Lewis Acid Research Articles

From Building Blocks to Catalysts: The Underinvestigated Potential of Boronic Acid Esters.

Lewis acids are crucial in chemistry, with applications in pharmaceuticals, agrochemicals, and materials science. In main-group chemistry, they offer alternatives to transition metals, prompting our study of halogenated boronic acid esters (BAEs). Although BAEs are well-known, their catalytic potential has been overlooked. Our investigation found their Lewis acidity superior to that of boron trifluoride and comparable to that of tris(pentafluorophenyl)borane. Additionally, their catalysis of the Sakurai allylation of aldehydes has been documented, paving the way for future advancements.

Electric-controlled adsorption and surface energy of cyclodextrins

By applying the electro-capillary rise method to control the solvent adsorption behavior for α-, β-, and γ–cyclodextrin, CD, respectively, this work showed that the voltage increase can enhance the adsorption of polar solvents, e.g. the water and formamide, while keep the adsorption of non-polar solvents, e.g. the hexane and diiodomethane, stabilization for these CDs. In terms of recorded dynamic adsorption curves, the electric-controlled surface properties of those CDs were furthermore estimated. Results showed that the increase of both the number of glucose units and voltage can enhance the surface free energy of CD due to the Lewis acid-base property of CD increased accordingly. The Lifshitz-van der Waals component has been found the main component of the surface energy of CD and was greater with the smaller number of glucose units uninfluenced by the electricity variety.

Engineering Lewis-Acid Defects on ZnO Quantum Dots by Trace Transition-Metal Single Atoms for High Glycerol-to-Glycerol Carbonate Conversion.

Efficient conversion of biomass wastes into valuable chemicals has been regarded as a sustainable approach for green and circular economy. Herein, a highly efficient catalytic conversion of glycerol (Gly) into glycerol carbonate (GlyC) by carbonylation with the commercially available urea is presented using low-cost transition metal single atoms supported on zinc oxide quantum dots (M1-ZnO QDs) as a catalyst without using any solvent. A facile one-step wet chemical synthesis allows various types of metal single atoms to simultaneously dope and introduce Lewis-acid defects in the ZnO QD structure. It is found that doping with a trace amount of isolated metal atoms greatly boosts the catalytic activity with Gly conversion of 90.7%, GlyC selectivity of 100.0%, and GlyC yield of 90.6%. Congruential results from both Density Functional Theory (DFT)and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS) studies reveal that the superior catalytic performance can be attributed to the enriched Lewis acid sites that endow optimal adsorption, formation of the intermediate for coupling between urea and Gly, and desorption of GlyC. Moreover, the tiny size of ZnO QDs efficiently promotes the accessibility of these active sites to the reactants.

23.81%-Efficiency Flexible Inverted Perovskite Solar Cells with Enhanced Stability and Flexibility via a Lewis Base Passivation.

Lewis base molecules bind the undercoordinated lead atoms at interfaces and grain boundaries, leading to the high efficiency and stability of flexible perovskite solar cells (PSCs). We demonstrated a highly efficient, stable, and flexible PSC via interface passivation using a Lewis base of tri(o-tolyl)phosphine (TTP). It not only induced an intimate interface contact and a complete deposition of the perovskite thin layers on hole transport layers (HTLs) but also led to a better perovskite with a raised crystallinity, fewer defects, and a better morphology, including fewer gullies, high uniformity, and low roughness. Furthermore, the TTP treatments induced a good alignment of energy levels among the perovskites, HTLs, and C60. The resultant flexible inverted PSCs exhibited a high power conversion efficiency (PCE) of 23.81%, which is one of the highest PCEs among these flexible inverted PSCs. Moreover, the optimized flexible PSCs exhibited high storage stability, superior operation stability, and enhanced mechanical flexibility. This study presents an effective method to substantially raise the PCE, stability, and mechanical flexibility of the flexible inverted perovskite photovoltaics.

