Lewis Acid Research Articles

Solvent-free CO2 cycloaddition by one- and two-dimensional sodium coordination polymers based on hydrazonic ligands

We successfully synthesized and characterized four new coordination polymers of sodium: sodium 4-hydroxy-3-{[2-(pyridine-4-carbonyl)hydrazinylidene]methyl}benzene-1-sulfonate (NaL4-Py), sodium 3-[(2-benzoylhydrazinylidene)methyl]-4-hydroxybenzene-1-sulfonate (NaLPh), sodium 3-[(2-acetylhydrazinylidene)methyl]-4-hydroxybenzene-1-sulfonate (NaLMe), and sodium 3-[(2-carbamoylhydrazinylidene)methyl]-4-hydroxybenzene-1-sulfonate (NaLNH2). These polymers were synthesized through a 1:1 condensation reaction of hydrazide derivatives with sodium salicylaldehyde-5-sulfonate monohydrate (NaSalSO3) and were thoroughly characterized using various analytical techniques. The compounds NaLMe and NaLPh, crystallized in the P 1‾ and P21/n space groups, respectively, forming one-dimensional polymers. On the other hand, NaLNH2 and NaL4-Py crystallized in the Cmca and P 1‾ space groups, respectively, forming two-dimensional coordination polymers. The coordination environment of the sodium cation differed among the compounds, with the imine nitrogen atom interfering in NaL4-Py and NaLPh. The sulfonate group exhibited various coordination modes, resulting in the different dimensionalities of the polymers. Compounds NaL4-Py and NaLPh acted as heterogeneous Lewis acid catalysts for the solvent-free cycloaddition reaction of epichlorohydrine and CO2 into valued cyclic carbonate. High selective conversion (68 %) of epichlorohydrin to cyclic carbonate was obtained at mild conditions, 60 °C and 1 bar CO2, after 24 h of reaction.

One-Pot Conversion of Biomass Saccharides to γ-Valerolactone over a Versatile Tin-Containing Material.

The synthesis of biofuel γ-valerolactone (GVL) from accessible biomass is an attractive and challenging goal. Here, we report an efficient, one-pot, and mild strategy for the efficient production of GVL from various biomass saccharides without using any homogeneous acid as a co-catalyst and molecular hydrogen as a hydrogen donor. A versatile porous tin-containing material (Sn(M)-S) was designed as an individual heterogeneous catalyst. As high as 68.4% yield of GVL form glucose was achieved in the presence of ammonia borane as a solid hydrogen donor under mild conditions, with GVL yields of 76.2%, 68.9%, 62.5%, and 52.2% being obtained from fructose, sucrose, cellobiose, and cellulose, respectively. The synergistic effect of Sn and sulfonic acid group in Sn(M)-S not only provides appropriate Lewis acid sites to promote the isomerization of glucose into fructose but also affords abundant Brønsted sites for the following conversion steps. Moreover, Sn(M)-S(1) showed good stability and reusability during consecutive recycles.

Cone Angles Quantify and Predict the Affinity and Reactivity of Anion Complexes between Trifluoroborates and Rigid Macrocycles

AbstractSteric manipulation is a known concept in molecular recognition but there is currently no linear free energy relationship correlating sterics to the stability of receptor‐anion complexes nor to the reactivity of the bound anion. By analogy to Tolman cone angles in cation coordination chemistry, we explore how to define and correlate cone angles of organo‐trifluoroborates (R−BF3−) to the affinities observed for cyanostar‐anion binding. We extend the analogy to a rare investigation of the anion's reactivity and how it changes upon binding. The substituent on the anion is used to define the cone angle, θ. A series of 10 anions were studied including versions with ethynyl, ethylene, and ethyl substituents to tune steric bulk across the sp, sp2 and sp3 hybridized α‐carbons bearing 0, 1 and 2 hydrogen atoms. A linear relationship between affinity and cone angle is observed for anions bearing substituents larger than the −BF3− headgroup. This correlation predicted affinities of two new anions to within ±5 %. We explored how complexation affects the reactivity of fluoride exchange. The yield of fluoride transfer from R−BF3− to Lewis acid triphenylborane is correlated with cone angle. We predict that other rigid macrocycles, like commercially available bambusuril, could follow these trends.

