Lewis Acid Research Articles

Mechanochemical Synthesis of High-Entropy MOF-74 with Multiple Active Sites for CO2 Adsorption and Synergistic Conversion.

Compared with monometallic metal-organic frameworks (MOFs), the synergistic effect of multiple metals significantly enhances the catalytic performance of the CO2 cycloesterification reaction, leading to improved CO2 adsorption and catalytic conversion capabilities. To investigate this concept, a high-entropy MOF-74 (HE-MOF-74) with a uniform distribution of five distinct metal ions (Zn2+, Mg2+, Ni2+, Co2+, and Cu2+) was successfully synthesized using a straightforward mechanical ball milling technique and comprehensively characterized (including structural, morphological, and physicochemical properties). The results reveal that HE-MOF-74 exhibits significantly increased specific surface area and CO2 adsorption capacity compared with those of monometallic MOF-74. The presence of multiple unsaturated metal centers as Lewis acid sites, oxygen atoms linking the metals, and ligand-based hydroxyl groups serving as base sites enable efficient immobilization of CO2 into cyclic carbonate. This study introduces a novel synthetic approach for the green and efficient production of HE-MOF-74 and proposes a new application for CO2 utilization.

Antiaromatic 2-Azaboroles with π4σ2 Electronic Configuration.

Similar to pyridine, which is a structural analog of benzene, 2-azaborole can be viewed as a structural analog of borole, in which the CH group at the 2-position is replaced by an N atom. Due to its unique π4σ2 electronic configuration, it should exhibit Lewis acidity, antiaromaticity, as well as Lewis basicity simultaneously. However, this uniqueness also makes its synthesis and isolation particularly challenging. One anticipated issue is its readiness for self-dimerization. This work proposes 2-azaborole and targets the synthesis and characterization of its derivatives for the first time. By reacting benzoborirene C6H4{BN(SiMe3)2} with bulky nitriles, crystalline benzo-fused 2-azaboroles have been successfully achieved and fully characterized. The importance of steric hindrance has been experimentally verified, showing that insufficient kinetic protection results in the dimerization of benzo-fused 2-azaboroles to form BN-allenophanes, a class of 10-membered macrocyclic compounds featuring two BN-allene units. The unique electronic structure of 2-azaborole as well as the mechanism of dimerization has been corroborated by theoretical calculations. In addition, its ability to act both as a Lewis acid and a Lewis base is demonstrated through its reaction with 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (MeIiPr) and AlCl3, respectively, which also implies the potential of the 2-azaborole motif as a σ-donor ligand for main group and organometallic chemistry.

Ring-Opening Reactions of Imidazolidines and Hexahydropyrimidines with Grignard Reagents.

We herein report an unprecedented ring-opening of unstrained cyclic aminals such as imidazolidines and hexahydropyrimidines by the use of Grignard and cuprate reagents to give secondary sulfonamides bearing diversely substituted tertiary amines in the β- or γ-position. This synthetic procedure can be carried out in a one-pot fashion without collateral reactions that are commonly associated with sp3-organometallic multicomponent Mannich-type reactions, indicating the fundamental role of sulfonamide protection of the second nitrogen atom in the generation of the cyclic aminal and in the ring-opening process. Computational density functional theory (DFT) data point to the formation of a transient iminium ion intermediate, in which the Lewis acidity of the cationic component of the organometallic reagent triggers the ring-opening process by coordination. The presented method allows the nucleophilic decoration of diamines including those bearing an adjacent chiral center to the tertiary amine not easily achievable by means of alternative standard synthetic procedures.