Non-metallic Lewis acid sites enhance the activation of peracetic acid by cobalt-iron spinel for micropollutant decontamination: The critical role of Lewis acid sites as electron shuttles

Spinel is a common activator for peroxides, e.g., peracetic acid (PAA), in wastewater treatment, but suffers from insufficient electron transfer capacity at the active site and thus the limited activation capacity. Lewis acids are recognized for their capacity to interact with electrons and activate active sites, playing a crucial role in enhancing oxidant adsorption, shortening catalytic reaction distances, and increasing reaction kinetics. Herein, tellurium (Te), a typical Lewis acid, was introduced into cobalt-iron spinel (TCF) as an electron shuttler to improve the electron transfer capacity and thus boost the PAA activation. Results show that the degradation rate of sulfamethoxazole (SMX) with PAA activated via TCF is 4.0 times higher compared with cobalt-iron spinel (CF). Electron spin resonance (ESR) analysis and quenching experiments indicated acetylperoxyl radicals (CH3C(O)OO·) as the dominant reactive species in SMX degradation. Electrochemical tests revealed that the introduction of Lewis acid improved the charge transfer ability and charge utilization efficiency of the catalysts, exhibiting less electronic resistance and superior redox properties. Density functional theory (DFT) calculation suggests that the introduction of Lewis acid optimizes the electron distribution of the catalyst, and reduces the crucial reaction energy barriers for PAA activation. Additionally, the distinctive structural environment of Lewis acid renders TCF to exhibit excellent stability and resistance to complicated matrices in PAA activation. This work provides valuable insights into the Lewis acid regulating bimetallic catalyst performance and understanding the intrinsic interaction mechanisms, thereby establishing a critical groundwork for addressing prevailing wastewater treatment.

Iron-sulphur protein catalysed [4+2] cycloadditions in natural product biosynthesis

To the best of our knowledge, enzymes that catalyse intramolecular Diels-Alder ([4+2] cycloaddition) reactions are frequently reported in natural product biosynthesis; however, no native enzymes utilising Lewis acid catalysis have been reported. Verticilactam is a representative member of polycyclic macrolactams, presumably produced by spontaneous cycloaddition. We report that the intramolecular [4+2] cycloadditions can be significantly accelerated by ferredoxins (Fds), a class of small iron-sulphur (Fe-S) proteins. Through iron atom substitution by Lewis acidic gallium (Ga) iron and computational calculations, we confirm that the ubiquitous Fe-S cluster efficiently functions as Lewis acid to accelerate the tandem [4+2] cycloaddition and Michael addition reactions by lowering free energy barriers. Our work highlights Nature’s ingenious strategy to generate complex molecule structures using the ubiquitous Fe-S protein. Furthermore, our study sheds light on the future design of Fd as a versatile Lewis acid catalyst for [4+2] cycloaddition reactions.

Iron-sulphur protein catalysed [4+2] cycloadditions in natural product biosynthesis

To the best of our knowledge, enzymes that catalyse intramolecular Diels-Alder ([4+2] cycloaddition) reactions are frequently reported in natural product biosynthesis; however, no native enzymes utilising Lewis acid catalysis have been reported. Verticilactam is a representative member of polycyclic macrolactams, presumably produced by spontaneous cycloaddition. We report that the intramolecular [4+2] cycloadditions can be significantly accelerated by ferredoxins (Fds), a class of small iron-sulphur (Fe-S) proteins. Through iron atom substitution by Lewis acidic gallium (Ga) iron and computational calculations, we confirm that the ubiquitous Fe-S cluster efficiently functions as Lewis acid to accelerate the tandem [4+2] cycloaddition and Michael addition reactions by lowering free energy barriers. Our work highlights Nature’s ingenious strategy to generate complex molecule structures using the ubiquitous Fe-S protein. Furthermore, our study sheds light on the future design of Fd as a versatile Lewis acid catalyst for [4+2] cycloaddition reactions.

Lewis Acid-Base Pairs: The Bonding Rule of Closed-Shell M···M' Interactions (M = HgII/PdII; M' = AgI/AuI).

Metallophilic interactions are the widespread interactions in multimetal clusters to orientate closed-shell metal self-assembly form linear, facet, or block clusters. The closed-shell metal cation does not have empty valence orbitals, but is able to attract each other. It is still a conundrum to understand the resource in balancing the strong Coulomb repulsion between two cations. Most traditional descriptions attribute the counterintuitive attractions to London dispersion, Pauli repulsions, and ambiguous orbital interactions. However, neither the dispersion nor the unsourced donor-acceptor interaction can be applied to explain the saturability and directionality in multimetal clusters, where the M···M' structure is the basic molecular unit. Here, we clarify the origination of the covalency in closed-shell metallophilic interactions based on the study of heterobimetallic compounds composed of d10-d8 species (AgI/AuI-PdII) and d10-d10 species (AgI/AuI-HgII) obtained from experiments. The inner d electrons not only participate in the metallophilic interactions but also show different Lewis acidity and basicity in the formation of M···M' structures. The present work not only provides us a novel covalent perspective to visualize the closed-shell M···M' interactions but also unveils the truth of metallophilic interactions.