Isochalcogenourea-catalyzed asymmetric (4 + 2)-heterocycloadditions of allenoates

Abstract Allenoates are versatile reagents that can be used for numerous (formal) cycloaddition reactions under (chiral) Lewis base catalysis. Most commonly, the catalysts of choice are phosphines, amines, and N-heterocyclic carbenes. We have recently established the use of readily available chiral isochalcogenoureas as catalysts for asymmetric (4 + 2)-heterocycloadditions of allenoates with various vinylogous acceptors. This represents a complementary approach for allenoate activation and gives access to various highly functionalized chiral dihydropyrans with good to excellent enantioselectivities and diastereoselectivities.

Lewis Acid-Catalyzed Unusual (4+3) Annulation of para-Quinone Methides with Bicyclobutanes: Access to Oxabicyclo[4.1.1] octanes.

Over the past few years, there has been a surge of interest in the chemistry of bicyclobutanes (BCBs). Although BCBs have been used to synthesize bicyclo[2.1.1]hexanes and bicyclo[3.1.1]heptanes, the synthesis of bicyclo[4.1.1]octanes has remained elusive. Herein, we report the first Lewis acid-catalyzed unexpected (4+3) annulation of para-quinonemethides (p-QMs) with BCBs allowing the synthesis of oxabicyclo[4.1.1]octanes proceeding under mild conditions. With 5 mol % of Bi(OTf)3, the reaction afforded the (4+3) annulated product in high regioselectivity and good functional group compatibility via a simultaneous Lewis acid activation of BCBs and p-QMs. The reaction is likely initiated by the 1,6-addition of Lewis acid activated BCBs to p-QMs followed by the C2-selective intramolecular addition of the phenol moiety to the generated cyclobutyl cation intermediate. Moreover, detailed mechanistic studies provided insight into the mechanism of the reaction.

Vacancy-strengthened Cu/Co composite oxides for efficient removal of methylene blue via peroxymonosulfate activation: Synergism of multiple Lewis acid sites

The development of suitable catalysts poses a challenge for the peroxymonosulfate (PMS) activation system used in decontamination process. Here we constructed a series of vacancy-strengthened Cu/Co composite oxides (CuCoO) composed of CuCo2O4 and CuO containing multiple Lewis acid sites. The results demonstrate that the as-prepared CuCoO-2 exhibited exceptional activation performance, broad-ranging degradation capacity, remarkable reusability and stability. The optimized CuCoO-2+PMS system achieved complete removal of methylene blue (MB) in 40 min, while maintaining a degradation efficiency of above 86 % even after 5 cycles. A comprehensive survey revealed that hydroxyl radical (·OH), sulfate radical (SO4.-) and singlet oxygen (1O2) collectively contribute to the degradation of MB in the CuCoO-2+PMS system. Especially, 1O2 was the dominant active species. The synergism of various Lewis acid sites (Cu+/2+, Co2+/3+ and VO/N) as unsaturated surface-bound centers played a pivotal role in promoting non-radical-driven activation of PMS. Meanwhile, the generated VO/N facilitated enhanced metallic valence circulation in CuCoO-2, strengthening electron mediation between PMS and MB. Additionally, possible degradation pathways and toxicity evaluations of MB and its intermediates were proposed. In conclusion, this work offers valuable insights into PMS activation by vacancy-strengthened CuCoO and is of great significance for water purification.

High-Efficiency Solar Transformation of Sugars via a Heterogenous Gallium(III) Catalyst.