Metals doping effect of perovskite–type La-Mn oxides for NH3-SCR reaction at low temperature

Doping of perovskites with the heteroatoms has emerged as an additional lever, a dimension beyond structural perfection and compositional distinction, for the alteration of many properties of perovskites. In this work, a series of different metals were doped via citric acid sol–gel method to investigate their influences on the structure and catalytic performance of La-Mn perovskite oxides for NH3-SCR reaction. It is found that Zr doped La-Mn oxides (LMZr) displays better low-temperature deNOx performance (T90 = 118 °C) than other metal doped catalysts. Such a result is mainly attributed to the increase of Lewis acid sites, surface Mn4+ and adsorbed oxygen species. Besides, La-Mn perovskite doped with Sr, Ce, Y and Al can also obviously improve low-temperature deNOx activity and N2 selectivity. These doped metals mainly change the electron transfer between different cations, thus changing the valence distribution of Mn on the surface, meanwhile, metal doping changes the crystalline size of the catalysts, thereby affecting the adsorption and activation of NH3 and O2 involved in the removal of NO. Although other metal doping alters the tolerance of the La-Mn catalyst to SO2 and/or H2O, SO2 is inevitably oxidized to S6+ and deposits on the catalyst surface. Zr or Ce doping significantly enhances the tolerance to SO2 and/or H2O, but they have a different mechanism. Zr reduces the deposition rate of sulfate while Ce reacts with SO2 to form Ce2(SOx)3 to protect Mn active species. This paper highlights the importance of Lewis acid, Mn4+ content and adsorbed oxygen in the NH3-SCR reaction.

DNA: Novel Crystallization Regulator for Solid Polymer Electrolytes in High-Performance Lithium-Ion Batteries.

In this work, we designed a novel polyvinylidene fluoride (PVDF)@DNA solid polymer electrolyte, wherein DNA, as a plasticizer-like additive, reduced the crystallinity of the solid polymer electrolyte and improved its ionic conductivity. At the same time, due to its Lewis acid effect, DNA promotes the dissociation of lithium salts when interacting with lithium salt anions and can also fix the anions, creating more free lithium ions in the electrolyte and thus improving its ionic conductivity. However, owing to hydrogen bonding between DNA and PVDF, excess DNA occupies the lone pairs of electrons of the fluorine atoms on the PVDF molecular chains, affecting the conduction of lithium ions and the conductivity of the solid electrolyte. Hence, in this study, we investigated the effects of adding different DNA amounts to solid polymer electrolytes. The results show that 1% DNA addition resulted in the best improvement in the electrochemical performance of the electrolyte, demonstrating a high ionic conductivity of 3.74 × 10-5 S/cm (25 °C). The initial capacity reached 120 mAh/g; moreover, after 500 cycles, the all-solid-state batteries exhibited a capacity retention of approximately 71%, showing an outstanding cycling performance.

Lewis Acid Regulation Strategy for Constructing D-A-A Covalent Organic Frameworks with Enhanced Photocatalytic Organic Conversion.

Owing to their excellent photoelectric properties, donor-acceptor (D-A) type photocatalytic covalent organic frameworks (COFs) have attracted significant research interest in recent years. However, the limited D-A structural units of existing COFs restrict the development of novel and efficient photocatalytic COF materials. To solve this problem, we developed a series of D-A-A-type COFs utilizing a Lewis acid regulation strategy, in which Lewis acids act as the coordination centers, and pyridine and cyano groups act as ligands. Lewis acid sites in COFs serve as electron acceptors, facilitating the separation and transfer of photogenerated electron-hole pairs. This process is crucial for photocatalysis because it significantly increases the efficiency of the catalytic reaction by reducing the recombination rate of charge carriers. The developed Lewis acid-activated D-A-A COFs efficiently catalyzed the hydroxylation of various phenylboronic acid compounds under visible light. The developed catalysts are expected to contribute to increasing the fabrication efficiency of industrially important organic materials.

Gold Meets Selenium: Dual Activation of Selenium-Containing Propargyl Alcohols Towards the Synthesis of 2H-Chromenes and Mechanistic Insights.

Herein, we report the synthesis of seleno-substituted chromenes from selenoalkynes and phenols. In this cascade reaction, the applied gold catalyst not only functions as a π-acid, but also as a Lewis acid, enabling the propargylic substitution in the first step to connect the oxygen carbon bond. Under the optimal reaction condition a total of 26 chromenes were accessible by this modular access. During scale up experiments, the hydrolysis of the vinylselenium substructure to the corresponding chromenones was observed. By revisiting the electron-rich starting materials, four chromenones were produced following a one-pot reaction using a single gold catalyst. To better understand the interaction of gold and selenium, a series of nuclear magnetic resonance studies and high-resolution mass spectrometry studies were performed, which led to the proposal of a mechanism for this transformation.

