Showing papers on "Lewis acids and bases published in 2011"

Nb2O5·nH2O as a heterogeneous catalyst with water-tolerant Lewis acid sites.

Abstract: Niobic acid, Nb(2)O(5)·nH(2)O, has been studied as a heterogeneous Lewis acid catalyst. NbO(4) tetrahedra, Lewis acid sites, on Nb(2)O(5)·nH(2)O surface immediately form NbO(4)-H(2)O adducts in the presence of water. However, a part of the adducts can still function as effective Lewis acid sites, catalyzing the allylation of benzaldehyde with tetraallyl tin and the conversion of glucose into 5-(hydroxymethyl)furfural in water.

Synthesis and Characterization of a Neutral Tricoordinate Organoboron Isoelectronic with Amines

Abstract: Amines and boranes are the archetypical Lewis bases and acids, respectively. The former can readily undergo one-electron oxidation to give radical cations, whereas the latter are easily reduced to afford radical anions. Here, we report the synthesis of a neutral tricoordinate boron derivative, which acts as a Lewis base and undergoes one-electron oxidation into the corresponding radical cation. These compounds can be regarded as the parent borylene (H-B:) and borinylium (H-B(+.)), respectively, stabilized by two cyclic (alkyl)(amino)carbenes. Ab initio calculations show that the highest occupied molecular orbital of the borane as well as the singly occupied molecular orbital of the radical cation are essentially a pair and a single electron, respectively, in the p(π) orbital of boron.

Metal interactions at the biochar-water interface: energetics and structure-sorption relationships elucidated by flow adsorption microcalorimetry.

Abstract: Plant-derived biochars exhibit large physicochemical heterogeneity due to variations in biomass chemistry and combustion conditions However, the influence of biochar heterogeneity on biochar-metal interaction mechanisms has not been systematically described We used flow adsorption microcalorimetry to study structure-sorption relationships between twelve plant-derived biochars and two metals (K(+) and Cd(2+)) of different Lewis acidity Irrespective of the biochar structure, sorption of K(+) (a hard Lewis acid) occurred predominantly on deprotonated functional groups via ion exchange with molar heats of adsorption (ΔH(ads)) of -4 kJ mol(-1) to -8 kJ mol(-1) By comparison, although ion exchange could not be completely ruled out, our data pointed to Cd(2+) (a soft Lewis acid) sorption occurring predominantly via two distinct cation-π bonding mechanisms, each with ΔH(ads) of +17 kJ mol(-1) The first, evident in low charge-low carbonized biochars, suggested Cd(2+)-π bonding to soft ligands such as -C ═ O; while the second, evident in low charge-highly carbonized biochars, pointed to Cd(2+)-π bonding with electron-rich domains on aromatic structures Quantitative contributions of these mechanisms to Cd(2+) sorption can exceed 3 times that expected for ion exchange and therefore could have significant implications for the biogeochemical cycling of metals in fire-impacted or biochar-amended systems

Synthesis and reactions of N-heterocyclic carbene boranes.

Abstract: Boranes are widely used Lewis acids and N-heterocyclic carbenes (NHCs) are popular Lewis bases, so it is remarkable how little was known about their derived complexes until recently NHC-boranes are typically readily accessible and many are so stable that they can be treated like organic compounds rather than complexes They do not exhibit “borane chemistry”, but instead are proving to have a rich chemistry of their own as reactants, as reagents, as initiators, and as catalysts They have significant potential for use in organic synthesis and in polymer chemistry They can be used to easily make unusual complexes with a broad spectrum of functional groups not usually seen in organoboron chemistry Many of their reactions occur through new classes of reactive intermediates including borenium cations, boryl radicals, and even boryl anions This Review provides comprehensive coverage of the synthesis, characterization, and reactions of NHC-boranes

Activation of Carbonyl-Containing Molecules with Solid Lewis Acids in Aqueous Media

Abstract: Current interest in reacting carbonyl-containing molecules in aqueous media is primarily due to the growing emphasis on conversion of biomass to fuels and chemicals. Recently, solid Lewis acids have been shown to perform catalytic reactions with carbonyl-containing molecules such as sugars in aqueous media. Here, catalysis mediated by Lewis acids is briefly discussed, Lewis acid solids that perform catalysis in aqueous media are then described, and the review is concluded with a few comments on the outlook for the future.

