from advances in different areas, and developments in the fields of molecular biology, bioinformatics, and chemical engineering have accelerated the extension of the range of available transformations (E. L. Bell et al., Nat. Rev. Meth. Prim. 2021 , 1, 1–21). Recently, we surveyed advances in the expansion of the scope of biocatalysis via enzyme discovery and protein engineering (J. R. Marshall et al., Tetrahedron 2021 , 82, 131926). Herein, we focus on novel enzymes currently available to the broad synthetic community for the construction of new C−C, C−N and C−O bonds, with the purpose of providing the non-specialist with new and alternative tools for chiral and sustainable chemical synthesis.     

Facile C–N cleavage reactions of secondary and tertiary alkyl amines by P,O chelating rhodium and iridium complexes

The reactions of Cp*MCl(MDMPP-P,O) (1: M=Rh; 3: M=Ir; MDMPP-P,O=PPh₂(C₆H₃-2-O-6-MeO)) with secondary or tertiary alkyl amine (R₂NH or R₃N) in the presence of KPF₆ underwent a C–N cleavage of amine to afford primary amine complex [Cp*M(MDMPP-P,O) (RNH₂)](PF₆) (2: M=Rh; 4: M=Ir).     

Synthesis and characterization of precursor derived TiN@Si–Al–C–N ceramic nanocomposites for oxygen reduction reaction

The development of efficient and durable catalysts is critical for the commercialization of fuel cells, as the catalysts’ durability and reactivity dictate their ultimate lifetime and activity. In this work, amorphous silicon-based ceramics (Si–C–N and Si–Al–C–N) and TiN@Si–Al–C–N nanocomposites were developed using a precursor derived ceramics approach. In TiN@Si–Al–C–N nanocomposites, TiN nanocrystals (with sizes in the range of 5–12 nm) were effectively anchored on an amorphous Si–Al–C–N support. The nanocomposites were found to be mesoporous in nature and exhibited a surface area as high as 132 m²/g. The average pore size of the nanocomposites was found to increase with an increase in the pyrolysis temperature, and a subsequent graphitization of free carbon was observed as revealed from the Raman spectra. The ceramics were investigated for electrocatalytic activity toward the oxygen reduction reaction using the rotating disk electrode method. The TiN@Si–Al–C–N nanocomposites showed an onset potential of 0.7 V versus reversible hydrogen electrode for oxygen reduction, which seems to indicate a 4-electron pathway at the pyrolysis temperature of 1000°C in contrast to a 2-electron pathway exhibited by the nanocomposites pyrolyzed at 750°C via the Koutecky–Levich plot.     

Brønsted–Evans–Polanyi Relations for H<Subscript>2</Subscript>O<Subscript>2</Subscript> Synthesis on Gold Surfaces

Abstract  

By means of density functional theory calculations, the reaction mechanisms of H₂O₂ synthesis on three low index and two stepped Au surfaces have been investigated in detail. This study shows the activation energies of five elementary reaction steps of H₂O₂ synthesis, which include two hydrogenation and three decomposition steps of key species, are a function of reaction energies, which observe the Br?nsted–Evans–Polanyi rules on both the flat Au surfaces and the step edge sites of stepped Au surfaces. This study not only provides a simple method to estimate the reaction barriers of elementary steps of H₂O₂ synthesis by the reaction energies but also predicts the catalytic performances of Au nanoparticles applied in real catalysis.     

含Si–O–C界面层的C/Si–C–N复合材料的抗氧化性能(英文)

为了研究利用Si–O–C界面层来提高碳纤维增强陶瓷基复合材料的抗氧化性能,利用化学气相浸渗和聚合物浸渗裂解工艺制备了以Si–O–C为界面的碳纤维增强Si–C–N陶瓷基复合材料（C/Si–O–C/Si–C–N）和无界面层的碳纤维增强Si–C–N陶瓷基复合材料（C/Si–C–N）。研究了C/Si–O–C/Si–C–N和C/Si–C–N在600、900℃和1 200℃空气环境中的氧化行为。结果表明：采用Si–O–C界面层后可提高复合材料的抗氧化性能;Si–O–C界面层较高的氧化抗力是碳纤维增强Si–C–N复合材料抗氧化性能提高的主要原因。     