Selenylated Analogs of Tacrine: Synthesis, in Silico and in VitroStudies of Toxicology and Antioxidant Properties.

We present our results on the synthesis and preliminary in silico and in vitro studies of the toxicology and antioxidant properties of selenylated analogs of Tacrine. Initially, we synthesized 2-aminobenzonitriles containing an organic selenium moiety, resulting in sixteen compounds with various substituents linked to the portion derived from diorganyl diselenide. These compounds were then used as substrates in reactions with cyclic ketones, in the presence of 1.4 equivalents of trifluoroboroetherate as a Lewis acid, to synthesize selenylated analogs of Tacrine with yields ranging from 20% to 87%. In silico studies explored computational parameters related to antioxidant activity and hepatotoxicity. In vitro studies elucidated the antioxidant effects of Tacrine and its selenium hybrid (TSe) in neutralizing ABTS radicals, scavenging DPPH radicals, and reducing iron ions. Additionally, the acute oral toxicity of one synthesized compound was evaluated.

Chemoselective Synthesis of Cyanoformamides from Isocyanates and a Highly Reactive Nitrile Anion Reservoir

The direct addition of a nitrile anion to isocyanates is reported for a straightforward preparation of valuable cyanoformamides. Through the proper activation of an adequate silicon‐ate complex precursor (PhMe2SiCN) with a Lewis base (potassium tert‐amylate) under Barbier‐type conditions, the cyanide was instantaneously released, thus yielding the desired motif with full chemocontrol. No particular structural restriction was noticed for engaging in the transformation a wide series of commercially available isocyanates.

Overcoming Challenges in O-Nitration: Selective Alcohol Nitration Deploying N,6-Dinitrosaccharin and Lewis Acid Catalysis.

Nitrate esters hold pivotal roles in pharmaceuticals, energetic materials, and atmospheric processes, motivating the development of efficient synthesis routes. Here, we present a novel catalytic method for the synthesis of nitrates via the direct O-nitration of alcohols, addressing limitations of current traditional methods. Leveraging bench-stable and recoverable N,6-dinitrosaccharin reagent, our catalytic strategy employs magnesium triflate to achieve mild and selective O-nitration of alcohols, offering broad substrate scope and unprecedented large functional group tolerance (e.g. alkenes, alkynes, carbonyls). DFT mechanistic studies reveal a dual role of the magnesium catalyst in the activation of both the nitrating reagent and the alcohol substrate. They also unveil a barrierless proton transfer upon formation of a widely-accepted - yet elusive in solution - nitrooxonium ion intermediate. Overall, our work contributes to the development of mild, selective, and sustainable approaches to nitrates synthesis, with potential applications in drug discovery, materials science, and environmental chemistry.

Overcoming Challenges in O‐Nitration: Selective Alcohol Nitration Deploying N,6‐Dinitrosaccharin and Lewis Acid Catalysis

Nitrate esters hold pivotal roles in pharmaceuticals, energetic materials, and atmospheric processes, motivating the development of efficient synthesis routes. Here, we present a novel catalytic method for the synthesis of nitrates via the direct O‐nitration of alcohols, addressing limitations of current traditional methods. Leveraging bench‐stable and recoverable N,6‐dinitrosaccharin reagent, our catalytic strategy employs magnesium triflate to achieve mild and selective O‐nitration of alcohols, offering broad substrate scope and unprecedented large functional group tolerance (e.g. alkenes, alkynes, carbonyls). DFT mechanistic studies reveal a dual role of the magnesium catalyst in the activation of both the nitrating reagent and the alcohol substrate. They also unveil a barrierless proton transfer upon formation of a widely‐accepted — yet elusive in solution — nitrooxonium ion intermediate. Overall, our work contributes to the development of mild, selective, and sustainable approaches to nitrates synthesis, with potential applications in drug discovery, materials science, and environmental chemistry.

Overcoming Challenges in O-Nitration: Selective Alcohol Nitration Deploying N,6-Dinitrosaccharin and Lewis Acid Catalysis.

