Addition Of Lewis Acids Research Articles

Testing the Push-Pull Hypothesis: Lewis Acid Augmented N2 Activation at Iron.

We present a systematic investigation of the structural and electronic changes that occur in an Fe(0)-N2 unit (Fe(depe)2(N2); depe = 1,2-bis(diethylphosphino)ethane) upon the addition of exogenous Lewis acids. Addition of neutral boranes, alkali metal cations, and an Fe2+ complex increases the N-N bond activation (Δ νNN up to 172 cm-1), decreases the Fe(0)-N2 redox potential, polarizes the N-N bond, and enables -N protonation at uncommonly anodic potentials. These effects were rationalized using combined experimental and theoretical studies.

Chemical control of the viscoelastic properties of vinylogous urethane vitrimers

Vinylogous urethane based vitrimers are polymer networks that have the intrinsic property to undergo network rearrangements, stress relaxation and viscoelastic flow, mediated by rapid addition/elimination reactions of free chain end amines. Here we show that the covalent exchange kinetics significantly can be influenced by combination with various simple additives. As anticipated, the exchange reactions on network level can be further accelerated using either Brønsted or Lewis acid additives. Remarkably, however, a strong inhibitory effect is observed when a base is added to the polymer matrix. These effects have been mechanistically rationalized, guided by low-molecular weight kinetic model experiments. Thus, vitrimer elastomer materials can be rationally designed to display a wide range of viscoelastic properties.

Effects of Lewis Acids on Photoredox Catalysis

AbstractThe reactivity of organic photoredox catalysts in oxidation reactions is enhanced by the addition of Lewis acids, which bind to the catalyst to increase its oxidizing ability. Redox‐inactive metal ions, such as Sc3+, also act as Lewis acids and can bind to flavins to increase their one‐electron reduction potentials and photocatalytic stability. The Sc3+ ion can bind to acridine and pyrene to enhance their photoredox catalytic activity in the oxidation of substrates by dioxygen. The binding of Lewis acids to radical anions enhances their overall photocatalytic activity by prohibiting a back electron transfer from the radical anions of substrates to the oxidized photocatalysts. The photocatalytic asymmetric oxidation of terminal olefins by using water as the oxygen source with a mononuclear nonheme chiral manganese complex has been made possible by the addition of acetic acid, which is essential to afford the products with high enantioselectivity.

Role of Lewis acid additives in a palladium catalyzed directed C–H functionalization reaction of benzohydroxamic acid to isoxazolone

Metallic salts as well as protic additives are widely employed in transition metal catalyzed C-H bond functionalization reactions to improve the efficiency of catalytic protocols. In one such example, ZnCl2 and pivalic acid are used as additives in a palladium catalyzed synthesis of isoxazolone from a readily available benzohydroxamic acid under one pot conditions. In this article, we present some important mechanistic insights into the role of ZnCl2 and pivalic acid, gained by using density functional theory (M06) computations. Two interesting modes of action of ZnCl2 are identified in various catalytic steps involved in the formation of isoxazolone. The conventional Lewis acid coordination wherein zinc chloride (ZnCl2·(DMA)) binds to the carbonyl group is found to be more favored in the C-H activation step. However, the participation of a hetero-bimetallic Pd-Zn species is preferred in reductive elimination leading to Caryl-N bond formation. Pivalic acid helps in relay proton transfer in C-H bond activation through a cyclometallation deprotonation (CMD) process. The explicit inclusion of ZnCl2 and solvent N,N-dimethyl acetamide (DMA) stabilizes the transition state and also helps reduce the activation barrier for the C-H bond activation step. The electronic communication between the two metal species is playing a crucial role in stabilizing the Caryl-N bond formation transition state through a Pd-Zn hetero-bimetallic interaction.