Geminal phosphorus/aluminum-based frustrated Lewis pairs: C-H versus C≡C activation and CO2 fixation

Abstract: Catch it! Geminal phosphorus/aluminum-based frustrated Lewis pairs (FLPs) are easily obtained by hydroalumination of alkynylphosphines. These FLPs can activate terminal acetylenes by two competitive pathways, which were analyzed by DFT calculations, and they can bind carbon dioxide reversibly. Therefore, alongside polyfluorinated boranes, alanes are also ideal Lewis acids for FLP chemistry. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Effect of NH2 and CF3 functionalization on the hydrogen sorption properties of MOFs

Abstract: The hydrogen adsorption capacity and heat of adsorption at 77 K have been evaluated for several porous metal terephthalate MOFs (MIL-53(Fe), MIL-125(Ti) and UiO-66(Zr)), as well as in their –NH2 and –(CF3)2 functionalized isoreticular structures. The capacity of hydrogen is basically related to the textural properties of the solids and not to their composition. The heats of adsorption at low coverage are on the whole close to those usually reported for MOFs (6–7 kJ mol−1), except for the UiO-66(Zr) and MIL-53(Fe)-(CF3)2 analogues, whereas the presence of Lewis acid sites and/or a confinement effect enhances significantly the strength of interaction with hydrogen.

Catalytic Nazarov Cyclization: The State of the Art

Abstract: This transformation has received tremendous attention in the field of organic synthesis due to its ability to form synthetically versatile cyclopentenones. With appropriately substituted dienones, the orbital reorganization implants new stereocenters in the oxyallyl cation (3), which are retained in the pentacyclic product (5). In terms of the reaction mechanism (Scheme 1), the coordination of either a Brønsted or a Lewis acid (LA) promoter to the dienone (1) induces the electrocyclization of the pentadienyl cation (2). Consistent with the principle of conservation of orbital symmetry, a conrotatory closure proceeds to form an oxyallyl cation (3), having an anti relationship between R and R. This intermediate has the potential to go through diverse reaction pathways such as carbocationic rearrangements or interception from 3 with external nucleophiles. Traditionally, elimination of a proton generates a promoter bound enolate (4) that furnishes a cyclopentenone (5) upon reprotonation. Since the 2005 reviews of the Nazarov reaction by Frontier and Collison, Tius, and Pellissier, a number of new catalytic and stoichiometric reactions 15, 19–60] have been reported that address synthetic problems associated with this transformation. This review will discuss the catalytic advances in the Nazarov cyclization since 2005, including the design of new catalysts, formulation of new conditions, variations of the electrocyclization, tandem processes, strategies that result in enantioinduction, and application to natural product synthesis. The catalysts have been categorized into the following subclasses: Brønsted acids, organocatalysts, heterogeneous catalysts, Lewis acids including transition-metal catalysts, and a supramolecular host.

Proton-catalyzed, silane-fueled Friedel-Crafts coupling of fluoroarenes.

Abstract: The venerable Friedel-Crafts reaction appends alkyl or acyl groups to aromatic rings through alkyl or acyl cation equivalents typically generated by Lewis acids. We show that phenyl cation equivalents, generated from otherwise unreactive aryl fluorides, allow extension of the Friedel-Crafts reaction to intramolecular aryl couplings. The enabling feature of this reaction is the exchange of carbon-fluorine for silicon-fluorine bond enthalpies; the reaction is activated by an intermediate silyl cation. Catalytic quantities of protons or silyl cations paired with weakly coordinating carborane counterions initiate the reactions, after which proton transfer in the final aromatization step regenerates the active silyl cation species by protodesilylation of a quaternary silane. The methodology allows the high-yield formation of a range of tailored polycyclic aromatic hydrocarbons and graphene fragments.

