Stereoselective Research Articles

Preparation of high-strength silicon carbide by SLS-RS

One major limitation of selective laser sintering (SLS) plus reaction sintering (RS) SiC ceramics was the poor mechanical properties related to those of the traditional reaction sintered SiC due to the excess residual Si and C. TiC as a new carbon source in SLS was explored in this work to improve the performance of sintered SiC. It's found that TiC could react with liquid Si and form liquid TiSi2 during the processing, and then reduce the residual Si from 26.41 vol% to 0.53 vol% in the final products. As the content of TiC increased, the flexural strength of the sintered sample increased from 274.3 MPa to 434.6 MPa. The maximum value was reached when the TiC content was 25 wt%. Continuing to add TiC would generate more TiSi2, and the flexural strength of the sintered sample would decrease.

Improving the Electrochemical Glycerol-to-Glycerate Conversion at Pd Sites via the Interfacial Hydroxyl Immigrated from Ni Sites.

The electrochemical conversion of glycerol into high-value chemicals through the selective glycerol oxidation reaction (GOR) holds importance in utilizing the surplus platform chemical component of glycerol. Nevertheless, it is still very limited in producing three-carbon chain (C3) chemicals, especially glyceric acid/glycerate, through the direct oxidation of its primary hydroxyl group. Herein, Pd microstructure electrodeposited on the Ni foam support (Pd/NF) is designed and fabricated to achieve a highly efficient GOR, exhibiting a superior current density of ca. 120 mA cm-2 at 0.8 V vs. reversible hydrogen electrode (RHE), and high selectivity of glycerate at ca. 70%. The Faradaic efficiency of C3 chemicals from GOR can still be maintained at ca. 80% after 20 continuous electrolysis runs, and the conversion rate of glycerol can reach 95% after 10-h electrolysis. It is also clarified that the dual-component interfaces constructed by the adjacent Pd and Ni sites are responsible for this highly efficient GOR. Specifically, Ni sites can effectively strengthen the generative capacity of the active adsorbed hydroxyl (OHad) species, which can steadily immigrate to the Pd sites, so that the surface adsorbed glycerol species are quickly oxidized into C3 chemicals, rather than breaking the C-C bond of glycerol; thus, neither form the C2/C1 species. This study may yield fresh perspectives on the electrocatalytic conversion of glycerol into high-value C3 chemicals, such as glyceric acid/glycerate.

The Effect of the Tetraalkylammonium Cation in the Electrochemical CO2 Reduction Reaction on Copper Electrode.

Aprotic organic solvents such as acetonitrile offer a potential solution to promote electrochemical CO2 reduction over the competing hydrogen evolution reaction. Tetraalkylammonium cations (TAA+) are widely used as supporting electrolytes in organic media due to their high solubility and conductivity. The alkyl chain length of TAA+ cations is known to influence electron transfer processes in electrochemical systems by the adsorption of TAA+, causing modifications of the double layer. In this work, we elucidate the influence of the cation chain length on the mechanism and selectivity of the CO2RR reaction under controlled dry and wet acetonitrile conditions on copper cathodes. We find that the hydrophobic hydration character of the cation, which can be tuned by the chain length, has an effect on product distribution, altering the reaction pathway. Under dry conditions, smaller cations (TEA+) preferentially promote oxalate production via dimerization of the CO2 ·- intermediate, whereas formate is favored in the presence of water via protonation reaction. Larger cations (TBA+ > TPA+ > TEA+) favor the generation of CO regardless of water content. In situ FTIR analysis showed that TBA+ cations are able to stabilize adsorbed CO more effectively than TEA+, explaining why larger cations generate a higher proportion of CO. Our findings also suggest that higher cation concentrations suppress hydrogen evolution, particularly with larger cations, highlighting the role of cation chain length size and hydrophobic hydration shell.