Unusual Lewis-acid catalyzed formal (3+3)-cycloaddition of azomethine imines and nitrones to N-vinylpyrroles

The addition of Lewis acids change the reaction mechanism of the cycloaddition of N-vinylpyrroles with azomethine imine and C,N-diarylnitrones. The formal (3+3)-cycloaddition is observed instead of (3+2)-dipolar cycloaddition, which take place in the absence of catalysts. This unusual (3+3)-cycloaddition leads to heterocyclic compounds with pyrazolo[1,2-a]pyrrolo[1,2-d][1,2,4]triazine- and pyrrolo[2,1-d][1,2,5]oxadiazine cores, which are difficultly achievable by other methods.

Additive-Free Pd-Catalyzed α-Allylation of Imine-Containing Heterocycles.

An additive-free Pd-catalyzed α-allylation of different imino-group-ontaining heterocycles is reported. The activation of α-CH pronucleophiles (pKa (DMSO) > 25) occurs without the addition of strong bases or Lewis acids using only the Pd/Xantphos catalyst system. The reaction scope has been studied for various 5- and 6-membered nitrogen-containing heterocycles (yields up to 96%). Mechanistic investigations suggest an initial allylation of the imine-N followed by a Pd-catalyzed formal aza-Claisen rearrangement.

3-Methyleneisoindolin-1-one Assembly via Base- and CuI/l-Proline-Catalyzed Domino Reaction: Mechanism of Regioselective Anionic Cyclization.

Anionic cyclization of o-alkynylbenzamides is proposed as a crucial step in many heterocycle syntheses. The cyclization can produce three products: Z-3-methylenisoindolin-1-one (Z-5-exo), E-3-methylenisoindolin-1-one (E-5-exo), and isoquinolinone (6-endo). Under base catalysis, the selectivity is generally poor. However, a copper-involved domino reaction of coupling and cyclization gives surprising selectivity for the thermodynamically disfavored Z-5-exo product (Org. Lett. 2009, 11, 1309-1312). We study the selectivity of anionic cyclization in the presence of K2CO3 and copper-l-proline, using surveys of the experimental literature and density functional theory (DFT) calculations. The o-alkynylbenzamide is predicted to be readily deprotonated by many bases, with subsequent cyclization via nucleophilic attack of the amide N- to alkynyl. In the absence of copper, endo-exo selectivity is predicted to arise from substituent effects, while Z/E selectivity is a sensitive function of the tautomerization rate of an alkenyl anion intermediate. Most importantly, we predict that the remarkable selectivity of the copper-involved reaction occurs because copper-l-proline "locks" the alkene anion intermediates into the initially formed Z-5-exo configuration. Calculations on other metals suggest that soft Lewis acid additives provide a potential route to improved regiocontrol of other anionic cyclizations.

Lewis Acid Accelerated Aryl Ether Bond Cleavage with Nickel: Orders of Magnitude Rate Enhancement Using AlMe3.

Study of the kinetics of intramolecular aryl ether C-O bond cleavage by Ni was facilitated by access to a family of metal complexes supported by diphosphines with pendant aryl-methyl ethers. The nature of the aryl substituents was found to have little effect on the rate of cleavage. In contrast, soluble Lewis acidic additives accelerate the aryl ether cleavage dramatically. The effect of AlMe3 was studied in detail, and showed an increase in rate by several orders of magnitude. Low temperature NMR spectroscopy studies demonstrate quantitative coordination of ether to Al. From the Lewis acid-bound precursor, the activation parameters for ether cleavage are significantly lower. These findings provide a mechanistic basis for milder catalyst design for the activation of strong bonds.

Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles.

An acceptorless dehydrogenation of heterocycles catalyzed by frustrated Lewis pairs (FLPs) was developed. Oxidation with concomitant liberation of molecular hydrogen proceeded in high to excellent yields for N-protected indolines as well as four other substrate classes. The mechanism of this unprecedented FLP-catalyzed reaction was investigated by mechanistic studies, characterization of reaction intermediates by NMR spectroscopy and X-ray crystal analysis, and by quantum-mechanical calculations. Hydrogen liberation from the ammonium hydridoborate intermediate is the rate-determining step of the oxidation. The addition of a weaker Lewis acid as a hydride shuttle increased the reaction rate by a factor of 2.28 through a second catalytic cycle.

