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Various Lewis acid–Lewis base interactions are discussed as initiating chemical reactions and processes. For example, the hydrogen bond is often a preliminary stage of the proton transfer process or the tetrel and pnicogen bonds lead sometimes to the SN2 reactions. There are numerous characteristics of interactions being first stages of reactions; one can observe a meaningful electron charge transfer from the Lewis base unit to the Lewis acid; such interactions possess at least partly covalent character, one can mention other features. The results of different methods and approaches that are applied in numerous studies to describe the character of interactions are presented here. These are, for example, the results of the Quantum Theory of Atoms in Molecules, of the decomposition of the energy of interaction or of the structure-correlation method.

Prototypical Lewis acids, such as boranes, derive their reactivity from electronic unsaturation. Here, we report the Lewis acidity and catalytic application of electronically saturated phosphorus-centered electrophilic acceptors. Organofluorophosphonium salts of the formula [(C₆F₅) 3-x Ph x PF][B(C₆F₅)₄] (x = 0 or 1; Ph, phenyl) are shown to form adducts with neutral Lewis bases and to react rapidly with fluoroalkanes to produce difluorophosphoranes. In the presence of hydrosilane, the cation [(C₆F₅)₃PF]⁺ is shown to catalyze the hydrodefluorination of fluoroalkanes, affording alkanes and fluorosilane. The mechanism demonstrates the impressive fluoride ion affinity of this highly electron-deficient phosphonium center.

In contrast to the wealth of catalytic systems that are available to control the stereochemistry of thermally promoted cycloadditions, few similarly effective methods exist for the stereocontrol of photochemical cycloadditions. A major unsolved challenge in the design of enantioselective catalytic photocycloaddition reactions has been the difficulty of controlling racemic background reactions that occur by direct photoexcitation of substrates while unbound to catalyst. Here, we describe a strategy for eliminating the racemic background reaction in asymmetric [2 + 2] photocycloadditions of α,β-unsaturated ketones to the corresponding cyclobutanes by using a dual-catalyst system consisting of a visible light–absorbing transition-metal photocatalyst and a stereocontrolling Lewis acid cocatalyst. The independence of these two catalysts enables broader scope, greater stereochemical flexibility, and better efficiency than previously reported methods for enantioselective photochemical cycloadditions.

Terephthalic acid (PTA), a monomer in the synthesis of polyethylene terephthalate (PET), is obtained by the oxidation of petroleum-derived p-xylene. There is significant interest in the synthesis of renewable, biomass-derived PTA. Here, routes to PTA starting from oxidized products of 5-hydroxymethylfurfural (HMF) that can be produced from biomass are reported. These routes involve Diels-Alder reactions with ethylene and avoid the hydrogenation of HMF to 2,5-dimethylfuran. Oxidized derivatives of HMF are reacted with ethylene over solid Lewis acid catalysts that do not contain strong Brønsted acids to synthesize intermediates of PTA and its equally important diester, dimethyl terephthalate (DMT). The partially oxidized HMF, 5-(hydroxymethyl)furoic acid (HMFA), is reacted with high pressure ethylene over a pure-silica molecular sieve containing framework tin (Sn-Beta) to produce the Diels-Alder dehydration product, 4-(hydroxymethyl)benzoic acid (HMBA), with 31% selectivity at 61% HMFA conversion after 6 h at 190 °C. If HMFA is protected with methanol to form methyl 5-(methoxymethyl)furan-2-carboxylate (MMFC), MMFC can react with ethylene in the presence of Sn-Beta for 2 h to produce methyl 4-(methoxymethyl) benzenecarboxylate (MMBC) with 46% selectivity at 28% MMFC conversion or in the presence of a pure-silica molecular sieve containing framework zirconium (Zr-Beta) for 6 h to produce MMBC with 81% selectivity at 26% MMFC conversion. HMBA and MMBC can then be oxidized to produce PTA and DMT, respectively. When Lewis acid containing mesoporous silica (MCM-41) and amorphous silica, or Brønsted acid containing zeolites (Al-Beta), are used as catalysts, a significant decrease in selectivity/yield of the Diels-Alder dehydration product is observed.

The Lewis acidic nature of both [Zn4(µ3-OH)2(d-2,4-cbs)2(H2O)4].5H2On(Zn-CBS) and its ZnO nanostructures (ZnO_1, 3D microflower; ZnO_2, 3D polyhedron; and ZnO_3, 1D nanorod) was explored for the comparative study of the C–C and C-N bond forming reactions, such as Knoevenagel condensation, Friedel–Crafts alkylation and Strecker reaction, with various substrates. Notably, the nanorod (ZnO_3) is found to be an exceptionally efficient heterogeneous catalyst in comparison to its parent Zn-CBS for the Knoevenagel condensation reaction showing 100% conversion in 15 min with only 2 mol% catalyst in methanol at 25 °C. Similar catalytic results were obtained in the multicomponent Strecker reaction where ZnO_3 showed an enhanced catalytic activity in water as compared to Zn-CBS. However, for the Friedel–Crafts alkylation reaction, Zn-CBS was better than ZnO_3. These highly efficient catalysts are recyclable for three consecutive runs without any notable change in the catalytic activity. Their mechanism of action for all three reactions is also explained.