Synthesis of Lewis Acid–Base Adducts between Trialkyltriels and Heavy Bis(trimethylsilyl)pnictogenides

Synthesis of Lewis Acid–Base Adducts between Trialkyltriels and Heavy Bis(trimethylsilyl)pnictogenides

Bifunctional Lewis Base-Catalyzed (3 + 2) Cycloadditions of Pyrazolone-Derived MBH Carbonates with Arynes.

The (3 + 2) cycloaddition of arynes with allylic ylides remains a formidable challenge because both intermediates are highly reactive and prone to spontaneous quenching. Here, we report a (3 + 2) cycloaddition of pyrazolone MBH carbonates with arynes, enabling the efficient synthesis of diverse indene-fused spiropyrazolones. The key is employing a new bifunctional Lewis base catalyst to facilitate the cycloaddition of in situ generated allylic pyridinium ylides with arynes.

Pd-Catalyzed Dual C-H Activation/Cyclization: Convergent and Divergent Synthesis of 1-Azahelicenes.

Herein, we report a convergent synthesis of 1-azahelicenes using easily available quinoline derivatives and cyclic diaryliodonium salts as starting materials. This reaction undergoes a palladium-catalyzed dual C-H activation/cyclization process to give facile access to a wide range of 1-aza[5]helicenes and 1-aza[6]helicenes with abundant functional groups (including F, Cl, Br, I, CF3, SeR, SR, and heteroaryl) in moderate to excellent yields, thereby providing new opportunities to fine-tune the properties of the helicene backbone. In addition, the obtained products could be further transformed into helicene-based Lewis base catalysts and redox switch materials easily. Notably, one selected 1-azahelicene shined bright yellow light by aggregation. These features enlarge the chemical space of 1-azahelicenes and inspire further utilization in other areas of research.

Visible light-mediated organocatalyzed 1,3-aminoacylation of cyclopropane employing N-benzoyl saccharin as bifunctional reagent

The carboamination of unsaturated molecules using bifunctional reagents is considered an attractive approach for the synthesis of nitrogen-containing compounds. However, bifunctional C–N reagents have never been employed in the carboamination of cyclopropane. In this study, we use an N-heterocyclic carbene (NHC), N-benzoyl saccharin, as a bifunctional reagent and a photoredox catalyst for a dual-catalyzed 1,3-aminoacylation of cyclopropane. NHCs play multiple roles, functioning as Lewis base catalysts to activate C–N bonds, promoting the oxidative quenching process of PC*, and acting as efficient acyl radical transfer catalysts for the formation of C–C bonds. The oxidative quenching process between the excited-state PC* and acyl NHC adduct is the key to the photooxidation generality of aryl cyclopropanes.

FTIR traced modification of defect-engineered UiO-66 for enhanced accessibility of zirconium sites

This research focuses on identifying the accessibility of active sites within the defect-engineered UiO-66 framework. The task is particularly challenging due to reversible changes in the framework during dehydroxylation: the loss of μ3-OH groups with simultaneous reduction of the Zr coordination number and the possible creation of Zr4+ Lewis acid sites in defect MOFs. We used in-situ FTIR and XANES analyses, as well as interaction with probe molecules, to monitor the changes in Zr coordination and the host-guest interaction. The defects were introduced using benzoic acid as a modulator, which coordinated to Zr4+ in defective pores. Our results showed that the UiO-66 sample synthesized with benzoic acid contained defects, but these were concealed under benzoate residues and thus inaccessible. Standard washing and heating did not remove benzoate anions. Dehydroxylation of the sample leads to the development of “hidden” Lewis acidity: some Zr4+ sites were not able to form complexes with the weak base CO, but they interact with the stronger bases acetonitrile. Additionally, in-situ XANES analysis revealed that the effect of acetonitrile adsorption is similar to that of water rehydration. Treatment of the sample with HCl and DMF led to the replacement of benzoates with formate ions, exposing the bare Zr4+ sites within the defective pores. These cationic sites acted as true Lewis acids and were able to coordinate both CO and acetonitrile. Our findings emphasize that the active sites in UiO-66 highly depend on synthesis conditions and post-synthetic treatments. Comprehensive site-specific methods are crucial for accurately predicting and identifying these active sites.

