Nature of Lewis acidity in H-zeolites

It has been revealed that in crystalline aluminosilicates the three-coordinated aluminium ion belonging to the lattice is not the Lewis acid center. The latter is the off-lattice aluminium ion compensating the excess of the negative charge in the lattice or acting as cation valence-bonded to the oxygen anion of the SiO₄ tetrahedron.

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, , , . , , , SiO₄-, .

     

Lewis acidity of gallium halides

The Lewis acidity of GaF(3), GaF(2)Cl, GaFCl(2), and GaCl(3) in acid-base interactions has been studied by taking ammonia as their electron-donating counterpart. We have derived an unoccupied reactive orbital that shows the maximum localization on the Ga atomic center for each species. The orbital is located lower in energy compared to those in the corresponding boron and aluminum halides. In contrast to boron halides, the unoccupied reactive orbital of the acid site tends to be delocalized considerably on the halogens as the fluorines are substituted by chlorines in gallium halides. The trend observed in the effects of fluorine and chlorine on the acidity of the gallium halides is opposite to those found in the boron halides. This cannot be interpreted solely in terms of the electron-accepting strength of the gallium center, but can be understood by including electrostatic interactions and closed-shell repulsion with ammonia in the adducts. The origin of the difference in Lewis acidity of BCl(3), AlCl(3), and GaCl(3) has been clarified.     

Lewis acidity of gallium-containing pentasils

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2873–2874, December, 1990.     

Increased Lewis acidity in hafnium-substituted polyoxotungstates

Monolacunary polyoxotungstates [alpha(1)-P(2)W(17)O(61)](10-) and [alpha-PW(11)O(39)](7-) react with HfCl(4) to yield [alpha(1)-HfP(2)W(17)O(61)](6-) and [alpha-Hf(OH)PW(11)O(39)](4-), isolated as organo-soluble tetrabutylammonium (TBA) salts. Subsequent analyses, including mass spectrometry, show that they are stronger Lewis acids than (TBA)(5)H(2)[alpha(1)-YbP(2)W(17)O(61)]. The new polyoxotungstates catalyze Lewis acid mediated organic reactions, such as Mukaiyama aldol and Mannich-type additions. In particular, reactions with aldehydes, which were impossible with lanthanide polyoxotungstates, are made possible. Thus these modifications of the polyoxometalate composition allowed fine tuning of the Lewis acidity. The catalysts could be easily recovered and reused.     

Olefinic substitution reaction catalyzed by sulfur-containing polymeric palladium complexes

Silica-supported sulfur-containing polymeric palladium complexes were synthesized by the coordination of sulfur-containing polymeric ligands with palladium acetate or palladium chloride. The activated alkenes were reacted selectively with iodobenzene in the presence of tertiary amines and polymeric palladium complexes containing sulfur to give phenyl-substituted derivatives. High yields can be obtained under appropriate conditions with a low catalyst concentration. The sulfur-containing polymeric palladium complexes showed higher activities and better selectivities and stabilities than the corresponding phosphine-containing systems.     

Lewis acid-promoted hetero Diels-Alder cycloaddition of alpha-acetoxynitroso dienophiles

[reaction: see text] alpha-Acetoxynitroso compound 3 has been prepared as a new stable, isolable, and reactive dienophile in nitroso Diels-Alder reactions. The yield of the [4 + 2] cycloaddition of alpha-acetoxynitroso dienophile with 1,3-dienes could be enhanced in the presence of 20 mol % Lewis acid. An unexpected retro hetero-Michael reaction from 26 was observed, leading to the cleavage of the N-O bond of the cycloadduct. This tandem nitroso Diels-Alder/retro hetero-Michael sequence has been used with cyclic and acyclic 1,3-dienes.     