Cellulose hydrothermal conversion promoted by heterogeneous Bronsted and Lewis acids: Remarkable efficiency of solid Lewis acids to produce lactic acid

Abstract: Crystalline cellulose treated in hydrothermal conditions (190 °C, 24 h) is partially solubilised, 30%, into water soluble oligosaccharides/polymers with the formation of small amounts of glucose and 5-HMF. In the presence of solid Bronsted catalysts such as Cs2HPW12O40 and HY zeolite, the extent of the cellulose depolymerisation was not changed when no leaching occurred. However, a quite different products distribution was obtained, in favour of further transformations of glucose and HMF in levulinic and formic acids. On the opposite, solid Lewis acids such as tungstated zirconia (ZrW) and tungstated alumina (AlW) exhibited a remarkable promoting effect on the cellulose depolymerisation which was raised up to 45% while an unexpected decrease of the proportion of water soluble oligosaccharides/polymers was observed. Yields of 27 mol% and 18.5 mol% in lactic acid were achieved on AlW and ZrW, respectively. Moreover, these tungsten based Lewis acids exhibited a good stability and recyclability. The efficiency of the solid Lewis acids ZrW and AlW to produce lactic acid directly from crystalline cellulose was explained by a positive synergy between water autoprotolysis responsible of the cellulose depolymerisation into soluble intermediates which are further converted on the solid Lewis catalyst surface.

Transition-Metal-Free Diboration Reaction by Activation of Diboron Compounds with Simple Lewis Bases

Abstract: Synthetically useful diboronic reagents are efficiently prepared by reaction of nucleophilic substrates and nucleophilic reagents in the presence of methanol and a Lewis base.

Hydroarylation of alkynes catalyzed by nickel.

Abstract: Nickel catalysts derived from bis(1,5-cyclooctadiene)nickel [Ni(cod)2] and trialkylphosphines effect hydroarylation of alkynes through functionalization of C–H bonds of arenes including benzo-fused five-membered heteroarenes, pyridine-N-oxides, pyridines, 2-pyridones, and perfluoroarenes The reactions proceed with excellent stereo- and regioselectivity to give disubstituted arylethenes in good yields Use of Lewis acid (LA) co-catalysts is crucial for success in reactions of imidazoles, pyridines, and 2-pyridones; it is possible that coordination of the LA to the nitrogen or oxygen functionalities of such substrates increases the reactivity of their C–H bonds towards nickel(0) species DOI 101002/tcr201100023

Mechanistic Studies on Ring-Opening Polymerization of Benzoxazines: A Mechanistically Based Catalyst Design

Abstract: Ring-opening polymerization behavior of a model p-cresol–aniline-based benzoxazine under various conditions was investigated in detail by 1H NMR analysis in deuterated DMSO. An improved mechanistic scheme was proposed to explain the observed experimental results. In light of this mechanistic scheme, several bifunctional catalysts were studied, and lithium iodide was found to be a very active and effective catalyst. Lithium cation is the Lewis acid part and effectively coordinates with oxygen or nitrogen atoms and promotes ring-opening of benzoxazines. Iodide ion is the nucleophilic part with good leaving ability and can react with the ring-opened imminium intermediates to prevent its rapid recombination with the phenolate.

Lewis Acid Adducts of Narrow Band Gap Conjugated Polymers

Abstract: We report on the interaction of Lewis acids with narrow band gap conjugated copolymers containing donor and acceptor units. Examination of the widely used poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b′]dithiophene)-2,6-(diyl-alt-benzo[2,1,3]thiadiazole)-4,7-diyl] (1) shows weaker binding with B(C6F5)3 when compared with a small molecule that contains a cyclopenta-[2,1-b:3,4-b′]dithiophene (CDT) unit flanked by two benzo[2,1,3]thiadiazole (BT) fragments. Studies on model compounds representative of 1, together with a comparison between B(C6F5)3 and BBr3, indicate that the propensity for Lewis acid coordination is decreased because of steric encumbrance surrounding the BT nitrogen sites. These observations led to the design of chromophores that incorporate an acceptor unit with a more basic nitrogen site, namely pyridal[2,1,3]thiadiazole (PT). That this strategy leads to a stronger B−N interaction was demonstrated through the examination of the reaction of B(C6F5)3 with two small molecules bis(4,4-bis(...