A Procedure for Solid-Phase Extractions Using Metal-Oxide-Coated Silica Column in Lipidomics.

Lipid enrichment is indispensable for enhancing the coverage of targeted molecules in mass spectrometry (MS)-based lipidomics studies. In this study, we developed a simple stepwise fractionation method using a titanium- and zirconium-dioxide-coated solid-phase extraction (SPE) silica column that separates neutral lipids, phospholipids, and other lipids, including fatty acids (FAs) and glycolipids. Chloroform was used to dissolve the lipids, and neutral lipids, including steryl esters, diacylglycerols, and triacylglycerols, were collected in the loading fraction. Second, methanol with formic acid (99:1, v/v) was used to retrieve FAs, ceramides, and glycolipids, including glycosylated ceramides and glycosylated diacylglycerols, by competing for affinity with the Lewis acid sites on the metal oxide surface. Finally, phospholipids strongly retained via chemoaffinity interactions were eluted using a solution containing 5% ammonia and high water content (45:50 v/v, 2-propanol:water), which canceled the electrostatic and chelating interactions with the SPE column. High average reproducibility of <10% and coverage of ∼100% compared to those of the non-SPE samples were demonstrated by untargeted lipidomics of human plasma and mouse brain, testis, and feces. The advantage of our procedure was showcased by characterizing minor lipid subclasses, including dihexosylceramides containing very long-chain polyunsaturated FA in the testis, monogalactosyl and digalactosyl monoacylglycerols in feces, and acetylated and glycolylated derivatives of gangliosides in the brain that were not detected using conventional solvent extraction methods. Likewise, the value of our method in biology is maximized during glycolipidome profiling in the absence of neutral lipids and phospholipids that cover more than 80% of the chromatographic peaks.

Unravelling the Crucial of Spatial Al Distribution to Realize Precise Alkali‐Treatment for Target Acid‐Catalyzed Reactions

Constructing mesoporous structure within zeolites by alkali‐treatment is an effective protocol to improve their diffusion properties. However, undesirable changes in Brönsted acid site (BAS) densities always offset this advantage in acid‐catalyzed reactions. In this context, the crucial roles of spatial aluminum distribution were unraveled during alkali‐treatment of MFI zeolite and the desirable BAS density was achieved in obtained hierarchical samples for the target reactions. Various characterization methods, particularly the multiple one‐ and two‐dimensional magic‐angle‐spinning (MAS) NMR techniques, were performed to track the alkali‐treatment processes. For the sample with a more uniform spatial Al distribution, more tetrahedral Al sites would fall off and migrate around the Si‐OH in zeolite as Al(OH)4‐. Those re‐deposited Al(OH)4‐ sites were easily transformed into NMR‐invisible Al sites during the calcination process, which contributed negligibly to both Brönsted and Lewis acidities, thus being referred to“acid‐free”Al species. While most tetrahedral Al sites were preserved after the alkali‐treatment of sample with non‐uniform Al distribution and the BAS density gradually increased with treatment time. According to the requirements of typical acid‐catalyzed reactions, such as catalytic cracking of 1,3,5‐triisopropylbenzene and methanol‐to‐olefins, the desired hierarchical zeolite catalysts were developed by matching the amounts of extracted Si and generated“acid‐free”Al during the precise alkali‐treatment.

Quinoline Synthesis: Nanocatalyzed Green Protocols-An Overview.