Nitrate esters hold pivotal roles in pharmaceuticals, energetic materials, and atmospheric processes, motivating the development of efficient synthesis routes. Here, we present a novel catalytic method for the synthesis of nitrates via the direct O-nitration of alcohols, addressing limitations of current traditional methods. Leveraging bench-stable and recoverable N,6-dinitrosaccharin reagent, our catalytic strategy employs magnesium triflate to achieve mild and selective O-nitration of alcohols, offering broad substrate scope and unprecedented large functional group tolerance (e.g. alkenes, alkynes, carbonyls). DFT mechanistic studies reveal a dual role of the magnesium catalyst in the activation of both the nitrating reagent and the alcohol substrate. They also unveil a barrierless proton transfer upon formation of a widely-accepted - yet elusive in solution - nitrooxonium ion intermediate. Overall, our work contributes to the development of mild, selective, and sustainable approaches to nitrates synthesis, with potential applications in drug discovery, materials science, and environmental chemistry.

Reactivity and Steric Parameters from 2D to 3D Bulky Pyridines: Increasing Steric Demand at Nitrogen with Chiral Azatriptycenes

AbstractSterically hindered pyridines embedded in a three‐dimensional triptycene framework have been synthesized, and their resolution by chiral HPLC enabled access to unprecedented enantiopure pyridines exceeding the known steric limits. The design principles for new axially chiral pyridine derivatives are then described. To rationalize their associations with Lewis acids and transition metals, a comprehensive determination of the steric and electronic parameters for this new class of pyridines was performed. This led to the general parameterization of the steric parameters (percent buried volume %VBur, Tolman cone angle θ, and He8_steric descriptor) for a large set of two‐ and three‐dimensional pyridine derivatives. These parameters are shown to describe quantitatively their interactions with carbon‐ and boron‐centered Lewis acids and were used to predict the ΔG° of association with the prototypical B(C6F5)3 Lewis acid widely used in frustrated Lewis pair catalysis. This first parameterization of pyridine sterics is a fundamental basis for the future development of predictive reactivity models and for guiding new applications of bulky and chiral pyridines in organocatalysis, frustrated Lewis pairs, and transition‐metal catalysis.

Insight into the Metal-Involving Chalcogen Bond in the PdII/PtII-Based Complexes: Comparison with the Conventional Chalcogen Bond.

The metal-involving Ch···M chalcogen bond and the conventional Ch···O chalcogen bond between ChX2 (Ch = Se, Te; X = CCH, CN) acting as a Lewis acid and M(acac)2 (M = Pd, Pt; Hacac = acetylacetone) acting as a Lewis base were studied by density functional theory calculations. It has been observed that the nucleophilicity of the PtII complexes is higher than that of the corresponding PdII complexes. As a result, the PtII complexes tend to exhibit a more negative interaction energy and larger orbital interaction. The strength of the chalcogen bond increases with the increase of the chalcogen atom and the electronegativity of the substituent on the Lewis acid and vice versa. The metal-involving chalcogen bond shows a typical weak closed-shell noncovalent interaction in the (HCC)2Ch···M(acac)2 complexes, while it exhibits a partially covalent nature in the (NC)2Ch···M(acac)2 complexes. The conventional Ch···O chalcogen bond displays the character of a weak noncovalent interaction, and its strength is generally weaker than that of metal-involving Ch···M interactions. It could be argued that the metal-involving chalcogen bond is primarily determined by the correlation term, whereas the conventional chalcogen bond is mainly governed by the electrostatic interaction.

A thermodynamic model of the surface hydroxylation of γ-Al2O3.

Rational design of γ-alumina-based catalysts relies on an extensive understanding of the distribution of hydroxyl groups on the surface of γ-alumina and their physicochemical properties, which remain unclear and challenging to determine experimentally due to the structural complexity. In this work, by means of DFT and thermodynamic calculations, various hydroxylation modes of γ-alumina (110) and (100) surfaces at different OH coverages were evaluated, based on which a thermodynamic model to reflect the relationship between temperature and the surface structure was established and the stable hydroxylation modes under experimental conditions were predicted. This enables us to identify the experimentally measured IR spectra. The effect of hydroxyl coverages on the surface Lewis acidity was then analyzed, showing that the presence of hydroxyl groups could promote the Lewis acidity of neighboring Al sites. This work provides fundamental insights into the molecular level understanding of the surface properties of γ-alumina and benefits the rational design of alumina-based catalysts.