Identification of K+-determined reaction pathway for facilitated kinetics of CO2 electroreduction

Cations such as K+ play a key part in the CO2 electroreduction reaction, but their role in the reaction mechanism is still in debate. Here, we use a highly symmetric Ni-N4 structure to selectively probe the mechanistic influence of K+ and identify its interaction with chemisorbed CO2−. Our electrochemical kinetics study finds a shift in the rate-determining step in the presence of K+. Spectral evidence of chemisorbed CO2− from in-situ X-ray absorption spectroscopy and in-situ Raman spectroscopy pinpoints the origin of this rate-determining step shift. Grand canonical potential kinetics simulations - consistent with experimental results - further complement these findings. We thereby identify a long proposed non-covalent interaction between K+ and chemisorbed CO2−. This interaction stabilizes chemisorbed CO2− and thus switches the rate-determining step from concerted proton electron transfer to independent proton transfer. Consequently, this rate-determining step shift lowers the reaction barrier by eliminating the contribution of the electron transfer step. This K+-determined reaction pathway enables a lower energy barrier for CO2 electroreduction reaction than the competing hydrogen evolution reaction, leading to an exclusive selectivity for CO2 electroreduction reaction.

Clotrimazole Identified as a Selective UGT2B4 Inhibitor Using Canagliflozin-2'-O-Glucuronide Formation as a Selective UGT2B4 Probe Reaction.

UGT2B4 is a highly expressed drug-metabolizing enzyme in the liver contributing to the glucuronidation of several drugs. To enable quantitatively assessing UGT2B4 contribution toward metabolic clearance, a potent and selective UGT2B4 inhibitor that can be used for reaction phenotyping was sought. Initially, a canagliflozin-2'-O-glucuronyl transferase activity assay was developed in recombinant UGT2B4 and human liver microsomes (HLM) [±2% bovine serum albumin (BSA)]. Canagliflozin-2'-O-glucuronidation (C2OG) substrate concentration at half-maximal velocity value in recombinant UGT2B4 and HLM were similar. C2OG formation intrinsic clearance was five- to seven-fold higher in incubations containing 2% BSA, suggesting UGT2B4 susceptibility to the inhibitory unsaturated long-chain fatty acids released during the incubation. Monitoring for C2OG formation, 180 compounds were evaluated for UGT2B4 inhibition potency in the presence and absence of 2% BSA. Compounds that exhibited an apparent UGT2B4 IC50 of < 1 μM in HLM with 2% BSA were evaluated for inhibition of UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B7, UGT2B10, UGT2B15, and UGT2B17 catalytic activities to establish selectivity suitable for supporting UGT reaction phenotyping. In this study, clotrimazole was identified as a potent UGT2B4 inhibitor (HLM apparent IC50 of 11 to 35 nM ± 2% BSA). Moreover, clotrimazole exhibited selectivity for UGT2B4 inhibition (>24-fold) over the other UGT enzymes evaluated. Additionally, during this study it was discovered that the previously described UGT2B7 inhibitors 16α- and 16β-phenyllongifolol also inhibit UGT2B4. Clotrimazole, a potent and selective UGT2B4 inhibitor, will prove essential during UGT reaction phenotyping. SIGNIFICANCE STATEMENT: To mechanistically evaluate drug interactions, it is essential to understand the contribution of individual enzymes to the metabolic clearance of a drug. The present study describes the development of a UGT2B4 activity assay that enabled the discovery of the highly selective and potent UGT2B4 inhibitor clotrimazole. Clotrimazole can be used in UGT reaction phenotyping studies to estimate fractional contribution of UGT2B4.

Unraveling C–Selective Ring‐Opening of Phosphiranes with Carboxylic Acids and Other Nucleophiles: A Mechanistically‐Driven Approach

Phosphiranes are weak Lewis bases reacting with only a limited number of electrophiles to produce the corresponding phosphiranium ions. These salts are recognized for their propensity to undergo reactions with oxygen pronucleophiles at the phosphorus site, leading to the formation of phosphine oxide adducts. Building on a thorough mechanistic understanding, we have developed an unprecedented approach that enables the selective reaction of carboxylic acids, and other nucleophiles, at the carbon site of phosphiranes. This method involves the photochemical generation of highly reactive carbenes, which react with 1‐mesitylphosphirane to yield ylides. The latter undergoes a stepwise reaction with carboxylic acids, resulting in the production of the desired phosphines. In addition to DFT calculations, we have successfully isolated and fully characterized the key intermediates involved in the reaction.

