Hard Lewis Acid Research Articles

Interactions of Nanosized Al2O3 and ZnO with Poly(ethylene oxide)−NaSCN Polymer Electrolytes

The effect of nanosized Al2O3/ZnO fillers on the interactions in PEO-NaSCN polymer electrolytes has been studied by FT-IR, XRD, and DTA measurements. The experimental results are discussed according to the grain boundary effect, the principle of Lewis acid-base, and epitaxial effect. It is shown that the hard Lewis acid centers on the surface of Al2O3 exhibit strong interactions with ether oxygens in PEO, even with ether oxygens that coordinated to Na+. Therefore, the modes of the interaction of Al2O3 with PEO-NaSCN electrolytes are dependent on the salt content. Al2O3 cannot influence the ionic association in P(EO)8NaSCN electrolyte, but do reduce significantly the solvating ability of PEO toward NaSCN in P(EO)60NaSCN electrolyte. However, NaSCN in P(EO)20NaSCN-30%Al2O3 nanocomposite appears as a state that resembles the situation of NaSCN in PEO amorphous phase. In contrast with Al2O3, the epitaxial effect of ZnO is not found in the interested systems where only weaker interaction is observed between ZnO and ether oxygen, and the modes of interaction of ZnO with PEO-NaSCN electrolytes are hardly related to the salt content. In addition, the soft Lewis acid groups on ZnO surface exhibit stronger complexation with SCN- in PEO-NaSCN electrolytes, and both can form the complex anion ZnO...SCN-.

An Exceedingly Efficient and Chemoselective Esterification with Activated Alcohols Using AlCl3/NaI/CH3CN System

A combination of hard Lewis acid AlCl3 and a soft nucleophile NaI in acetonitrile has been used as an efficient system for the selective esterification reaction between aromatic carboxylic acid and alcohol.

An Exceedingly Efficient and Chemoselective Esterification with Activated Alcohols Using AlCl3/NaI/CH3CN System.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Controlling the softness–hardness of Pd by strong metal–zeolite interaction: cyclisation of diallylmalonate as a test reaction

The cycloisomerisation of diallylmalonate to form cyclopentenes is demonstrated here to be a specific test reaction for hard Lewis acid sites in palladium-containing zeolites. We have found a correlation of the cycloisomerisation efficiency for a series of palladium-supported zeolites with the hardness of their framework as previously reported by quantum chemical calculations. Thus, the catalytic activity decreases and eventually disappears when the softness of the zeolite framework increases, the optimum in the activity being achieved when palladium is hosted in hard zeolite frameworks. In this way, the cycloisomerisation yield decreases in the series HY (Si/Al 15)≈HBeta (Si/Al 12.5)>HZSM-5 (Si/Al 15)>(Si)Beta (Si/Al ∞)>H,NaBeta (Si/Al 12.5)>HNaY (Si/Al 2.6)>NaY (Si/Al 2.6)>NaBeta (Si/Al 12.5)≈(Si,Ge)Beta.

Controlling the softness?hardness of Pd by strong metal?zeolite interaction: cyclisation of diallylmalonate as a test reaction

The cycloisomerisation of diallylmalonate to form cyclopentenes is demonstrated here to be a specific test reaction for hard Lewis acid sites in palladium-containing zeolites. We have found a correlation of the cycloisomerisation efficiency for a series of palladium-supported zeolites with the hardness of their framework as previously reported by quantum chemical calculations. Thus, the catalytic activity decreases and eventually disappears when the softness of the zeolite framework increases, the optimum in the activity being achieved when palladium is hosted in hard zeolite frameworks. In this way, the cycloisomerisation yield decreases in the series HY (Si/Al 15)≈HBeta (Si/Al 12.5)>HZSM-5 (Si/Al 15)>(Si)Beta (Si/Al ∞)>H,NaBeta (Si/Al 12.5)>HNaY (Si/Al 2.6)>NaY (Si/Al 2.6)>NaBeta (Si/Al 12.5)≈(Si,Ge)Beta.