Investigation of Polysulfide Adsorption on FeS2 Additive in Sulfur Cathode of Li–S Battery by Ex situ UV–Visible Spectroscopy

The performance of lithium–sulfur (Li–S) rechargeable batteries is strongly dependent on the entrapment of the higher‐order intermediate polysulfides at the sulfur cathode. An attracting way of preventing the polysulfide shuttle is by introducing a polar host which can form a Lewis acid–base complex with polysulfides. Herein, the Li–S battery by incorporating iron sulfides (FeS2) as a polar Lewis acid to entrap higher‐order polysulfides at the cathode center is investigated. FeS2/S cathode demonstrates largely improved retention of capacity compared to C/S cathode (capacity fading per cycle of 0.12% and 0.80% for FeS2/S and C/S respectively) and good rate performance in Li–S batteries compared to conventional carbon–sulfur (C/S) cathode. This is attributed to the decrease in polysulfide dissolution and better retention of active sulfur in the cathode during battery cycling which is due to the polar FeS2 additive that well anchors polysulfides. The effect of FeS2 in preventing the shuttle mechanism is demonstrated by ex situ UV–vis spectroscopy and ex situ Raman spectroscopy studies.

Investigation of Polysulfide Adsorption on FeS2 Additive in Sulfur Cathode of Li–S Battery by Ex situ UV–Visible Spectroscopy

The performance of lithium–sulfur (Li–S) rechargeable batteries is strongly dependent on the entrapment of the higher‐order intermediate polysulfides at the sulfur cathode. An attracting way of preventing the polysulfide shuttle is by introducing a polar host which can form a Lewis acid–base complex with polysulfides. Herein, the Li–S battery by incorporating iron sulfides (FeS2) as a polar Lewis acid to entrap higher‐order polysulfides at the cathode center is investigated. FeS2/S cathode demonstrates largely improved retention of capacity compared to C/S cathode (capacity fading per cycle of 0.12% and 0.80% for FeS2/S and C/S respectively) and good rate performance in Li–S batteries compared to conventional carbon–sulfur (C/S) cathode. This is attributed to the decrease in polysulfide dissolution and better retention of active sulfur in the cathode during battery cycling which is due to the polar FeS2 additive that well anchors polysulfides. The effect of FeS2 in preventing the shuttle mechanism is demonstrated by ex situ UV–vis spectroscopy and ex situ Raman spectroscopy studies.

New Polymeric Disulfonimide for Heterogeneous Silicon Lewis Acid Catalysis

AbstractA new heterogeneous polymeric disulfonimide was very easily synthesized from simple commercially available reagents. The new cost-effective catalytic material exhibited a tremendously enhanced reactivity in a benchmark Mukaiyama aldol reaction via silicon Lewis acid activation when compared with common acidic resins. Moreover, the reported polymeric disulfonimide exhibits outstanding robustness, as confirmed by its good thermal stability and excellent recyclability.

Divergent Synthesis of Sulfur‐Containing Bridged Cyclobutanes by Lewis Acid Catalyzed Formal Cycloadditions of Pyridinium 1,4‐Zwitterionic Thiolates and Bicyclobutanes

AbstractBridged cyclobutanes and sulfur heterocycles are currently under intense investigation as building blocks for pharmaceutical drug design. Two formal cycloaddition modes involving bicyclobutanes (BCBs) and pyridinium 1,4‐zwitterionic thiolate derivatives were described to rapidly expand the chemical space of sulfur‐containing bridged cyclobutanes. By using Ni(ClO4)2 as the catalyst, an uncommon higher‐order (5+3) cycloaddition of BCBs with quinolinium 1,4‐zwitterionic thiolate was achieved with broad substrate scope under mild reaction conditions. Furthermore, the first Lewis acid‐catalyzed asymmetric polar (5+3) cycloaddition of BCB with pyridazinium 1,4‐zwitterionic thiolate was accomplished. In contrast, pyridinium 1,4‐zwitterionic thiolates undergo an Sc(OTf)3‐catalyzed formal (3+3) reaction with BCBs to generate thia‐norpinene products, which represent the initial instance of synthesizing 2‐thiabicyclo[3.1.1]heptanes (thia‐BCHeps) from BCBs. Moreover, we have successfully used this (3+3) protocol to rapidly prepare thia‐BCHeps‐substituted analogues of the bioactive molecule Pitofenone. Density functional theory (DFT) computations imply that kinetic factors govern the (5+3) cycloaddition reaction between BCB and quinolinium 1,4‐zwitterionic thiolate, whereas the (3+3) reaction involving pyridinium 1,4‐zwitterionic thiolates is under thermodynamic control.