Unraveling C-Selective Ring-Opening of Phosphiranes with Carboxylic Acids and Other Nucleophiles: A Mechanistically-Driven Approach.

Phosphiranes are weak Lewis bases reacting with only a limited number of electrophiles to produce the corresponding phosphiranium ions. These salts are recognized for their propensity to undergo reactions with oxygen pronucleophiles at the phosphorus site, leading to the formation of phosphine oxide adducts. Building on a thorough mechanistic understanding, we have developed an unprecedented approach that enables the selective reaction of carboxylic acids, and other nucleophiles, at the carbon site of phosphiranes. This method involves the photochemical generation of highly reactive carbenes, which react with 1-mesitylphosphirane to yield ylides. The latter undergoes a stepwise reaction with carboxylic acids, resulting in the production of the desired phosphines. In addition to DFT calculations, we have successfully isolated and fully characterized the key intermediates involved in the reaction.

Nickel‐Catalyzed Electrochemical Cross‐Electrophile C(sp2)−C(sp3) Coupling via a NiII Aryl Amido Intermediate

AbstractCross‐electrophile coupling (XEC) between aryl halides and alkyl halides is a streamlined approach for C(sp2)−C(sp3) bond construction, which is highly valuable in medicinal chemistry. Based on a key NiII aryl amido intermediate, we developed a highly selective and scalable Ni‐catalyzed electrochemical XEC reaction between (hetero)aryl halides and primary and secondary alkyl halides. Experimental and computational mechanistic studies indicate that an amine secondary ligand slows down the oxidative addition process of the Ni‐polypyridine catalyst to the aryl bromide and a NiII aryl amido intermediate is formed in situ during the reaction process. The relatively slow oxidative addition is beneficial for enhancing the selectivity of the XEC reaction. The NiII aryl amido intermediate stabilizes the NiII–aryl species to prevent the aryl–aryl homo‐coupling side reactions and acts as a catalyst to activate the alkyl bromide substrates. This electrosynthesis system provides a facile, practical, and scalable platform for the formation of (hetero)aryl–alkyl bonds using standard Ni catalysts under mild conditions. The mechanistic insights from this work could serve as a great foundation for future studies on Ni‐catalyzed cross‐couplings.

Nickel-Catalyzed Electrochemical Cross-Electrophile C(sp2)-C(sp3) Coupling via a NiII Aryl Amido Intermediate.

Cross-electrophile coupling (XEC) between aryl halides and alkyl halides is a streamlined approach for C(sp2)-C(sp3) bond construction, which is highly valuable in medicinal chemistry. Based on a key NiII aryl amido intermediate, we developed a highly selective and scalable Ni-catalyzed electrochemical XEC reaction between (hetero)aryl halides and primary and secondary alkyl halides. Experimental and computational mechanistic studies indicate that an amine secondary ligand slows down the oxidative addition process of the Ni-polypyridine catalyst to the aryl bromide and a NiII aryl amido intermediate is formed in situ during the reaction process. The relatively slow oxidative addition is beneficial for enhancing the selectivity of the XEC reaction. The NiII aryl amido intermediate stabilizes the NiII-aryl species to prevent the aryl-aryl homo-coupling side reactions and acts as a catalyst to activate the alkyl bromide substrates. This electrosynthesis system provides a facile, practical, and scalable platform for the formation of (hetero)aryl-alkyl bonds using standard Ni catalysts under mild conditions. The mechanistic insights from this work could serve as a great foundation for future studies on Ni-catalyzed cross-couplings.

Precursor‐Mediated Direct Growth of Defect‐Rich Hierarchical Nanocarbons for Electrocatalytic Hydrogen Peroxide Production†