Heterocyclic compounds are of great interest in our daily lives. They are widely distributed in nature and are synthesized in laboratories. Heterocycles play an important role in the metabolism of all living cells, including vitamins and coenzyme precursors like thiamine and riboflavin. Furthermore, heterocyclic systems are essential building blocks for creating innovative materials with intriguing electrical, mechanical, and biological properties. Also, more than 85% of all biologically active chemical entities comprise a heterocycle. As a result, heterocycle synthesis piqued researchers' curiosity, and in recent decades, chemists have concentrated more on nitrogen-containing cyclic nuclei in structures. Quinoline and its derivatives exhibit several biological functions, including antimicrobial, anticancer, antimalarial, anti-inflammatory, antihypertensive, and antiasthmatic effects. In addition, over a hundred quinoline-based drugs are available to treat a variety of disorders. Because of its biological importance, researchers developed one-pot synthetic methods employing effective acid/base catalysts (Lewis acids, Brønsted acids, and ionic liquids), reagents, and transition-metal-based catalysts. These methods have some downsides, including longer reaction times, harsher reaction conditions, creation of byproducts, costly catalysts, use of hazardous solvents, an unacceptable economic yield, and catalyst recovery. Researchers' focus has switched to creating environmentally friendly and effective methods for the synthesis of quinoline derivatives as a result of these methodologic shortcomings. Because of its special qualities, the use of nanocatalysts or nanocomposites offers an option for the effective synthesis of quinolines. This review focuses on the published research articles on nanocatalysts to synthesize substituted quinoline derivatives. This review covers all contributions until May 2024, focusing on quinoline ring building and mechanistic issues. With the aid of this review, we anticipate that synthetic chemists will be able to develop more effective methods of synthesizing quinolines.

Unravelling the Crucial of Spatial Al Distribution to Realize Precise Alkali-Treatment for Target Acid-Catalyzed Reactions.

Constructing mesoporous structure within zeolites by alkali-treatment is an effective protocol to improve their diffusion properties. However, undesirable changes in Brönsted acid site (BAS) densities always offset this advantage in acid-catalyzed reactions. In this context, the crucial roles of spatial aluminumdistribution were unraveled during alkali-treatment of MFI zeolite and the desirable BAS density was achieved in obtained hierarchical samples for the target reactions. Various characterization methods, particularly the multiple one- and two-dimensional magic-angle-spinning (MAS) NMR techniques, were performed to track the alkali-treatment processes. For the sample with a more uniform spatial Al distribution, more tetrahedral Al sites would fall off and migrate around the Si-OH in zeolite as Al(OH)4-. Those re-deposited Al(OH)4- sites were easily transformed into NMR-invisible Al sites during the calcination process, which contributed negligibly to both Brönsted and Lewis acidities, thus being referred to"acid-free"Al species. While most tetrahedral Al sites were preserved after the alkali-treatment of sample with non-uniform Al distribution and the BAS density gradually increased with treatment time. According to the requirements of typical acid-catalyzed reactions, such as catalytic cracking of 1,3,5-triisopropylbenzene and methanol-to-olefins, the desired hierarchical zeolite catalysts were developed by matching the amounts of extracted Si and generated"acid-free"Al during the precise alkali-treatment.

Interface Synergy of Exposed Oxygen Vacancy and Pd Lewis Acid Sites Enabling Superior Cooperative Photoredox Synthesis

AbstractPhoto‐driven cross‐coupling of o‐arylenediamines and alcohols has emerged as an alternative for the synthesis of bio‐active benzimidazoles. However, tackling the key problem related to efficient adsorption and activation of both coupling partners over photocatalysts towards activity enhancement remains a challenge. Here, we demonstrate an efficient interface synergy strategy by coupling exposed oxygen vacancies (VO) and Pd Lewis acid sites for benzimidazole and hydrogen (H2) coproduction over Pd‐loaded TiO2 nanospheres with the highest photoredox activity compared to previous works so far. The results show that the introduction of VO optimizes the energy band structure and supplies coordinatively unsaturated sites for adsorbing and activating ethanol molecules, affording acetaldehyde active intermediates. Pd acts as a Lewis acid site, enhancing the adsorption of alkaline amine molecules via Lewis acid‐base pair interactions and driving the condensation process. Furthermore, VO and Pd synergistically promote interfacial charge transfer and separation. This work offers new insightful guidance for the rational design of semiconductor‐based photocatalysts with interface synergy at the molecular level towards the high‐performance coproduction of renewable fuels and value‐added feedstocks.