New enzyme catalysts are usually engineered by repurposing the active sites of natural proteins. Here we show that design and directed evolution can be used to transform a non-natural, functionally naive zinc-binding protein into a highly active catalyst for an abiological hetero-Diels–Alder reaction. The artificial metalloenzyme achieves >104 turnovers per active site, exerts absolute control over reaction pathway and product stereochemistry, and displays a catalytic proficiency (1/KTS = 2.9 × 1010 M−1) that exceeds all previously characterized Diels–Alderases. These properties capitalize on effective Lewis acid catalysis, a chemical strategy for accelerating Diels–Alder reactions common in the laboratory but so far unknown in nature. Extension of this approach to other metal ions and other de novo scaffolds may propel the design field in exciting new directions.A de novo designed zinc-binding protein has been converted into a highly active, stereoselective catalyst for a hetero-Diels–Alder reaction. Design and directed evolution were used to effectively harness Lewis acid catalysis and create an enzyme more proficient than other reported Diels–Alderases.

Fe/Co metal–organic frameworks (Fe/Co-MOFs) were synthesized by a solvothermal method using terephthalic acid (BDC) and N , N -dimethylformamide (DMF), and the removal of tetracycline (TC) was accomplished with high efficiency. We explored the influence of Co content on the characteristics of the bimetallic MOFs. The structures of the Fe/Co-MOFs were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), and nitrogen physisorption analysis. The results showed that Co 2+ was well loaded in the Fe-MOFs. Due to the different coordination behaviors of Co 2+ and Fe 3+ with BDC, the Fe/Co-MOFs formed a new 3D structure with a spike ball-like morphology. The morphology changed dramatically, leading to a large specific surface area, and the activity of the Lewis acid sites was greatly enhanced. Through FTIR and XPS analysis before and after adsorption, the adsorption process was mainly affected by coordination action. The largest TC adsorption capacity was observed when the Fe/Co ratio was equal to 1:3, and the saturated adsorption capacity was 139.8 mg g −1 . Graphical abstract

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. Native enzymes using Lewis acid catalysis in intramolecular [4+2] cycloaddition remain unreported. Herein, the authors report the ferredoxins catalyzed [4+2] cycloaddition in polycyclic macrolactam verticilactam biosynthesis using a transition-metal center.

In this concept article, we consider the notion of ‘frustrated Lewis pairs’ (FLPs). While the original use of the term referred to steric inhibition of dative bond formation in a Lewis pair, work in the intervening decade demonstrates the limitation of this simplistic view. Analogies to known transition metal chemistry and the applications in other areas of chemistry are considered. In the light of these findings, we present reflections on the criteria for a definition of the term ‘frustrated Lewis pair’. Segregation of the Lewis acid and base and the kinetic nature of FLP reactivity are discussed. We are led to the conclusion that, while an all-inclusive definition of FLP is challenging, the notion of ‘FLP chemistry’ is more readily recognized. This article is part of the themed issue ‘Frustrated Lewis pair chemistry’.

In this work, the formation of levulinic acid (LA) as one of the top-twelve chemical building blocks from glucose was studied. In particular, the formulations of heterogeneous acid catalysts based on SBA-15, MCM-41 mesoporous silica was carried out and their performance in catalytic conversion of glucose to LA were assessed and compared with commercial H-Beta-25 (SiO2/Al2O3 = 25) microporous zeolite. The high surface area, suitable porosity, balanced acid sites were considered as the main factors of a proper catalytic performance. Thus, essential modification of mesoporous SBA-15 and MCM-41 materials was carried out by introducing Al in their structures for Lewis acid sites improvement. Alumina was introduced to SBA-15 by post synthesis evaporation impregnation method while it was embedded inside the MCM-41 mesoporous material during the synthesis. In addition, Brønsted acidity was introduced via post-synthesis grafting of sulfonic acid groups. The textural and morphological features and acidity of the materials were investigated using N2 physisorption, SEM, EDX, TEM, XRD and pyridine-FTIR. All catalysts were tested for aqueous glucose conversion in an autoclave at 180 °C. Al-MCM-SO3H has shown the best performance with 54% of LA yield after 4 h reaction. According to Py-FTIR introduction of alumina and sulfonic acid groups improved weak and medium Lewis and Brønsted acid sites. However, the Brønsted to Lewis acid site ratio (B/L) was higher for Al-MCM-SO3H compared to SBA-Al-SO3H leading the reaction pathway to LA. H-Beta-25 zeolite displayed a poor performance because of harsh medium and strong acid sites catalyzing humins formation.