The preparation and properties of zirconia-doped Y–Si–Al–O–N oxynitride glasses and glass-ceramics

Two series (N-9 and N-18 series) of zirconia-doped Y–Si–Al–O–N oxynitride glasses and glass-ceramics were designed. Nominal compositions of the glass samples in equivalent percent (eq%) are xZr: (24–0.25x)Y: (15–0.25x)Al: (61–0.5x)Si: 91O: 9 N and xZr: (24–0.25x)Y: (15–0.25x)Al: (61–0.5x)Si: 82O: 18 N (x=0, 2, 4, 6), respectively. The obtained samples were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Densities, Vickers hardness, fracture toughness, glass transition temperature, and thermal expansion coefficient data were established for each sample. Effect of Zr and N content on glass network structure, thermal and mechanical properties was investigated. It was found that the addition of zirconia is effective in preparing Y–Si–Al–O–N oxynitride glasses with lower glass transition temperature and higher hardness.     

A novel polyborosilazane for high-temperature amorphous Si–B–N–C ceramic fibres

Polyborosilazane synthesised from BCl₃, HMeSiCl₂, and Me₃SiNHSiMe₃ is easy to cross-link for dehydrogenation of Si–H and N–H, which limits its practical applications for Si–B–N–C fibres on an industrial scale. Therefore, in this context, MeSiCl₃ was used instead of HMeSiCl₂ to synthesise a novel polyborosilazane with limited cross-linking density to fabricate Si–B–N–C fibres. The polyborosilazane synthesised from BCl₃, MeSiCl₃, and Me₃SiNHSiMe₃ exhibits good melt-processability and 1 km long polyborosilazane fibre can be obtained by melt spinning. Prior to pyrolysis, chemical curing with vapour HSiCl₃ at 80 °C was utilised to make the λ green fibres infusible. The as-cured fibres were subsequently pyrolyzed at 1200 °C in nitrogen atmospheres to provide Si–B–N–C ceramic fibres with ca. 1.5 GPa in tensile strength, ca. 160 GPa in Young's modulus, ca. 12 μm in diameter and keeping amorphous up to 1700 °C, which makes them to be promising reinforcements in ceramic matrix composites for high temperature applications.     

Preparation of Ni–Mo–C/Ti(C,N) coated powders and its influence on the microstructure and mechanical properties of Ti(C,N)-based cermets

Ni–Mo–C/Ti(C,N) coated powders, namely Ni–Mo alloy and Mo₂C coated Ti(C,N) composite powders, were synthesized by using a heterogeneous precipitation and thermal reduction method, then pressed and vacuum sintered to fabricate cermets. The chemical composition, microstructure and phases of the composite powders and the microstructure and properties of sintered cermets were experimentally investigated. The results show that a fine and uniform microstructure of (Ti,Mo)(C,N)-Ni cermets without the conventional core-rim structure is obtained. The phases formed during the preparation of the coated powders as well as the cermets were analyzed by means of a X-ray diffraction (XRD) technique. The XRD result confirms the formation of the Ni₃Ti phase in the cermets. Due to the formation of the non-magnetic Ni₃Ti and the dissolution of Mo in Ni binder phase, the magnetic properties are strongly retarded. The fracture of the cermets is mainly characterized by inter-granular and dimple fractures. Better mechanical properties can be obtained in comparison with conventionally fabricated ones.     

A porous metal–organic framework as active catalyst for multiple C–N/C–C bond formation reactions

A 3D porous metal–organic framework {[Cu(4-tba)₂](solvent)}_n (1⋅S) is assembled via 4-(1H-1,2,4-triazol-1-yl)benzoic acid (Htba) and Cu(II) nodes, which shows the [2 + 2] roto-translational interpenetrating network. Interestingly, 1 displays high CO₂ adsorption selectivity over CH₄/H₂/O₂/Ar/N₂ gases, and acts an efficient catalyst precursor in some C–N/C–C bond formation reactions, including Chan–Lam coupling reaction of phenylboronic acid with imidazole, Suzuki–Miyoura coupling reaction of phenylboronic acids with aryl halides, and Heck coupling reaction of styrene with aryl halides.     