Catalytic Enantioselective Friedel–Crafts Allenylation

AbstractThe first enantioselective Friedel‐Crafts (FC) allenylation reaction for the creation of central chirality is developed under cooperative Ir(I)/(phosphoramidite,olefin) and Lewis acid catalysis. This enantioconvergent reaction utilizes racemic allenylic alcohol as the electrophile and shows compatibility with a variety of electron‐rich arenes and heteroarenes. The resulting highly enantioenriched (up to &gt;99.5 : 0.5 e.r.) 1,1‐disubstituted allenylic methanes, bearing a benzylic carbon stereocenter, are obtained with complete regiocontrol – both on (hetero)arenes as well as on the allenylic fragment. This protocol allows for the enantioselective formal introduction of a 4‐carbon alkyl chain into (hetero)arenes, along with the creation of a benzylic stereocenter. Judicious synthetic elaborations not only lead to formal enantioselective FC alkylation products of less electron‐rich arenes but also of substituted arenes in ortho‐ and even meta‐selective fashion. An intramolecular version of this FC allenylation is shown to proceed with promising enantioselectivity under the same catalytic conditions. Mechanistic studies revealed the involvement of dynamic kinetic asymmetric transformation (DyKAT) of racemic allenylic alcohols in this reaction.

Rutile-phase Sn-Ti Mixed Oxides as Acid Catalysts for the Condensation of C2-C3 Oxygenated Compounds in Water.

The design of new cost-effective and water-resistant acid catalysts is crucial for valorizingaqueous side-streams to increase the efficiency and competitiveness of bio-refineries in a sustainable way. In this work, Sn-Ti mixed oxides are prepared via a green and scalable co-precipitation method. This synthesis procedure allows obtaining homogeneous rutile-phase Sn-Ti mixed oxides with enhanced textural properties and a higher amount of Lewis acid sites. More importantly, the catalytic behavior of these Sn-Ti mixed oxides is investigated by using an aqueous mixture of C2-C3 representative oxygenated compounds, closer to real industrial conditions and differing from usual individual probe molecules studies performed even in the absence of water. Catalytic experiments combined and correlated with spectroscopic measurements (i.e., XRD, TEM, FT-IR with pyridine adsorption-desorption and N2 adsorption) are performed to better understand the properties of different Sn-Ti catalysts. These materials show promising results in the condensation of light oxygenated compounds present in aqueous fractions. Homogeneous and robust rutile-phase structures with inherent hydrophobic characteristics turn these Sn-Ti materials into active and highly resistant catalysts capable to transform these low-value oxygenated compounds into a mixture of hydrocarbons and aromatics useful for blending with automotive fuels, especially under complex acidic aqueous environments and moderate process conditions.

Catalytic Enantioselective Friedel-Crafts Allenylation.

The first enantioselective Friedel-Crafts (FC) allenylation reaction for the creation of central chirality is developed under cooperative Ir(I)/(phosphoramidite,olefin) and Lewis acid catalysis. This enantioconvergent reaction utilizes racemic allenylic alcohol as the electrophile and shows compatibility with a variety of electron-rich arenes and heteroarenes. The resulting highly enantioenriched (up to >99.5 : 0.5 e.r.) 1,1-disubstituted allenylic methanes, bearing a benzylic carbon stereocenter, are obtained with complete regiocontrol - both on (hetero)arenes as well as on the allenylic fragment. This protocol allows for the enantioselective formal introduction of a 4-carbon alkyl chain into (hetero)arenes, along with the creation of a benzylic stereocenter. Judicious synthetic elaborations not only lead to formal enantioselective FC alkylation products of less electron-rich arenes but also of substituted arenes in ortho- and even meta-selective fashion. An intramolecular version of this FC allenylation is shown to proceed with promising enantioselectivity under the same catalytic conditions. Mechanistic studies revealed the involvement of dynamic kinetic asymmetric transformation (DyKAT) of racemic allenylic alcohols in this reaction.