Extremely limited research exploring the photocatalytic potential of main group metals, such as aluminum, gallium, and tin, has been undertaken due to their weak light harvesting properties. This study reports the efficient transformation of sugars to 5-hydroxymethylfurfural (HMF) with high yield employing an original heterogeneous photocatalyst comprising a gallium(III) complex immobilized on an alumina support. Under visible light irradiation, the reaction rate of HMF formation is ~143 times higher than the equivalent thermal reaction performed in the absence of light. The turnover number (TON) of the heterogeneous gallium(III) photocatalyst was as high as 1500, which was ca. two orders of magnitude higher than the TON of the homogeneous gallium(III) system. It is proposed that photoirradiation significantly enhances the Lewis acidity of the catalyst by forming a semi-coordination state between gallium(III) and N-donor ligands, enabling the increased interaction of reactant sugar molecules with gallium(III) active sites. Consistent with this, the photoresponsive coordination of the gallium(III) complex and the abstraction of the hydroxy group by the metal under irradiation with visible light is observed by NMR spectroscopy for the first time. These findings demonstrate that efficient photocatalysts derived from the main group elements can facilitate biomass conversion using visible light.

Desulfurization reactions of thiophene and cyclohexane over Zn and Nb modified zeolites in FCC process

In refineries, one of the most important production units is Fluid Catalytic Cracking (FCC). In them, the incorporation of additives to the catalyst is a very interesting option to reduce the sulfur content of gasoline, and thus obtain commercial fuel specifications. In this study, the transformation of thiophene into a cyclohexane (HC) stream for its in-situ desulfurization has been investigated, using catalysts based on niobium or zinc (6 wt%) incorporated into three types of zeolites: HUSY, HBeta and HZSM-5. Catalytic tests were carried out at 773 K in an automatically controlled fixed-bed tubular reactor. The reaction products were analyzed online by gas chromatography, with FID detector for hydrocarbons and SCD for sulfur compounds, and the catalysts were characterized by XRD, FTIR, N2 adsorption isotherms, HRTEM, XRF, 29Si and 27Al solid state NMR, NH3-TPD and FTIR-pyridine techniques. The results indicate that the addition of metals did not significantly alter the textural and structural properties of the zeolites, and that the Nb/HB and Nb/HY catalysts were the most effective in the conversion of thiophene, and selective toward paraffins and H2S mainly. The performance of these materials was attributed to the existence of an optimal balance between the Brønsted and Lewis acid sites, which increased the hydrogen transfer coefficient (HTC) during the cyclohexane transformation, resulting in higher desulfurization activity. The addition of zinc drastically increased the density of the Lewis acid sites and decreased the HTC, favoring the production of aromatics and thiophene condensation reactions, thus transferring sulfur to the gasoline streams. In general, the catalysts investigated are of significant interest as additives for sulfur reduction in FCC gasoline streams, either by producing easily removable H2S or by transferring sulfur to heavier fractions.

High‐Efficiency Solar Transformation of Sugars via a Heterogenous Gallium(III) Catalyst

AbstractExtremely limited research exploring the photocatalytic potential of main group metals, such as aluminum, gallium, and tin, has been undertaken due to their weak light harvesting properties. This study reports the efficient transformation of sugars to 5‐hydroxymethylfurfural (HMF) with high yield employing an original heterogeneous photocatalyst comprising a gallium(III) complex immobilized on an alumina support. Under visible light irradiation, the reaction rate of HMF formation is ~143 times higher than the equivalent thermal reaction performed in the absence of light. The turnover number (TON) of the heterogeneous gallium(III) photocatalyst was as high as 1500, which was ca. two orders of magnitude higher than the TON of the homogeneous gallium(III) system. It is proposed that photoirradiation significantly enhances the Lewis acidity of the catalyst by forming a semi‐coordination state between gallium(III) and N‐donor ligands, enabling the increased interaction of reactant sugar molecules with gallium(III) active sites. Consistent with this, the photoresponsive coordination of the gallium(III) complex and the abstraction of the hydroxy group by the metal under irradiation with visible light is observed by NMR spectroscopy for the first time. These findings demonstrate that efficient photocatalysts derived from the main group elements can facilitate biomass conversion using visible light.

