Showing papers on "Lewis acids and bases published in 1992"

Homogeneous olefin polymerization catalyst by ligand abstraction with lewis acids

Abstract: Addition polymerization catalysts having a limiting charge separated structure corresponding to the formula: LMX* XA-, wherein: L is a derivative of a substituted delocalized n-bonded group imparting a constrained geometry to the metal active site and containing up to 50 nonhydrogen atoms; M is a metal of Group 4 or the Lanthanide series of the Periodic Table of the Elements; X independently each occurrence is hydride or a hydrocarbyl, silyl or germyl group having up to 20 carbon, silicon or germanium atoms; and A- is an anion of a Lewis acid, A, having relative Lewis acidity greater than or equal to that of phenylbis(perfluorophenyl)borane, said anion being compatible with the metal cation, are prepared by contacting a derivative of a Group 4 or Lanthanide metal corresponding to the formula: LMX2, wherein L, M, and X are as previously defined, with the Lewis acid, A.

Low-temperature Fourier-transform infrared investigation of the interaction of CO with nanosized ZSM5 and silicalite

Abstract: Nanosized ZSM5 zeolites with microcrystal dimensions in the 20–120 nm range have been characterized by means of IR spectroscopy and HRTEM microscopy. The vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-ZSM5 samples of different crystallite dimensions has been investigated. For the sake of comparison the spectra of silicalite samples containing different concentrations of sodium and aluminium are also shown. For this purpose high-purity silicalite samples were prepared following a novel synthesis route.Carbon monoxide (a very weak Lewis base) was used to probe the acidity present on the external and internal surfaces of the zeolites through formation of 1 : 1 adducts with silanols (both internal and external), Bronsted-acid groups (both framework and extraframework), Na+ ions, and Lewis Al3+ centres (in extraframework and framework positions). The IR-active CO stretching modes of the complexes are shifted to higher wavenumber with respect to the free molecule; the positive shift can be used to estimate the acid strength. CO that was physically adsorbed in the zeolite channels has also been investigated.

Homogeneous Ziegler-Natta polymerization of functionalized monomers catalyzed by cationic group IV metallocenes

Abstract: Ziegler-Natta catalysts are remarkable in their ability to polymerize {alpha}-olefins to high molecular weight, stereoregular polyolefins. One of the major limitations of conventional Ziegler-Natta catalysts is their intolerance to Lewis bases; catalysts based on titanium halides and alkylaluminum cocatalysts are poisoned by most types of monomers containing ethers, esters, amines, and carboxylic acids. The absence of functionality in hydrocarbon polymers seriously affects their adhesive properties, affinity for dyes, permeability, and compatibility with more polar polymers. Previous attempts to polymerize sterically hindered amines, esters and amides, alkyl halides, and carboxylic acids using catalysts derived from TiCl{sub 3} and AlR{sub 3-n}Cl{sub n} have achieved limited success due to the severe loss of catalytic activity in the presence of these monomers. This work reports that cationic, group four metallocenes are active catalysts for the homo-polymerization of {alpha}-olefins containing silyl-protected alcohols and tertiary amines. Employing different monomers and conditions, a table shows the starting monomer, reaction time and temperature, and spectroscopic analysis of the end products. A major advanatage of these metallocene-based catalysts is that the ligand system can be modified to proved the optimal combination of catalystic activity, stereospecificity, and tolerance to functionality. 32 refs., 1 tab.

Synthetic fluorine chemistry

Abstract: Partial table of contents: Lewis Acid Properties of Fluorinated Noble Gas Cations (G. Schrobilgen). Controlled Replacement of Fluorine by Oxygen in Fluorides and Oxyfluorides (K. Christe, et al.). Transition Metal Derivatives of Strong Protonic Acids and Superacids (F. Aubke, et al.). Fluorine Stabilized Carbon--Sulfur Multiple Bonding (K. Seppelt). A New Synthetic Procedure for the Preparation and Manufacture of Perfluoropolyethers (R. Lagow, et al.). Universal Synthesis of Perfluorinated Organic Compounds (J. Adcock). Electrophilic Fluorination Reactions with F 2 and Some Reagents Directly Derived from It (S. Rozen). Fluorination with Onium Poly(Hydrogen Fluorides): The Taming of Anhydrous Hydrogen Fluoride for Synthesis (G. Olah & X. Li). Index.

Carbohydrates as chiral templates : stereoselective synthesis of (R)- and (S)-α-aminophosphonic acid derivatives

Abstract: The stereoselective synthesis of diethyl (S)- or (R)-α-[(O-pivaloyl-hexapyranosyl) amino]benzylphosphonates is achieved via Lewis acid catalyzed addition of diethyl phosphite to O-pivaloylated N-benzylidene -β-D-galactosylamine or N-benzylidene-α-D-arabinopyranosylamine. The process can also be performed by a one-pot procedure selectively giving (S)-aminophosphonic acid derivatives from galactosylamine and (R)-aminophosphonic acid derivatives from β-L-fucosylamine as the chiral auxiliaries.