A Novel Tandem [4+ + 2] Cycloaddition—Elimination Reaction: 2‐Alkenyl‐4,4‐dimethyl‐1,3‐oxathianes as Synthetic Equivalents for α,β‐Unsaturated Thioaldehydes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A novel tandem [4 ++2] cycloaddition–elimination reaction: 2-alkenyl-4,4-dimethyl-1,3-oxathianes as synthetic equivalents for α,β-unsaturated thioaldehydes

The first tandem cationic [4 ++2] polar cycloaddition–elimination reaction of 1-thia-1,3-butadienyl cations B with olefins to afford directly 3,4-dihydro-2 H-thiopyrans D is described. The cations B were easily accessible by treatment of monothioacetals A , particularly the 2-alkenyl-4,4-dimethyl-1,3-oxathianes 1 , with a hard Lewis acid. In this novel reaction, 2-alkenyl-4,4-dimethyl-1,3-oxathianes were utilized as synthetic equivalents for highly reactive α,β-unsaturated thioaldehydes. The effect of geminal dimethyl substituents on the oxathianes 1 and the mechanistic aspect of the reaction are considered. The reaction's asymmetric version was also investigated using a chiral oxathiane 4 derived from (−)-(1 R,2 R,5 R)-2-(1-mercapto-1-methylethyl)-5-methylcyclohexanol. Although the enantioselectivities were moderate, the whole process can be done under odorless conditions.

Potential binding modes of beryllium with the class II major histocompatibility complex HLA-DP: a combined theoretical and structural database study

In an effort to understand the molecular basis of chronic beryllium disease (CBD), a study of the chemical relationship between beryllium, antigen, and the major histocompatibility complex II, HLA-DP, was undertaken. A homology model of the HLA-DP protein was developed. An analysis of the sequences of HLA-DPB1 and HLA-DPA1 alleles most common among CBD patients revealed several carboxylate rich regions in the peptide-binding cleft. These regions contain many hard Lewis base sites that may provide bonding opportunities for beryllium, a hard Lewis acid. Quantum chemistry calculations and structural database results support the presence of beryllium clusters, bridged by carboxylate, hydroxo, and/or oxo ligands, in the HLA-DP binding cleft. These results strongly suggest that beryllium clusters are an integral part of the antigen, and may even act solely as antigen. This work provides an initial model for thinking about beryllium interactions with proteins relevant to CBD and other metal-induced diseases.

Efficient Detachment of N‐Benzyl Carbamate Group.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mixed-mode reversed-phase and ion-exchange separations of cationic analytes on polybutadiene-coated zirconia

The retention and selectivity of the chromatographic separation of basic (cationic) analytes on a polybutadiene-coated zirconia (PBD–ZrO 2) stationary phase have been studied in greater detail than in previous studies. These separations are strongly influenced by the chemistry of the accessible surface of zirconia. In the presence of buffers which contain hard Lewis bases (e.g., phosphate, fluoride, carboxylic acids) zirconia’s surface becomes negatively charged due to adsorption of the buffer anion at the hard Lewis acid sites. Consequently, under most conditions (e.g., neutral pH), cationic analytes undergo both hydrophobic and cation-exchange interactions. This mixed-mode retention process generally leads to greater retention factors for cations relative to those on silica-based reversed phases despite the lower surface areas of the zirconia phase, but, more importantly, adsorption of hard Lewis bases can be used to control the chromatographic selectivity for cationic analytes on these zirconia-based stationary phases. In contrast to our prior work, here we show that when mixed-mode retention takes place, both retention and selectivity are easily adjusted by changing the type of hard Lewis base buffer anion, the type of buffer counter-ion (e.g., sodium, potassium, ammonium), the pH, and the ionic strength of the eluent as well as the type and amount of organic modifier.

Efficient Detachment ofN-Benzyl Carbamate Group

The efficient deprotection of N-benzyl carbamate (N-Cbz) group under the mild conditions with a combination of hard Lewis acid and soft nucleophile is described.

Metal complexation with Langmuir monolayers of histidyl peptide lipids.

Langmuir monolayers made from peptide-lipid molecules represent a novel direction in the research areas of biomimetic interfaces and two-dimensional supramolecular chemistry. Peptide structures and molecular recognition activities toward other guest molecules have been the focus of previous study. This study reports the investigation of metal complexation to histidine-containing peptide lipids in the organized Langmuir, Langmuir-Schaefer, or Langmuir-Blodgett films. Three peptide lipids PEP1-PEP3, with a histidine amino acid incorporated in the middle of the peptide, were designed and synthesized. The monolayer structures and metal-binding activities of each peptide lipid and their 1:1:1 molar ratio mixture were studied by thermodynamic and spectroscopic techniques. It was found that hard Lewis acid type metal cations such as K+ and Mg2+, and borderline or soft metal cations such as Zn2+, Cu2+, and Cd2+ exhibit clearly different binding activity toward peptide-lipid monolayers. The conformational changes of peptides upon binding with Cu2+ and Zn2+ were partially revealed by FT-IR spectroscopic studies. Furthermore, by adding a fluorescent-probe lipid to the peptide monolayer, dramatic fluorescence change was observed when Cu2+ or Zn2+ bound to the Langmuir and Langmuir-Schaefer films of peptide-lipid monolayers. Metal-protein complexation plays a crucial role in the function and activity of proteins and enzymes. Investigation of metal complexation to organized peptide Langmuir monolayers may provide an alternative approach for the development of artificial metalloproteins and novel supramolecular systems or materials.