Comprehensive SummaryCarbon‐based nanomaterials show great potential in selective electrochemical oxygen reduction reaction (ORR) through two‐electron (2e−) pathway for H2O2 production, which provides an eco‐friendly alternative to industrial energy‐intensive anthraquinone process. However, it still remains challenging to directly construct topological defects, which makes it difficult to study the working mechanism on 2e– ORR. Herein, we propose a precursor‐mediated chemical vapor deposition (CVD) approach for direct growth of topological defect‐rich hierarchical nanocarbons. Boric acid (H3BO3) is introduced into the precursor for disturbing the nucleation and growth through decomposing B‐containing species, which can in situ induce the formation of pentagon defects. The topological defect is found to be capable of introducing lattice strain, which can modify the electronic structure of nanocarbons and promote the key intermediate (*OOH) formation, thus greatly enhancing the 2e– ORR performance. Experimentally, the 2e– ORR selectivity shows a positive correlation to the topological defect density, where the average H2O2 selectivity reaches above 90% over a wide potential range with optimized concentration of H3BO3 as mediator. Moreover, in a flow cell, the hierarchical nanocarbons achieve a high H2O2 production rate of 998 mmol·gcatalyst−1·h−1 over 20 h of continuous electrocatalysis with stable current density (&gt;100 mA·cm–2) and Faradaic efficiency (&gt; 90%). This work provides a straightforward method for the synthesis of active metal‐free carbon‐based catalyst for sustainable H2O2 production.

Harnessing the potential of acyl triazoles in bifunctional cobalt-catalyzed radical cross-coupling reactions

Persistent radicals facilitate numerous selective radical coupling reactions. Here, we have identified acyl triazole as a new and versatile moiety for generating persistent radical intermediates through single-electron transfer processes. The efficient generation of these persistent radicals is facilitated by the formation of substrate-coordinated cobalt complexes, which subsequently engage in radical cross-coupling reactions. Remarkably, triazole-coordinated cobalt complexes exhibit metal-hydride hydrogen atom transfer (MHAT) capabilities with alkenes, enabling the efficient synthesis of diverse ketone products without the need for external ligands. By leveraging the persistent radical effect, this catalytic approach also allows for the development of other radical cross-coupling reactions with two representative radical precursors. The discovery of acyl triazoles as effective substrates for generating persistent radicals and as ligands for cobalt catalysis, combined with the bifunctional nature of the cobalt catalytic system, opens up new avenues for the design and development of efficient and sustainable organic transformations.

Programming Fluid Motion Using Multi-Enzyme Micropump Systems.

In the presence of appropriate substrates, surface-anchored enzymes can act as pumps and propel fluid through microchambers. Understanding the dynamic interplay between catalytic reactions and fluid flow is vital to enhancing the accuracy and utility of flow technology. Through a combination of experimental observations and numerical modeling, we show that coupled enzyme pumps can exhibit flow enhancement, flow suppression, and changes in the directionality (reversal) of the fluid motion. The pumps' ability to regulate the flow path is due to the reaction selectivity of the enzymes; the resultant fluid motion is only triggered by the presence of certain reactants. Hence, the reactants and the sequence in which they are present in the solution and the layout of the enzyme-attached patches form an "instruction set" that guides the flowing solution to specific sites in the system. Such systems can operate as sensors that indicate concentrations of reactants through measurement of the trajectory along which the flow demonstrates a maximal speed. The performed simulations suggest that the solutal buoyancy mechanism causes fluid motion and is responsible for all of the observed effects. More broadly, our studies provide a new route for forming self-organizing flow systems that can yield fundamental insight into nonequilibrium, dynamical systems.

Anchoring Cs+ Ions on Carbon Vacancies for Selective CO2 Electroreduction to CO at High Current Densities in Membrane Electrode Assembly Electrolyzers

AbstractElectrolyte cations have been demonstrated to effectively enhance the rate and selectivity of the electrochemical CO2 reduction reaction (CO2RR), yet their implementation in electrolyte‐free membrane electrode assembly (MEA) electrolyzer presents significant challenges. Herein, an anchored cation strategy that immobilizes Cs+ on carbon vacancies was designed and innovatively implemented in MEA electrolyzer, enabling highly efficient CO2 electroreduction over commercial silver catalyst. Our approach achieves a CO partial current density of approximately 500 mA cm−2 in the MEA electrolyzer, three‐fold enhancement compared to pure Ag. In situ Raman and theoretical analyses, combined with machine learning potentials, reveal anchored Cs induces an electric field that significantly promotes the adsorption of *CO2− intermediates through performing muti‐point energy calculations on each structure. Furthermore, reduced adsorption of *OH intermediates effectively hampers competing hydrogen evolution reaction, as clarified by disk electrode experiments and density functional theory studies. Additionally, coupling our system with commercial polysilicon solar cells yields a notable solar‐to‐CO energy conversion efficiency of 8.3 %. This study opens a new avenue for developing effective cation‐promoting strategy in MEA reactors for efficient CO2RR.