Enhanced Lewis acidity by aliovalent cation doping in metal fluorides

A model regarding the generation of acidity in binary metal fluorides has been proposed and its validity has been examined for several binary fluoride systems with the general compositions MF₃/M′F₃ and MF₂/M′F₃. In accordance with this hypothesis, the binary systems (CrF₃/AlF₃, CrF₃/FeF₃ and AlF₃/VF₃) do not show acidities larger than the sum of the acidities of the component fluorides. The hypothesis predicts the generation of Lewis acidity when MF₂ is the major component (host) and generation of Brønsted acidity when MF₃ acts as the host for the MF₂/M′F₃. The experimental results (surface acidity and catalytic activity) confirmed the predictions made from this hypothesis for binary combinations MgF₂/ M′F₃ (M′=Cr, Al, Fe, V). The application of this model is discussed in terms of other parameters: ionic radii and the fluoride affinity of the metal fluorides involved.     

Sweet (hetero)aromatics: glycosylated templates for the construction of saccharide mimetics

Mono- and diglycosylated aromatics and heteroaromatics may serve as building blocks for the construction of metabolically stable mimetics of oligosaccharides. Methods for their preparation from monosaccharidic precursors by direct C-glycosylation, dipolar cycloaddition or Larock cyclization are described.     

Enhancement of large magnetoresistances in ruthenocuprates by Ta substitution

An unexpected enhancement of the large negative magnetoresistance (MR) observed in RuSr(2)Nd(0.95)Y(0.15)Ce(0.9)Cu(2)O(10-delta) up to -47% at 4 K and 9 T is evidenced upon dilution of the Ru magnetic order by substitution of Ta for Ru; this enhancement of -MR scales with the cell volume.     

Regulating sulfur removal efficiency of fuels by Lewis acidity of ionic liquids

It is urgent to develop a new deep desulfurization process of fuels as the environmental pollution increases seriously. In this work, a series of Lewis acidic ionic liquids (ILs) [C₄³MPy]Cl/nZnCl₂ (n=1, 1.5, 2, 3) were synthesized and used in extraction and catalytic oxidative desulfurization (ECOD) of the fuels. The effects of the Lewis acidity of ILs, the molar ratio of H₂O₂/sulfur, temperatures, and different substrates including dibenzothiophene (DBT), benzothiophene (BT) and thiophene (TS), on sulfur removal were investigated. The results indicated that [C₄³MPy]Cl/₃ZnCl₂ presented near 100% DBT removal of model oil under conditions of 323 K, H₂O₂/DBT molar ratio 6:1. Kinetics for the removal of DBT, BT and TS by the [C₄³MPy]Cl/₃ZnCl₂-H₂O₂ system at 323 K is first-order with the apparent rate constants of 1.1348, 0.2226 and 0.0609 h^-1, and the calculated apparent activation energies for DBT, BT and TS were 61.13, 60.66, and 68.14 kJ/mol from 298 to 308 K, respectively. After six cycles of the regenerated [CC₄³MPy]Cl/₃ZnCl₂, the sulfur removal had a slight decrease. [CC₄³MPy]Cl/₃ZnCl₂ showed a good desulfurization performance under optimal conditions.     

Determination of the Lewis acidity of ionic liquids by means of an IR spectroscopic probe

Pyridine and ethanenitrile can be used as molecular probes to measure the Lewis acidities of ionic liquids by monitoring the shift of IR absorption bands near 1450 cm(-1) for pyridine and in the range 2250-2340 cm(-1) for ethanenitrile.     

Lewis acid-assisted nucleophilic substitution of fullerene epoxide

[Structure: see text] A 1,4-bis(phenyl)-1,4-dihydro[60]fullerene resulting from an efficient nucleophilic substitution has been obtained by reaction of a fullerene epoxide, C60O, with nucleophilic aromatic compounds in the presence of boron trifluoride etherate as a Lewis acid.     