Introducing Alkyl Selenocyanates as Bifunctional Reagents in Photoredox Catalysis: Divergent Access to Ambident Isomers of -SeCN.

Owing to their diverse biological activities and versatility as synthetic precursors, organoselonocyanes categorize themselves as vital compounds. However, a limited reagent pool restricts their utility. In the present work, alkyl selenocyanates are hereby established as new bifunctional reagents for the simultaneous transfer of an alkyl group in addition to -SeCN. These reagents, when merged with photocatalysis, provide a key to accessing organoselenocyanates from feedstock olefins in an efficient and atom-economic fashion. A route to the analogous isoselenocyanate isomers facilitated by Lewis acid catalysis is also reported, presenting a divergent strategy for accessing both ambident isomers of -SeCN in an efficient manner.

A MOF@MOF S-scheme Heterojunction with Lewis Acid-Base Sites Synergistically Boosts Cocatalyst-Free CO2Cycloaddition.

The photocatalytic cycloaddition reaction between CO2 and epoxide is one of the most promising green routes for CO2 utilization, for which high performance photocatalysts are intensely desired. Herein, we have constructed an S-scheme heterojunction of MIL-125@ZIF-67 modified by amino groups, which achieves a cyclic carbonate yield of as high as 99% without employing any co-catalyst, outperforming the previously reported photocatalysts. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and in-situ electron paramagnetic resonance (EPR) spectroscopy reveal the important role of photogenerated electron migration from Lewis acid (Co) sites to the O atom of epoxide in triggering its ring-opening (the rate-determining step of CO2 cycloaddition reaction) under the assistance of photogenerated hole. Synergistically and concurrently, the Lewis base (amino groups) sites activate CO2 to CO2*, facilitating the following CO2 cycloaddition. Such a synergistic effect provides a most favorable approach to design efficient heterogeneous photocatalysts with dual/multiple-active sites for CO2 cycloaddition reaction.

High Selectivity MEMS C2H2 Sensor for Transformer Fault Characteristic Gas Detection**

AbstractAcetylene (C2H2), as an important characteristic gas in transformer fault diagnosis, should be accurately detected and effectively distinguished from other dissolved gases (H2, CH4, C2H6, C2H4, CO, CO2), which is crucial to determine whether the fault occurs and the fault type, but also faces challenges now. The rational design and employment of rare earth and noble metals are expected to address this issue. In this work, SnO2‐3 at% Sm2O3‐1 at% PdO based MEMS gas sensor was prepared to achieve high performance detection of C2H2 which has a response value of 56 to 50 ppm C2H2, response/recovery time of 2 s/136 s, lower detection limit of 1 ppm, power consumption of 15.5 mW, and weak cross sensitivity to other transformer fault characteristic gases. Lewis acids and bases theory was used to explain the reason why rare earth Sm is a benefit element to improve selectivity to C2H2. The formation of oxygen vacancies and hetero junctions was used to explain the increased sensitivity of the material. This study proved the feasibility of rare earth and noble metals as potential additives to enable advanced gas‐sensitive materials for highly selective transformer fault characteristic gas C2H2 detection.

Borane-Catalyzed Enantioselective α-Alkylation of Unactivated 2-Alkylbenzoxazoles with Electron-Deficient Olefins.

Chiral borane-catalyzed reactions have recently emerged as a powerful tool for the enantioselective production of chiral scaffolds. In this study, we demonstrated for the first time that a chiral bisborane catalyst can be used for the α-functionalization of 2-alkylazaarenes; specifically, we accomplished unprecedented highly enantioselective α-alkylation of unactivated 2-alkylbenzoxazoles with electron-deficient olefins. The strong Lewis acidity and the steric bulk of the bisborane catalyst were essential to the observed reactivity and selectivity.