Anchoring Cs+ Ions on Carbon Vacancies for Selective CO2 Electroreduction to CO at High Current Densities in Membrane Electrode Assembly Electrolyzers.

Electrolyte cations have been demonstrated to effectively enhance the rate and selectivity of the electrochemical CO2 reduction reaction (CO2RR), yet their implementation in electrolyte-free membrane electrode assembly (MEA) electrolyzer presents significant challenges. Herein, an anchored cation strategy that immobilizes Cs+ on carbon vacancies was designed and innovatively implemented in MEA electrolyzer, enabling highly efficient CO2 electroreduction over commercial silver catalyst. Our approach achieves a CO partial current density of approximately 500 mA cm-2 in the MEA electrolyzer, three-fold enhancement compared to pure Ag. In situ Raman and theoretical analyses, combined with machine learning potentials, reveal anchored Cs induces an electric field that significantly promotes the adsorption of *CO2 - intermediates through performing muti-point energy calculations on each structure. Furthermore, reduced adsorption of *OH intermediates effectively hampers competing hydrogen evolution reaction, as clarified by disk electrode experiments and density functional theory studies. Additionally, coupling our system with commercial polysilicon solar cells yields a notable solar-to-CO energy conversion efficiency of 8.3 %. This study opens a new avenue for developing effective cation-promoting strategy in MEA reactors for efficient CO2RR.

Semiconducting Properties of Delaminated Titanium Nitride Ti4N3Tx MXene with Gate-Tunable Electrical Conductivity.

MXenes have garnered significant attention due to their atomically thin two-dimensional structure with metallic electronic properties. However, it has not yet been fully achieved to discover semiconducting MXenes to implement them into gate-tunable electronics such as field-effect transistors and phototransistors. Here, a semiconducting Ti4N3Tx MXene synthesized by using a modified oxygen-assisted molten salt etching method under ambient conditions, is reported. The oxygen-rich synthesis environment significantly enhances the etching reaction rate and selectivity of Al from a Ti4AlN3 MAX phase, resulting in well-delaminated and highly crystalline Ti4N3Tx MXene with minimal defects and high content of F and O, which led to its improved hydrophobicity and thermal stability. Notably, the synthesized Ti4N3Tx MXene exhibited p-type semiconducting characteristics, including gate-tunable electrical conductivity, with a current on-off ratio of 5 × 103 and a hole mobility of ∼0.008 cm2 V-1 s-1 at 243 K. The semiconducting property crucial for thin-film transistor applications is evidently associated with the surface terminations and the partial substitution of oxygen in the nitrogen lattice, as corroborated by density functional theory (DFT) calculations. Furthermore, the synthesized Ti4N3Tx exhibits strong light absorption characteristics and photocurrent generation. These findings highlight the delaminated Ti4N3Tx as an emerging two-dimensional semiconducting material for potential electronic and optoelectronic applications.

Solid-Phase Synthesis of Diverse Macrocycles by Regiospecific 2-Pyridone Formation: Scope and Applications.

This study introduces a novel solid-phase macrocyclization method generating 2-pyridone rings. This method relies on the intramolecular condensation between secondary and tertiary dimethoxy-propionic amide units. This selective reaction leads to the formation of a single well-defined regioisomer. The method demonstrates remarkable efficiency in producing diverse peptidic and nonpeptidic bioactive targets, paving the way for the development of innovative macrocycle libraries featuring the 2-pyridone unit.