Infrared study of the influence of reducible iron(III) metal sites on the adsorption of CO, CO2, propane, propene and propyne in the mesoporous metal–organic framework MIL-100

Abstract: The present study illustrates the importance of the oxidation state of iron within the mesoporous iron trimesate [{Fe3O(H2O)2F0.81(OH)0.19}{C6H3(CO2)3}2] denoted MIL-100(Fe) (MIL= Material from Institut Lavoisier) during adsorption of molecules that can interact with the accessible metal sites through π-back donation. Adsorption of CO has been first followed by FTIR spectroscopy to quantify the Lewis acid sites in the dehydrated Fe(III) sample, outgassed at 150 °C, and on the partially reduced Fe(II/III), outgassed at 250 °C. The exposure of MIL-100(Fe) to CO2, propane, propene and propyne has then been studied by FTIR spectroscopy and microcalorimetry. It appears that π-back donating molecules are strongly adsorbed on reduced iron(II) sites despite the weaker Lewis acidity of cus Fe2+ sites compared to that of Fe3+ ones, as shown by pyridine adsorption.

Molecular mechanical study of halogen bonding in drug discovery

Abstract: A halogen bond is a noncovalent bond between a halogen atom (X) and a Lewis base (Y). This type of bond is attributed to the anisotropic distribution of the charge density on the halogen atom, resulting in the formation of a positive cap (called the σ-hole) centered on the A–X axis. The current research is the first reported molecular mechanical study of halogen bonding, the positive region centered on the halogen atom was represented by an extra-point (EP) of charge. The correlation between the X–EP distance and the X…Y bond length was explored to determine the optimal position of the EP. A test set of 27 halogen-containing molecules complexed to various Lewis bases was studied using molecular mechanical potentials. The molecular mechanical minimized halogen bond lengths and binding energies were in good agreement with the corresponding quantum mechanical values. The EP inclusion on the halogen atom resulted in an improvement in the accuracy of the electrostatic-potential derived charges. The solvation free energies of halobenzene molecules relative to benzene were calculated with and without EP inclusion to assess the accuracy of the developed approach. Molecular mechanical study of halo derivatives of benzotriazole complexed to cyclin-dependent protein kinase 2 (CDK2) was performed, and MM-PB(GB)SA binding energies were calculated as a case study in finding potent halogenated inhibitors that can serve as antitumor drugs. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011

Acid properties of solid acid catalysts characterized by solid-state 31P NMR of adsorbed phosphorous probe molecules.

Abstract: A brief review is presented on acidity characterization of solid acid catalysts by means of solid-state phosphor-31 magic-angle-spinning nuclear magnetic resonance (31P MAS NMR) spectroscopy using phosphor-containing molecules as probes. It is emphasized that such a simple approach using 31P MAS NMR of adsorbed phosphorous probe molecules, namely trimethylphosphine (TMP) and trialkylphosphine oxides (R3PO), represents a unique technique in providing detailed qualitative and quantitative features, viz. type, strength, distribution, and concentration of acid sites in solid acid catalysts. In particular, it will be shown that when applied with a proper choice of probe molecules with varied sizes and results obtained from elemental analysis, the amounts and locations (intracrystalline vs. extracrystalline) of different types (Bronsted vs. Lewis) of acid sites may be determined. In addition, by incorporating the NMR results with that obtained from theoretical density functional theory (DFT) calculations, correlations between the 31P chemical shifts (δ31P) and acidic strengths of Bronsted and Lewis acid sites may also be derived, facilitating a suitable acidity scale for solid acid catalysts.