A study of the Lewis acid-base interactions of vinylphosphonic acid-modified polybutadiene-coated zirconia.

Polybutadiene-coated zirconia (PBD-ZrO2) is very useful for reversed-phase separations under a wide variety of conditions. Its excellent chemical (pH = 1-13) and thermal (up to 150 degrees C) stability distinguish it from silica-based reversed phases. Just as with silica-based phases, zirconia's surface chemistry significantly influences the chromatography of certain classes of analytes. Zirconia's hard Lewis acid sites can be chromatographically problematic. Analytes such as carboxylic acids strongly interact with these sites on PBD-ZrO2 and do not elute. Addition of phosphate or other strong, hard Lewis bases to the eluent brings about elution, but the resulting peak is often tailed and broad. Typically, cationic solutes are more retained in the presence of phosphate or fluoride due to adsorption of the Lewis base additives and the concomitant development of a negative charge on the surface. This Coulombic interaction can be used to optimize selectivity, but the reversed-phase-cation-exchange retention can produce broad peaks with excessive retention. As an alternative to adding Lewis bases to the eluent, we studied the effect of permanently modifying PBD-ZrO2 by covalently attaching vinylphosphonic acid (VPA) to PBD which was predeposited in the pores of zirconia. We have investigated the chromatography of acids, bases, and small peptides on VPA-modified PBD-ZrO2 (VPA-PBD-ZrO2) and compared it to PBD-ZrO2. VPA-PBD-ZrO2 is a reversed-cation-exchange phase with properties quite different from PBD-ZrO2. The chemical stability of both phases led us to explore how low-pH (1.5-3), ultralow-pH (0), and high-pH (12) eluents effect the retention properties of these mixed-mode phases. Ultralow-pH eluents effectively separate small peptides on both phases. This approach gives lower retention, without sacrificing resolution, and much higher efficiency for small peptides than previously reported.

Hard Acid and Soft Base Stabilisation of Di- and Trimercury Cations in Benzene Solution – A Spectroscopic, X-ray Scattering, and Quantum Chemical Study

Hg2Cl2 dissolves in GaCl3/benzene solution to yield Hg22+ and chlorogallate(III) ions, GanCl3n+1−. In such solutions, Hg22+ can be reduced to Hg32+ by metallic mercury. Solubility measurements show that one mol of Hg is oxidised per mol of Hg22+. The Hg32+ ion gives a strong band at 110 cm−1 in the Raman spectrum and Hg–Hg correlations at about 2.60 and 5.15 Å in the radial distribution function obtained by liquid X-ray scattering. – Hg32+ can also be synthesised in high yield by direct oxidation of metallic mercury by GaIII in GaCl3/benzene solution. In contrast, mercury is insoluble in neat liquid GaCl3 and only sparingly soluble in GaCl3/KCl melts. It therefore seems likely that the thermodynamic stabilisation of subvalent mercury species in benzene solution not only relies on the traditional acid stabilisation provided by the hard Lewis acid GaCl3, but also on a “soft-base stabilisation” provided by interactions between the aromatic molecules and the cations. Evidence for such specific interactions between Hgm2+ cations and C6H6 are observed in the Raman spectra: The totally symmetric C6H6 band at 991 cm−1 is found to split in the presence of Hgm2+ ions and to give new peaks at 978 (m = 2) and 982 (m = 3) cm−1. – In order to further elucidate the cluster–arene interactions, ab initio and density functional calculations were performed for the model compounds Hgm(C6H6)22+ and HgmCl2(C6H6)2, m = 2 and 3. The calculations show that both models represent coordinations modes which are feasible for Hgm2+ ions. However, the calculated vibrational frequencies for the Hgm(C6H6)22+ models with η1/quasi-η3 coordination of the benzene molecules along the Hg–Hg vector are most consistent with the body of experimental and literature data. The counterions are thus suggested to occupy secondary coordination sites.