Reactivity of electrogenerated thiocyanogen in the thiocyanation of pyrazolo[1,5-a]pyrimidines

Selective thiocyanation (yield 70–90%) at the 3-position of pyrazolo[1,5- a ]pyrimidines was performed with thiocyanogen electrochemically generated in situ from NH 4 SCN in MeCN on a Pt anode. For substrates with higher oxidation potential addition of Lewis acids was necessary.

Protodeboronation of Heteroaromatic, Vinyl, and Cyclopropyl Boronic Acids: pH-Rate Profiles, Autocatalysis, and Disproportionation.

pH-rate profiles for aqueous-organic protodeboronation of 18 boronic acids, many widely viewed as unstable, have been studied by NMR and DFT. Rates were pH-dependent, and varied substantially between the boronic acids, with rate maxima that varied over 6 orders of magnitude. A mechanistic model containing five general pathways (k1-k5) has been developed, and together with input of [B]tot, KW, Ka, and KaH, the protodeboronation kinetics can be correlated as a function of pH (1-13) for all 18 species. Cyclopropyl and vinyl boronic acids undergo very slow protodeboronation, as do 3- and 4-pyridyl boronic acids (t0.5 > 1 week, pH 12, 70 °C). In contrast, 2-pyridyl and 5-thiazolyl boronic acids undergo rapid protodeboronation (t0.5 ≈ 25-50 s, pH 7, 70 °C), via fragmentation of zwitterionic intermediates. Lewis acid additives (e.g., Cu, Zn salts) can attenuate (2-pyridyl) or accelerate (5-thiazolyl and 5-pyrazolyl) fragmentation. Two additional processes compete when the boronic acid and the boronate are present in sufficient proportions (pH = pKa ± 1.6): (i) self-/autocatalysis and (ii) sequential disproportionations of boronic acid to borinic acid and borane.

Postsynthetic Systematic Electronic Tuning of Organoplatinum Photosensitizers via Secondary Coordination Sphere Interactions

In this work we show that postsynthetic addition of borane Lewis acids to Lewis base decorated organoplatinum photosensitizers induces significant changes in the optical and electrochemical properties. In particular, the charge transfer (CT) energies of these chromophores are significantly modified by these outer-sphere interactions. The direction of the CT shift depends on the site of Lewis acid binding, which occurs either at the diimine ligand in bipyrazine-linked molecules or at an ancillary acetylide ligand in pyridyl-substituted bis(acetylide) molecules. The magnitude of the shift depends on the Lewis acidity of the borane and the number of equivalents added and is comparable to the perturbation brought on by covalent substituent modification of supporting ligands in related complexes. This approach offers a new means of tuning the properties of organometallic phosphors that complements the traditional approach of covalent modification and other postsynthetic modification strategies.

Electrochemical Stabilization of Self-Extinguishing Electrolyte Solutions with Trimethyl Phosphate By Adding Lewis Acids