Synthesis of 3-methyl-3-buten-1-ol by supercritical CO2 in coordination with HZSM-5-catalyzed formaldehyde-isobutene Prins reaction.

The reaction solvent and catalyst play essential roles in the Prins reaction for the synthesis of 3-methyl-3-buten-1-ol (MBO) from formaldehyde and isobutene. The reactivity of the solid base-catalyzed Prins condensation reaction by formaldehyde and isobutene in supercritical CO2 was investigated using CsH2PO4-modified HZSM-5. We found that the alkaline sites of the alkali-loaded catalyst could extract the α-H on isobutene to generate olefin carbon-negative ions, while the supercritical CO2 with weak Lewis acidity could activate formaldehyde to carbon-positive ions, which can combine more easily with carbon-negative isobutene to react, thus improving the reactivity of the reaction system.

One-Pot Preparation of Nitro-Functionalized Bimetallic UiO-66(Zr-Hf) with Hierarchical Porosity for Oxidative Desulfurization Performance.

Preparation of multifunctional metal-organic frameworks (MOFs) offers new opportunities to obtain ultrahigh synergistic catalytic performance for heterogeneous reactions; however, the application of a one-pot method for preparing multifunctional MOFs remains challenging. Herein, we develop a one-pot green route for synthesizing bimetallic nitro-functionalized UiO-66(Zr-Hf)-NO2 with hierarchical porosity under solvent-free conditions. The optimal UiO-66(Zr-Hf0.6)-NO2 shows an ultrahigh enhancement of oxidative desulfurization (ODS) efficiency to oxidize sulfur compounds (1000 ppm sulfur) in a model fuel at 40 °C within 12 min due to the introduction of more active Hf sites in the nodes, the increased Lewis acidity of Zr/Hf-O nodes by the electron-withdrawing NO2 group, and the enhanced diffusion rates by the mesopores. The turnover frequency (TOF) over UiO-66(Zr-Hf0.6)-NO2 at 40 or 50 °C reaches 145.3 or 217.0 h-1 that surpasses the TOF of most reported MOF-based catalysts in the ODS reaction. Quenching and electron paramagnetic resonance experiments confirm that the formed Hf-OH on the Zr/Hf-O nodes can easily decompose the oxidant (H2O2) for generating a Hf-OOH-active intermediate and dominate the ODS efficiency. This contribution provides a one-pot solvent-free avenue to synthesize multifunctional MOFs for enhancing their catalytic activities for targeted applications.

Role of self-assembled molecules’ anchoring groups for surface defect passivation and dipole modulation in inverted perovskite solar cells

Abstract Inverted perovskite solar cells have gained prominence in industrial advancement due to their easy fabrication, low hysteresis effects, and high stability. Despite these advantages, their efficiency is currently limited by excessive defects and poor carrier transport at the perovskite-electrode interface, particularly at the buried interface between the perovskite and transparent conductive oxide (TCO). Recent efforts in the perovskite community have focused on designing novel self-assembled molecules (SAMs) to improve the quality of the buried interface. However, a notable gap remains in understanding the regulation of atomic-scale interfacial properties of SAMs between the perovskite and TCO interfaces. This understanding is crucial, particularly in terms of identifying chemically active anchoring groups. In this study, we used the star SAM ([2-(9H-carbazol-9-yl)ethyl] phosphonic acid) as the base structure to investigate the defect passivation effects of eight common anchoring groups at the perovskite-TCO interface. Our findings indicate that the phosphonic and boric acid groups exhibit notable advantages. These groups fulfill three key criteria: they provide the greatest potential for defect passivation, exhibit stable adsorption with defects, and exert significant regulatory effects on interface dipoles. Ionized anchoring groups exhibit enhanced passivation capabilities for defect energy levels due to their superior Lewis base properties, which effectively neutralize local charges near defects. Among various defect types, iodine vacancies are the easiest to passivate, whereas iodine-substituted lead defects are the most challenging to passivate. Our study provides comprehensive theoretical insights and inspiration for the design of anchoring groups in SAMs, contributing to the ongoing development of more efficient inverted perovskite solar cells.