Substituted cyclopentadienyl compounds. Part V. NMR and extended Hückel studies of the isomeric preferences of some main group metal cyclopentadienide compounds and a reinvestigation of the crystal structure of thallium(I) tricyanovinylcyclopentadienide

Solution NMR studies have been performed on a series of mono-substituted cyclopentadienide salts, [M +(C 5H 4X −)] (M=Li, Na, K, Cs, Tl; X=CONMe 2, COOMe, COC 6H 5, COMe, CHO, COCO 2C 2H 5, NO 2 and C(CN)C(CN) 2). Population of a fulvenoid isomer in which the dominant bonding mode involves interaction between the cation and the ring substituent heteroatom is enhanced when X is strongly electron accepting and the cation is a hard Lewis acid. An alternate isomer involving a bonding interaction between the cyclopentadienyl carbon atoms and the metal ion predominates when X is weakly accepting and the cation is a soft Lewis acid. A range of compounds between these two isomeric extremes show fluxional behaviour in dimethyl sulphoxide (DMSO) solvent. Within the series of alkali metal salts and for a given X group, the maximum barrier for interconversion of isomers is achieved when M is potassium. This phenomenon is attributed to the balance between Lewis character and solvation of the cation in DMSO. Molecular orbital calculations based on the extended Hückel method were performed to examine the effect of the chemical nature of X and M on the relative energies of the isomers. These suggest that the energy separation is more sensitive to the nature of X than of M, but that the electronegativity of M also plays a role by its effect in shifting the energies into critical π-MO energy ranges. The single crystal X-ray structure of a representative derivative, Tl[C 5H 4C(CN)C(CN) 2] has been reinvestigated. It shows a complex 3D structure in which each thallium is bonded to four nitrogen atoms and a single cyclopentadienide ring, a situation entirely consistent with the behaviour of this compound in solution in which the fulvenoid isomer is dominant.

Platinum- and Acid-Catalyzed Enyne Metathesis Reactions: Mechanistic Studies and Applications to the Syntheses of Streptorubin B and Metacycloprodigiosin

Formal total syntheses of the antibiotics metacycloprodigiosin (2) and streptorubin B (3) are described, which are known to exhibit promising immunomodulating properties. The key step en route to their meta-bridged pyrrole core structures 5 and 7, respectively, consists of a metathesis reaction of electron-deficient enynes catalyzed by either platinum halides, hard Lewis acids, or HBF4. This transformation expands the pre-existing cycloalkene of the substrates by two C atoms, forges the bicyclic pyrrolophane structure of the targets, and simultaneously forms a bridgehead alkene function. The products of this skeletal rearrangement are converted into the targets by a sequence comprising (i) a stepwise reduction of their enone entity to the corresponding saturated alcohols and (ii) an aromatization of the N-tosylated dihydropyrroles 20 and 34 thus obtained via elimination of potassium sulfinate on exposure to KAPA (potassium 3-aminopropylamide). A careful analysis of the minor byproducts formed in the enyne...

Reactions of 3-Thia(Selena)Pentane-1,5-Diones With Lewis Acids

3-Thia(selena)pentane-1,5-diones react with Lewis acids by heteroatom. In these reactions soft Lewis acids such as tin tetrachloride and antimony pentachloride form adducts but hard Lewis acid such as phosphorus pentachloride oxidizes chalcogen atom of 3-thia(selena)-pentane-1,5-dione. Carbon analogues of these diketones form pyrylium salts in the reactions with these Lewis acids.

Extraction of Sr(II), Ag(I) and Pb(II) by some oxa-tia- and tia-crown ethers

Liquid-liquid extraction of several cations by some oxa-tia- and tia-crown ethers was studied. Sr(II), Pb(II) and Ag(I) were taken as hard, borderline and soft Lewis acids, respectively. Trace amounts of210Sr and110mAg were used as chemical yield monitors. The results were explained by the Pearson theory. Some of the examined compounds posses high affinity towards Ag(I) and Pb(II) extraction. The distribution coefficent for Ag(I) reached the value of 140.

Reactions of 3-Thia(Selena)Pentane-1,5-Diones with Lewis Acids

3-Thia(selena)pentane-1,5-diones react with Lewis acids by heteroatom. In these reactions soft Lewis acids such as tin tetrachloride and antimony pentachloride form adducts but hard Lewis acid such as phosphorus pentachloride oxidizes chalcogen atom of 3-thia(selena)-pentane-1,5-dione. Carbon analogues of these diketones form pyrylium salts in the reactions with these Lewis acids.

Ring-Opening Cycloaddition of Aziridines to Ketenimines.

The Lewis acid-catalyzed addition of aziridines to ketenimines gave substituted pyrrolidonimines in 47-87% yields. The hard Lewis acid LiClO(4) proved to be superior to the soft [(PhCN)(2)PdCl(2)], affording higher yields under milder conditions. The reaction is regioselective and occurs with complete stereoselectivity using [(PhCN)(2)PdCl(2)] and with a small amount of racemization in the case of LiClO(4).