Porphyrin-based carbon dots as fluorescence probe for sensing of Fe3+ ion and S2- anion in aqueous solution

In this work, a facile bottom-up approach was employed to synthesize the porphyrin-citric acid-derived fluorescent carbon dots (Por-CA-CDs) using a single-step hydrothermal method. As-prepared Por-CA-CDs were well characterized using spectroscopic techniques such as FT-IR, high-resolution transmission electron microscopy, P-XRD, UV-Vis absorption, and photoluminescence spectroscopy. The synthesized Por-CA-CDs exhibited blue fluorescence with excellent photostability and diameter ranging from 2.4 to 6.4 nm. Por-CA-CDs show excitation-dependent emission behavior, with a quantum yield of 26% and were used for sensing of Fe3+ and S2− ions in an aqueous solution. Por-CA-CDs demonstrated a robust and selective reaction to Fe3+ compared to the other cations utilized in this investigation. The sensing capability of Por-CA-CDs with Fe3+ ions was evaluated with fluorescence spectroscopy, and results show a “Turn-Off” emission mode upon binding of Fe3+ with Por-CA-CDs. In metal ion sensing studies, the fluorescence intensity of Por-CA-CDs exhibited significant quenching upon the addition of Fe3+ at concentrations ranging from 0 to 230 μM, with a detection limit (LOD) of 3.2 × 10−8 M. The quenching efficiency and Stern-Volmer quenching constant were computed as 99.45% and 4.4 × 106 M−1, respectively. Interestingly, fluorescence of Por-CA-CDs can be regained with the addition of S2− (0-160 µM) with LOD of 4.3 × 10−6 M and Stern-Volmer quenching constant of 6 × 105 M−1. Most importantly, the toxicity assay revealed good reliability for samples in deionized, tap, and river water, suggesting applications for Fe3+ monitoring in complex environments.

Derivatives of (5-Oxooxazolidin-4-yl)acetic acids in aspartic acid α-carboxyl protection: a word of caution

Isomeric products could be isolated in a reaction of pentafluorophenyl ester of (S)-2-(3-((benzyloxy)-carbonyl)-5-oxooxazolidin-4-yl)acetic acid with aminoacid esters. One product is an expected dipeptide, the isomeric one is an N-hydroxymethylamino succinimide – a product of methylene bridge cleavavge. Steric bulkness of aminoacid esters favors the formation of a peptide, however the reaction selectivity is quite unpredicatable. Moreover, the precise structure assignement requires high-temperature NMR experiments.

Study of Mesoporous Zr-TiO2 Catalyst with Rich Oxygen Vacancies for N-Methylmorpholine Oxidation to N-Methylmorpholine-N-oxide.

A series of Zr-TiO2 catalysts were prepared using a facile sol-gel method and were used for N-methylmorpholine (NMM) oxidation to N-methylmorpholine-N-oxide (NMMO). The structure features of Zr-TiO2 catalysts were studied in detail through a variety of characterization methods, such as XRD, SEM, N2 adsorption-desorption isotherms, XPS, EPR, and O2-TPD. As-obtained 5%Zr-TiO2 catalysts had superior catalytic performance and stability with a 97.6% NMMO yield at 40 °C, which related to Zr doping, a higher surface area, more oxygen vacancies, and oxygen chemisorption on the catalytic surface. This work provides an efficient preparation strategy of TiO2-based catalysts for selective oxidation reactions by a facile method.

Process optimization and scale-up of toluene nitration in a microchannel reactor using HNO3-AC2O as nitrating agent

Aromatic nitro compounds are an important class of basic chemical raw materials, while traditional nitration methods may cause safety accidents and environmental pollution due to strong exothermicity and presence of sulfuric acid. In this work, aiming to alleviate these issues, the nitration of toluene was conducted in a microchannel reactor with nitric acid and acetic anhydride. The effect of temperature, molar ratio of nitric acid to toluene, mass fraction of acetic anhydride, and residence time on the conversion and selectivity of reaction were systematically investigated, and response surface methodology was used to optimize the process. A mononitrotoluene yield of 99.21 % can be achieved under optimum condition. Meanwhile, in order to better understand the exothermic behavior, toluene nitration by HNO3-AC2O was investigated in the semi-batch calorimeter. Finally, the optimized process was scaled up and effect of volumetric flow rate was investigated. At the selected maximum volumetric flow rate, the overtemperature of the system was 10.9 °C with a 98.3 % yield of mononitrotoluene. Continuous flow mode had a space-time yield nearly two orders of magnitude greater than semi-batch mode. This work can provide guidance for safe, efficient and green synthesis of mononitrotoluene.