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 p-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.

Formal High-Order Cycloadditions of Donor-Acceptor Cyclopropanes with Cycloheptatrienes.

The design of various cycloaddition/annulation processes is one of the most intriguing challenges in the development of donor-acceptor (D-A) cyclopropane chemistry. In this work, a new class of formal high-order [6+n]-cycloaddition and annulation processes of D-A cyclopropanes with cycloheptatriene systems has been designed and reported, to fill a significant gap in the chemistry of D-A cyclopropanes. The reactivity of methylated cycloheptatrienes from Me1 to Me5 as well as unsubstituted cycloheptatriene was studied in detail under GaCl3 activation conditions, which makes it possible to efficiently generate gallium 1,2-zwitterionic complexes or 1,3-zwitterionic intermediates starting from D-A cyclopropanes, while other Lewis acids are ineffective and non-selective. New examples of formal [6+2]-, [6+3]-, [6+4]-, [6+1]-, and [4+2]-cycloaddition and annulation reactions with cycloheptatrienes along with more complex processes were discovered. Cycloheptatriene itself can also successfully act as a hydride anion donor, which allows the ionic hydrogenation of D-A cyclopropanes to be performed under mild conditions. As a result, a number of efficient and highly diastereoselective protocols for the synthesis of seven-membered carbocycles has been developed.

Hydride Shuttle Catalysis: From Conventional to Inverse Mode.

Hydride shuttle catalysis has emerged as a powerful synthetic platform, enabling the selective formation of C-C bonds to yield sp3-rich structures. By virtue of the compelling reactivity of sterically encumbered Lewis acids from the frustrated Lewis pair regime, hydride shuttle catalysis enables the regioselective functionalization of alkyl amines at either the α- or β-position. In contrast to classical Lewis acid reactivity, the increased steric hindrance prevents interaction with the Lewis basic amine itself, instead leading to reversible abstraction of a hydride from the amine α-carbon. The created positive charge facilitates the occurrence of transformations before hydride rebound or a similar capture event happen. In this Perspective, we outline a broad selection of transformations featuring hydride shuttle catalysis, as well as the recently developed approach of inverse hydride shuttle catalysis. Both strategies give rise to a wide array of functionalized amines and offer elegant approaches to otherwise elusive bond formations.

Understanding the Electronic Effects of Lewis Acid Catalysts in Accelerating Polar Diels-Alder Reactions.

The electronic effects of Lewis acid (LA) catalysts in reducing the activation energies of polar Diels-Alder (P-DA) reactions have been studied within Molecular Electron Density Theory. To this end, a quantum topological energy partitioning scheme, namely, the Relative Interacting Atomic Energy (RIAE) analysis, is applied to the transition state structures (TSs) and the ground state of the reagents of two different LA-catalyzed P-DA reactions. Analyses of the ξEtotalX total energies of the two interacting frameworks f(X) show that the electronic energy stabilization of the electrophilic frameworks, resulting from the global electron density transfer (GEDT), is the cause of an effective decrease of the activation energies. On the other hand, an in-depth analysis of the ξEintraA intra-atomic energies of the atoms belonging to the electrophilic ethylenic framework in the LA-catalyzed P-DA reactions of cyclopentadiene with acrolein indicates that the strong electronic stabilization of the carbonyl carbon, resulting from the GEDT taking place at the TSs, is the main factor responsible for the decrease of the activation energies in these LA-catalyzed P-DA reactions. Finally, the increase in GEDT at the TSs of these P-DA reactions causes an increase in the larger C-C distance, resulting from the stabilization of the electrophilic framework, thereby decreasing the suggested Pauli repulsion.