A new family of cinchona-derived amino phosphine precatalysts: application to the highly enantio- and diastereoselective silver-catalyzed isocyanoacetate aldol reaction.

Abstract: A new class of readily accessible chiral amino-phosphine precatalysts derived from 9-amino(9-deoxy) epicinchona alkaloids has been developed. In combination with Ag(I) salts, these amino-phosphines performed as effective cooperative Bronsted base/Lewis acid catalysts in the asymmetric aldol reaction of isocyanoacetate nucleophiles. Under optimal conditions, high diastereoselectivities (up to 98%) and enantioselectivities (up to 98%) were obtained.

Transition metals as Lewis bases: "Z-type" boron ligands and metal-to-boron dative bonding

Abstract: While the vast majority of inorganic chemistry involves electron donation from main-group atoms to metals, an intriguing yet flip-side exists: where Lewis-basic metals donate electron density to Lewis-acidic main-group atoms (most often boron). These so-called “Z-type” ligands, along with other less clear-cut complexes, are examples of this metal-ligand role reversal. This perspective article offers an introduction to metal-to-boron dative bonding, and attempts to correlate spectroscopic and structural data from the complexes..

Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid–Base Pairs that are Reactive Towards the CH Bond Activation of Methane

Abstract: The activation of methane is still a great challenge today, because it constitutes one of the largest hydrocarbon resources on earth. Many oxides catalyze processes involving the activation of C H bonds. Of these, g-alumina (g-Al2O3) is one of the most active: when treated beforehand at temperatures above 400 8C, it catalyses H/D exchange reactions of D2/CH4 and CH4/CD4 mixtures at room temperature with unexpectedly low activation energies (17–30 kJmol ). These reactions involve a very small number of active sites (defects) generated by the high-temperature pretreatment. Such a reactivity was attributed to the Lewis acidity of surface Al atoms, yielding Al CH3 and O H species, and more recently specifically to surface three-coordinate AlIII centers leading to the formation of four-coordinate AlIV CH3 moieties. However, it is not clear why AlIII, the expected most Lewis acidic site, would exist in realistic conditions, that is, on a hydroxylated alumina surface. The predominant termination of g-Al2O3 particles is the (110) facet (70–83%); [9] complete dehydration would require temperatures (> 900 8C) much higher than the window of stability of g-Al2O3. [11] Even after treatments at 400–500 8C, the OH density is 2–5 OH/nm 2 on g-Al2O3; [7,12] therefore, the strongest AlIII sites should be completely hydroxylated and thus their reactivity annihilated. Therefore, several questions emerge: what is the real nature of the active site for C H bond activation on alumina? Is it possible to have the two strong antagonists, highly Lewis acidic sites and a strong Lewis base (H2O), on the surface simultaneously without a direct annihilation between them? If these strong Lewis sites still exist, how would surface hydroxylation affect their reactivity towards CH4? Herein we provide answers to these questions and show the unexpected role of water by combining experiments and first-principle calculations. We underline that the reactive site is best described as an (Al,O) Lewis acid–base pair where oxygen basicity, apart from Al acidity, is also a key factor to reactivity. We first investigated the influence of thermal treatment of alumina on the density of sites involved in the formation of Al CH3 species through reaction with CH4 at 150 8C (Figure 1a). The density of sites gives a volcano-type curve as a function of the g-Al2O3 pretreatment temperature, reaching a maximum of 0.03 reactive Al sites per nm at about 700 8C. Below 400 8C, no site is generated, while at higher temperatures (> 800 8C) their density decreases. By comparison, both the hydroxy group coverage qOH and the specific surface area SBET decrease with increasing temperature (Figure 1b and Supporting Information, Figure S1): qOH decreases exponentially while SBET falls first slowly and then sharply above 800 8C. These phenomena are associated with a progressive change of the alumina bulk structure (g!d!q), as indicated by X-ray powder diffraction studies (Supporting Information, Figure S2). The nature of the active sites was studied by DFT calculations with a specific focus on low energy metastable terminations of partially hydrated alumina surfaces. The most abundant (110) surface was considered: its hypothetic fully dehydrated surface unit cell (s0) exposes three different aluminum Lewis acid centers: one three-coordinate (AlIII) and two types of four-coordinate (AlIVa and AlIVb) sites, with decreasing intrinsic Lewis acidity according to AlIII>AlIVb> AlIVa. It also exposes twoand threefold-coordinated O atoms with intrinsic Lewis basicity O2>O3 (Supporting Information, Figure S3a). This (110) termination has a rather high surface energy (1.5 J m ) and strongly interacts with water. 16] At a simulated qOH of about 3OH nm 2 (1 H2O per surface unit cell), close to the measured OH density at 500 8C, the OH group preferentially occupies the most Lewis acidic AlIII site (Supporting Information, Figure S3b). [15] However, from the various structures explored, a configuration with OH bridging two AlIVa centers, keeping AlIII free, is only 44 kJmol 1 less stable (s1, Figure 2a), making the [*] Dr. R. Wischert, Prof. Dr. C. Cop ret Universit de Lyon, CNRS Institut de Chimie de Lyon, C2P2, CPE Lyon 43, Bd. du 11 Novembre 1918, 69616 Villeurbanne Cedex (France)