Nanocarbon containing low carbon Al2O3–C refractories: Comparison between N220 and N990 nanocarbons

Continuous casting process is the majorly used solidification process in steel fabrication. The refractories used in this process are most commonly made up of alumina-carbon-based compositions. Generally, these functional refractories consist of about 30% residual carbon after coking. Improvements in steel industries, such as attaining clean steel and ultralow-carbon steel, require alumina-carbon refractories with low carbon content. In the present work, low carbon-containing Al₂O₃–C refractories are studied by using two different grade nanocarbons, namely, N220 and N990 with varying amounts, along with fixed 3-wt% graphite in the batch composition. The physical, mechanical, and thermomechanical properties along with the oxidation resistance are evaluated and compared. Phase analysis and microstructural developments at different temperatures were also characterized. Optimized compositions of both the nanocarbons are further studied for hot strength and oxidation resistance measurement. Based on all the obtained results, one batch composition is finalized for the thermal shock and corrosion testing. All the results are compared against a reference batch composition containing 25% graphite as a carbon source. The formation of in situ ceramic phases like aluminum carbide in nanocarbon-containing compositions provides a dense compact microstructure that improves strength, helps to inhibit oxidation, and contributes to corrosion resistance.     

Synthesis of a Novel Strong Brønsted Acidic Ionic Liquid and its Catalytic Activities for the Oxathioacetalization

The novel efficient procedure has been developed for the oxathioacetalization of carbonyl compounds and 2-mercaptoethanol at room temperature under solvent-free condition. The results showed that the novel multi-SO₃H functionalized ionic liquid was very efficient for the reactions with the good to excellent yields in short time. Operational simplicity, without need of any solvent, room temperature, low cost of the catalyst used, high yields, applicability to large-scale reactions, reusability and chemoselectivity are the key features of this methodology.     

Sol–Gel Pd Exhaust Catalysts and N2O Production

Al₂O₃, CeO₂–Al₂O₃, CeO₂–Tb₄O₇–Al₂O₃ and ZrO₂–Al₂O₃ supported Pd samples have been prepared by sol–gel methods. Extents and mechanisms of N₂O production in CO–NO and CO–NO–O₂ reactions on these have been considered. This occurs most selectively under oxidising (lean-burn) conditions or in the presence of CeO₂ and CeO₂–Tb₄O₇ promoters near the CO–NO light off temperature. Over Pd/ZrO₂–Al₂O₃ the CO–NO reaction at 573 K has CO and NO conversions that are second order with respect to p _CO and p _NO. Over this catalyst NO conversion is faster than that of CO until O_2(g) is added, causing CO conversion and N₂O production at 573 K to rise simultaneously. CeO₂ or CeO₂–Tb₄O₇ incorporation into a Pd/Al₂O₃ catalyst enhances N₂O production near the CO–NO light-off temperature in the absence of added O₂ without CO conversion being raised. There is current attention on pollution control opportunities through lean-burn conditions, Pd catalysts and oxygen storage capacity enhancement. The present work suggests that their role in N₂O production may need to be better understood and controlled. For the moment N₂O formation provides a window on mechanisms of TWC operation.     

Pyrolysis synthesis of Si–B–C–N ceramics and their thermal stability

Si–B–C–N ceramics were synthesized by co-pyrolyzing hybrid polymeric precursors of polycarbosilane (PCS) and polyborazine (PBN). The pyrolysis behavior and structural evolution of the hybrid precursor, the microstructure and composition of the prepared Si–B–C–N ceramics were fully investigated. It was found that the copyrolysis of hybrid polymeric precursors in Ar led to the release of CH₄, CH₃NH₂ and CH₃CN gases at temperatures ranging from 200 to 1100 °C, and finally resulted in the formation of amorphous Si–B–C–N ceramics. In particular, the Si–B–C–N ceramics formed from the hybrid precursor with PBN/PCS mass ratio of 1 could keep amorphous state up to the annealing temperature of 1800 °C with weight change of only 2.08%. But this amorphous ceramics would decompose to form crystalline SiC, BN and Si₃N₄ at 2000 °C. Additionally, compared with PCS-derived SiC ceramics, the Si–B–C–N ceramics showed improved anti-oxidation performance up to 1300 °C due to the formation of borosilicate layers covering the ceramics.