Bond dissociation energy and Lewis acidity of the xenon fluoride cation

This study focuses upon the Lewis acid reactivity of XeF(+) with various bases in the gas phase and the determination of the bond dissociation energy of XeF(+). The bond dissociation energy of XeF(+) has been measured by using energy-resolved collision-induced dissociation with neon, argon, and xenon target gases. Experiments with neon target yield a 298 K bond dissociation enthalpy of 2.81 +/- 0.09 eV, and those with argon target give a similar value at 2.83 +/- 0.12 eV. When using a xenon target, a significantly lower value of 1.95 +/- 0.16 eV was observed, which corresponds closely with previous measurements and theoretical predictions. It is proposed that the lighter target gases give inefficient excitation of the XeF(+) vibration leading to dissociation at energies higher than the BDE. Novel xenon-base adducts have been prepared in a flowing afterglow mass spectrometer by termolecular addition to XeF(+) and by reaction of base with XeF(+)(H(2)O). New species have been characterized qualitatively by CID, and it is found that the products formed reflect the relative ionization energies of the fragments. Among the new xenon-containing species that have been prepared are the first examples of xenon carbonyls.     

A theoretical study of the effect of silicon substitution on hydrocarbon acidity

Ab initio molecular orbital calculations with double-zeta basis sets show the relative stabilities of three tautomers on the C₂SiH₄ energy hypersurface to be 3-silapropyne > 1-silaallene > 1-silapropyne. Comparison with literature values shows 1-silaallene to be more stable than 2-silaallene. Assuming deprotonation at carbon then the order of acidity is 1-silapropyne > 10-silaallene > 3-silapropyne > silaethane > silaethylene. For silaethylene and silaethane deprotonation occurs more easily at silicon than at carbon, while for both silapropynes and 1-silaallene carbon deprotonation is slightly favoured. The α-silyl group enhances the acidity of the adjacent methyl group and a silyl group in conjugation with a carboncarbon triple bond enhances the acidity of the alkynyl proton. The methyl, ethyl, and 2-silaethyl groups all weakly decrease the acidity of the alkynyl proton.     

Tuning of the optical properties and Lewis acidity of dibenzopnictogenaborins by modification on bridging main group elements

Dibenzophosphaborin 1 and dibenzoazaborine 7 have been synthesized from the corresponding dibromides and MesB(OMe)2. Dibenzophosphaborin P-sulfide 2, the P-selenide 3, and the phosphonium salt 4 were obtained by the reaction of 1 with S8, elemental selenium, and benzyl bromide, respectively. Crystallographic analysis of 1-3 and 7 showed that the dibenzophosphaborin framework does not have a planar structure, which is caused by the pyramidalization around the phosphorus atoms, unlike in 7. Compound 1 showed a blue-shifted and weak UV-vis absorption relative to 7, indicating a weak electronic interaction between the phosphorus lone pair electrons and the pi-orbitals on the dibenzopnictogenaborin framework. The longest absorption bands of 2 and 3 are attributable to the intramolecular electron transfer from the lone pair electrons on the chalcogen atoms to the pi orbitals on the dibenzophosphaborin framework, and the existence of these electronic transitions is supported by TD-DFT calculations. The UV-vis and 11B NMR spectra of 4 showed a temperature-dependent change due to the coordination of a counter bromide anion to the boron center, indicating a strong Lewis acidity of the boron center. We also investigated the complex formation ability of dibenzopnictogenaborin to fluoride or chloride anions. The complex formation constants increased in accordance with the decrease in LUMO energy levels calculated using the DFT method. Compound 4 exhibited an anomalously high complex formation ability among these compounds, due to the strong electron-withdrawing effect of the phosphonium cation moiety.     

Lewis acidity enhancement of organoboranes via oxidation of appended ferrocene moieties

The Lewis acidity of boron in diboradiferrocene 1 is strongly enhanced through oxidation of the iron atoms as evident from examination of X-ray structural parameters of the mixed-valent cation 1(+)PF(6) and further confirmed from the strong complexation of MeCN to the dication in 2(2+)(I(3))(2).     

Substituent effects on the Lewis acidity of 4,6-di-tert-butylchatechol boronate esters

In studying the factors which contribute to the Lewis acidity of organoboron compounds we investigated approaches to the design of robust, novel Lewis acids purposed for metal-free catalysis. Based on a sterically encumbered catechol motif, a series of boronate esters are shown to demonstrate modest Lewis acidities for the conventional Gutmann-Beckett test as an inquisitive investigation.     

Tuning Lewis acidity using the reactivity of "frustrated Lewis pairs": facile formation of phosphine-boranes and cationic phosphonium-boranes

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.