Electrocatalytic Carbon Dioxide Activation: The Rate‐Determining Step of Pyridinium‐Catalyzed CO2 Reduction

Abstract: The reactivity of reduced pyridinium with CO2 was investigated as a function of catalyst concentration, temperature, and pressure at platinum electrodes. Concentration experiments show that the catalytic current measured by cyclic voltammetry increases linearly with pyridinium and CO2 concentrations; this indicates that the rate-determining step is first order in both. The formation of a carbamate intermediate is supported by the data presented. Increased electron density at the pyridyl nitrogen upon reduction, as calculated by DFT, favors a Lewis acid/base interaction between the nitrogen and the CO2. The rate of the known side reaction, pyridinium coupling to form hydrogen, does not vary over the temperature range investigated and had a rate constant of 2.5 M−1 s−1. CO2 reduction followed Arrhenius behavior and the activation energy determined by electrochemical simulation was (69±10) kJ mol−1.

Capture of NO by a Frustrated Lewis Pair: A New Type of Persistent N‐Oxyl Radical

Abstract: Frustrated Lewis Pairs (FLPs) show a rapidly increasing spectrum of interesting chemical reactions. They have been reported to activate dihydrogen under mild conditions and to serve as metal free hydrogenation catalysts toward specific organic substrates. FLPs add to numerous unsaturated substrates such as alkenes and alkynes, carbonyl compounds, azides and even CO2 or N2O. [3,4] For instance, the intramolecular P/B-FLP 1 adds to nitrosobenzene to form the sixmembered heterocycle 2. Nitric oxide (NO) is an important messenger molecule and regulator in biological systems. We find that the reactive frustrated Lewis pair 1 cleanly and rapidly reacts with this essential small molecule to form the persistent heterocyclicNoxyl radical “P/B-FLP-NOC” (3). Compound 3 represents a novel type of N-oxyl radical related to the ubiquitous TEMPO radical (4) and its congeners (e.g. 5 (PINO) and 6, see Scheme 1). Herein we describe the synthesis, characterization, and O-based reactivity of this novel type of N-oxyl radical derived from a frustrated Lewis pair and nitric oxide. Treatment of a yellow solution of the FLP Mes2PCH2CH2B(C6F5)2 (1) [10] (in situ generated from Mes2PCH=CH2 and Piers borane [HB(C6F5)2] ) in fluorobenzene with 1 equiv NOgas gave rise to an intense green solution fromwhich blue crystals of the P/B-FLP-NOC product 3 were isolated in 58% yield by precipitation with pentane (Scheme 1). X-ray crystal structure analysis of 3 (Figure 1)