Introduction Safety, specifically flammability, is still an important concern in lithium-ion batteries, especially as they are now widely used in higher-power and energy applications, such as electric-load leveling and electric vehicle systems. Organophosphorus compounds, such as phosphates, phosphonates, and phosphazenes, are widely known as excellent flame retardants. In our previous research, an ethylene carbonate (EC)-based solution including 50vol% trimethyl phosphate (TMP) self-extinguished after ignition in air. However, TMP has a higher donor number than EC and tends to co-intercalate with lithium ions into the graphite negative electrode, resulting in continuous decomposition and large irreversible capacity loss during the initial cycle [1]. On the other hand, Takeuchi et al. have improved the compatibility of TMP with graphite by introducing a salt composed of a stronger Lewis acid than Li+, such as Ca2+[2]. In this study, we verified the electrochemical stability in a self-extinguishing electrolyte solutions containing TMP or dimethyl methylphosphonate (DMMP) with Lewis acids toward practical use of electrolyte solutions in lithium-ion batteries. The simple addition of Lewis acids is a unique method to improve electrochemical properties of graphite negative electrode. Under the addition of stronger Lewis acids than Li+, the interactions between Li+ and EC or Li+ and TMP should be varied. However, there has been no systematically studied. Therefore, we have conducted NMR studies to evaluate the interactions in the electrolyte solutions containing flame retardant solvents with stronger Lewis acids of Mg2+, Ca2+ and weaker Lewis acids of Na+, K+ than Li+. Here we report firstly how interactions between Li+and EC affect the first coulombic efficiencies of graphite electrode and also show the electrochemical properties of graphite electrode in the above electrolyte solutions. Results and Discussion Figure 1 shows the relationship between the coulombic efficiency in charge-discharge at constant current of 0.1 mA cm-2 in a graphite/Li metal half cell and the chemical shifts of 17O nucleus of carbonyl group C=O in EC in 1 mol dm-3 LiPF6 dissolved EC+EMC (1:2, v/v%) and EC+EMC+TMP (1:2:3, v/v/v%) containing no additives, 0.5 mol dm-3 Ca(TFSA)2, Mg(TFSA)2, NaTFSA, and KTFSA. The values of the chemical shift in Fig. 1 are the amount of the change from the chemical shift of 1 mol dm-3 LiPF6 EC+EMC (1:2, v/v%) (triangle). TMP has a higher donor number than EC. Therefore, when TMP was added in the EC+EMC electrolyte solution, the strength of EC-Li+ interaction decreased due to selective solvation of TMP with Li+. As a result, the chemical shift in the electrolyte solution containing TMP “No add.” (cubic) is 11.5 ppm larger than that not containing TMP (triangle). By adding 0.5 mol dm-3 Ca(TFSA)2, Mg(TFSA)2, and NaTFSA, however, the chemical shift in EC moves in the direction approaching the EC+EMC electrolyte solution (triangle). Therefore, the coulombic efficiencies are improved due to suppression of the co-intercalation of TMP with Li+ into the graphite anode because interaction between EC and Li+ is re-enhanced by the added additives. The less effectiveness to coulombic efficiency by adding of NaTFSA derived from the lower Lewis acidity of Na+ than Li+. Based on these results, the electrolyte solution added KTFSA consisting of K+ which is a weaker Lewis acid than Na+ was not expected to improve the coulombic efficiency. Although, the chemical shift of KTFSA moved less than that of NaTFSA in the direction approaching the EC+EMC electrolyte solution (triangle) as we expected, the coulombic efficiency was more improved than that of Ca(TFSA)2 or Mg(TFSA)2. In Fig. 1, we considered that this improvement in the electrolyte solution added KTFSA was not derived from the solvation of Li+, which was altered by the Lewis acid K+. Therefore, the factor derived from this improvement was elucidated by analysis of the electrode after charging or discharging in an electrolyte solution. Furthermore, we will reveal a roll of each Lewis acid to electrochemical stability of self-extinguishing electrolyte solutions in this presentation References [1] H. Nakagawa, M. Ochida, Y. Domi, T. Doi., S. Tsubouchi, T. Yamanaka., T. Abe, and Z. Ogumi, J. Power Sources., 212,148 (2012). [2] S. Takeuchi, S. Yano, T. Fukutsuka, K. Miyazaki, and T. Abe, J. Electrochem. Soc., 159, A2089 (2012). Figure 1

Synthesis of allyl sulfoxides from allylsilanes via silyl sulfinates

Allylsilanes underwent sila-ene reaction with sulfur dioxide in the presence of Lewis acid catalysts. The obtained Vogel’s silyl sulfinates were found to act as sulfinyl transfer agents in reactions with aryl, heteroaryl, alkyl, and allyl Grignard reagents proceeding with the expulsion of the trialkylsilyloxy group to give allyl sulfoxides in up to 83% yield. The nucleophilic attack of Grignard reagents was accelerated in toluene and in the presence of LiCl or ZnCl2 as Lewis acidic additives. The developed method allows the transformation of allylsilanes into allyl sulfoxides.

Selective Solvation of Lewis Acids in EC-Based Electrolyte Solution with Organophosphorus Compound

Introduction Safety is still an important concern in lithium-ion batteries. Safety issues have recently received greater attention because lithium-ion batteries are now being widely used in higher-power and larger-size applications such as electric-load leveling and electric vehicle systems. Phosphorus compounds, such as phosphate, phosphonate, and phosphazene, are widely known as excellent flame retardants. Correlation between a concentration of trimethylphosphate (TMP) in an ethylene carbonate (EC)-based solvent and a flame retardancy of the mixed solvent was quantitatively clarified by applying the limiting oxygen index (LOI) method. The EC-based solvent EC+EMC+TMP(1:2:3, v/v/v%) indicated that LOI is 24.5%, at which the solvent does not ignite in air. The aim of this research is to apply the electrolyte solution including 50vol% of TMP, which has high flame retardancy, for lithium ion batteries. TMP tends to co-intercalate with Li+ into the graphite anode, resulting in continuous decomposition of electrolyte solvents and a large irreversible capacity loss during the initial cycling [1]. In recent years, however, Takeuchi et al. have discovered a method for suppressing the co-intercalation of TMP with Li+ into the graphite. Introducing a salt that is composed of stronger Lewis acids than Li+, such as Ca2+, made TMP more compatible with graphite [2]. In this study, interaction of flame retardants and Lewis acids were examined in more detail through understanding of the correlation between a composition of these electrolyte solutions and the coulombic efficiency in the initial cycling. Results and Discussion Fig. 1 shows the relationship between the coulombic efficiency and the chemical shifts of 17O nucleus of the phosphate group P=O in TMP (or dimethyl methylphosphonate DMMP) and carbonyl group C=O in EC in 1 mol dm-3 LiPF6/ EC+EMC+TMP (or DMMP) (1:2:3, v/v/v%) containing no additives, 0.5 mol dm-3 Ca(TFSA)2, Mg(TFSA)2, or NaTFSA. The values of the chemical shift in Fig. 1 are the amount of the change from the chemical shift of 1 mol dm-3 LiPF6 EC+EMC (1:2, v/v%) (Flammable). The chemical shift of the C=O in EC in the electrolyte is smaller than that in a solvent only because Li+ magnetically shields the electron of the lone pair of O of the C=O in EC. Focusing on a change in the electrolyte solution containing TMP, the chemical shift in the electrolyte solution containing TMP “No additives” (cubic) is 11.5 ppm larger than that not containing TMP “Flammable” (triangle). By containing TMP that has a higher donor, Li+ is solvated with TMP not only EC. Therefore, Li+ concentration in the vicinity of EC decreases. As a result, the effect of magnetic shielding on the 17O nucleus of the C=O in EC also reduces. By adding 0.5 mol dm-3 Ca(TFSA)2, Mg(TFSA)2, and NaTFSA, the chemical shift in EC moves in the direction approaching the “Flammable” (triangle). Therefore, the coulombic efficiencies are improved due to suppression of the co-intercalation of TMP with Li+ into the graphite anode because interaction between EC and Li+ is re-enhanced by the added additives. The improvement of coulomb efficiency was also observed in an electrolytic solution containing DMMP. Furthermore, all the plots in Fig. 1 are mostly arranged along a straight diagonal line going up-down left to right regardless of the species of the flame retardants and the additives. This indicates that the strength of the magnetic shielding to the 17O nucleus of the C=O in EC correlates with the coulombic efficiency. Even after the addition of the same Lewis acid, the degree of change in the chemical shift in TMP was larger than that in DMMP. This means that it is difficult to selectively form a solvation structure of EC with Li+ by adding Lewis acids in DMMP because the magnetic shielding to DMMP is not as low as that to TMP. This difference between TMP and DMMP is attributed to the different electron-donating ability of the DMMP and TMP. As a result of the above, it was elucidated that each flame retardant formed a selective-coordinating solvation structure with each Lewis acid.

Decarboxylative Generation of 2-Azaallyl Anions: 2-Iminoalcohols via a Decarboxylative Erlenmeyer Reaction.

Condensation between the tetrabutylammonium salt of 2,2-diphenylglycine and aldehydes results in a decarboxylative Erlenmeyer reaction, affording 1,2-diaryl-2-iminoalcohols as a mixture of diastereomers in good yields. The diastereomeric ratio shifts over time, with the anti diastereomer and the syn oxazolidine tautomer serving as the kinetic and thermodynamic products, respectively. Addition of Lewis acids can catalyze the rates of reaction and product equilibration. The results highlight the stereochemical promiscuity of 1,2-diaryl-2-iminoalcohols in the presence of Lewis acids and Brønsted bases.

Unprotected Xylose‐Derived Nitrone in Stereodivergent Synthesis of 4‐Hydroxypiperidine Enantiomers: Weak Lewis Acid Induced Alteration of Stereochemistry in 1,3‐Dipolar Cycloaddition

AbstractA one‐pot, two‐step synthesis of chiral 7‐oxa‐1‐azabicyclo[2.2.1]heptane derivatives from unprotected D‐xylose was achieved by an intramolecular 1,3‐dipolar cycloaddition of the N‐(γ‐alkenyl)‐substituted nitrone intermediate. The stereochemical course of the cycloaddition was altered by the addition of an achiral Lewis acid. The bicyclic byproducts were employed in a concise synthesis of two new enantiomeric piperidine‐type iminosugars.

Iron catalyzed CO2 hydrogenation to formate enhanced by Lewis acid co-catalysts.

A family of iron(ii) carbonyl hydride complexes supported by either a bifunctional PNP ligand containing a secondary amine, or a PNP ligand with a tertiary amine that prevents metal-ligand cooperativity, were found to promote the catalytic hydrogenation of CO2 to formate in the presence of Brønsted base. In both cases a remarkable enhancement in catalytic activity was observed upon the addition of Lewis acid (LA) co-catalysts. For the secondary amine supported system, turnover numbers of approximately 9000 for formate production were achieved, while for catalysts supported by the tertiary amine ligand, nearly 60 000 turnovers were observed; the highest activity reported for an earth abundant catalyst to date. The LA co-catalysts raise the turnover number by more than an order of magnitude in each case. In the secondary amine system, mechanistic investigations implicated the LA in disrupting an intramolecular hydrogen bond between the PNP ligand N-H moiety and the carbonyl oxygen of a formate ligand in the catalytic resting state. This destabilization of the iron-bound formate accelerates product extrusion, the rate-limiting step in catalysis. In systems supported by ligands with the tertiary amine, it was demonstrated that the LA enhancement originates from cation assisted substitution of formate for dihydrogen during the slow step in catalysis.

Advanced selective non-invasive ketone body detection sensors based on new ionophores

New molecules and methods were examined that can be used to detect trace level ketone bodies. Diseases such as type 1 diabetes, childhood hypo-glycaemia-growth hormone deficiency, toxic inhalation, and body metabolism changes are linked with ketone bodies concentration. Here we introduce, selective ketone body detection sensors based on small, environmentally friendly organic molecules with Lewis acid additives. Density functional theory (DFT) simulation of the sensor molecules (Bromo-acetonaphthone tungstate (BANT) and acetonaphthophenyl ether propiono hydroxyl tungstate (APPHT)), indicated a fully relaxed geometry without symmetry attributes and specific coordination which enhances ketone bodies sensitivity. A portable sensing unit was made in which detection media containing ketone bodies at low concentration and new molecules show color change in visible light as well as unique irradiance during UV illumination. RGB analysis, electrochemical tests, SEM characterization, FTIR, absorbance and emission spectroscopy were also performed in order to validate the ketone sensitivity of these new molecules.

Catalytic Asymmetric Alkylation of Acylsilanes

AbstractThe highly enantioselective addition of Grignard reagents to acylsilanes is catalyzed by copper diphosphine complexes. This transformation affords α‐silylated tertiary alcohols in up to 97 % yield and 98:2 enantiomeric ratio. The competing Meerwein–Ponndorf–Verley reduction is suppressed by the use of a mixture of Lewis acid additives. The chiral catalyst can be recovered as a copper complex and used repeatedly without any loss of catalytic activity.