Perfluoropolyethers: intramolecular disproportionation

Abstract: Thermal degradation of commercial perfluoropolyethers (Fomblin Z and Y, Krytox, and Demnum) and PFPEO (perfluoropoly(ethylene oxide)) in the presence of Lewis acids (metal oxides and halides) was examined. It is shown that the degradation is dominated by the intramolecular disproportionation process of the following scheme RCF 2 OCF 2 (CH 2 ) 3 OR → RCOF + CF 3 (CH 2 ) 3 OR. The reaction occurs when the two oxygen atoms flanking a monomer unit of polyether chains interact with a Lewis acid site(s). The differentail charges induced at the carbons immediately outside and inside the unit cause a fluorine transdfer and effect the disproportionation process

Acid-catalysed addition of N-aryl imines to dihydrofuran. Postulated dependence of the reaction mechanism on the relative face of approach of reactants

Abstract: Imines 1a and 1b react with dihydrofuran (DHF) under Lewis acid catalysis via Diels–Alder-type addition to form the tetrahydroquinoline derivatives 3a, b and 4a, b. Besides these compounds, the methanol adduct 2 of 1a, or 1a in the presence of methanol, gives the methanol-containing tetrahydrofuran derivative 7a and the hexahydrofuro[3,2-b]furan derivatives 5 and 6. In the presence of methanol, 1b gives 3b and 4b, and also 7b and 8. The products 3a, b, 5, 6 and 7a, b originate from approach of the Si(or Re) face of 1a, b onto the Si(or Re) face of DHF; 4a, b and 8 derive from interaction of the Si(or Re) face of 1a, b with the Re(or Si) face of DHF. The dependence of the product distribution on the polarity of the solvent suggests that a concerted mechanism predominates in the former mode and a zwitterionic one in the latter. In the addition of 1a or 2 the mechanistic preference may be exclusive.

UO2-salenes: neutral receptors for anions with a high selectivity for dihydrogen phosphate

Abstract: The design and synthesis of neutral macrocyclic receptors that complex cations in apolar solvents with a high degree selectivity are based on the ability to organize nucleophilic binding sites (Lewis bases) in a complementary array. Although macrocycles with quaternary ammonium salts are known to complex anions, selectivity is not simply introduced. Macrocyclic and acyclic ligands that contain Lewis acidic binding sites such as boron, silicon, tin, and mercury bind anions, but these structures lack the possibility of subtle structural variation that is the basis for the selectivity in cation complexation. In this communication we report a novel class of neutral receptors for anions that contain an immobilized Lewis acidic binding site (UO{sub 2}){sup 2+} and in which there are ample opportunities to introduce specific secondary binding sites. This concept leads to high thermodynamic stabilities combined with high selectivities. Neutral anion receptors are very important for application in sensors and membrane transport experiments. This unique combination of a Lewis acidic uranyl center and the presence of amide NH groups which can form a favorable H-bond with a coordinated anion guest, in a preorganized receptor, can lead to highly specific anion recognition. 20 refs., 2 figs., 1 tab.

Asymmetric tandem Mannich-Michael reactions of amino acid ester imines with Danishefsky's diene

Abstract: Imines 1 derived from aromatic, aliphatic, and functionalized aldehydes and various amino acid esters react with Danishefsky's diene under Lewis acid catalysis via a tandem Mannich-Michael mechanism to give cyclic 6-substituted 2,3-didehydro4-piperidinones in good to high yields and with diastereomeric ratios reaching from 92:8 up to 97:3. The chiral auxiliary is removed by conversion of the α C atom of the amino acid into an acetalic center, employing a Curtius reaction as the key step

Homogeneous catalysis: a ruthenium-based Lewis-acid catalyst for the Diels-Alder reaction

Abstract: In order to circumvent some of the inherent drawbacks of using traditional Lewis acids to catalyze the Diels-Alder and other Lewis-acid-promoted reactions, a ruthenium-based catalyst has been devised. The readily prepared catalyst is trans-[Ru(salen)(NO)(H 2 O)]SbF 6 , where salen is the N,N'-bis(salicylidene)ethylenediamine dianon ligand. It has has been fully characterized; it is stable to oxygen and can be handled under normal laboratory conditions

Dimethyl phosphate hydrolysis at neutral pH

Abstract: [(cyclen)Co(OH 2 ) 2 ] 3+ (1) hydrolyzes dimethyl phosphate with a second-order rate constant of 6.2×10 -7 M -1 s -1 at 60 o C, pD 6.3 (cyclen: 1,4,7,10-tetraazaclyclododecane). In constant, [(tetren)Co(OH 2 )] 3+ (2) does not hydrolyze the diester to any observable extent, even when the reaction solution is heated to 100 o C for 1 month (tetren: tetraethylenepentamine). The mechanism for 1-promoted hydrolysis of dimethyl phosphate involves joint Lewis acid activation and metal hydroxide activation

Electron-transfer oxidation of ketene silyl acetals and other organosilanes. Mechanistic insight into Lewis acid mediated electron transfer

Abstract: Kinetic studies on photoinduced and thermal electron-transfer oxidation of a variety of organosilanes in acetonitrile at 298 K are reported in terms of the electron-transfer rate constants (k et ) with a series of oxidants having the known one-electron reduction potentials (E 0 red ). The Rehm-Weller Gibbs energy relationship is applied to determine the fundamental parameters for the electron-transfer oxidation, i.e., the one-electron oxidation potentials (E 0 ox ) and the intrinsic barrier for the electron-transfer oxidation (ΔG 0 )

The isomerization of α‐pinene oxide with Brønsted and Lewis acids

Abstract: α-Pinene oxide (2) is a very reactive substrate which isomerizes rapidly under the influence of acid catalysts. A large number of products can be formed from a single intermediate. Selectivity to the industrially important campholenic aldehyde can be achieved with metal-halogen compounds, especially zinc chloride and zinc bromide. A possible explanation for this behaviour is offered, stipulating a stabilized carbenium-ion intermediate. Protonic catalysts are typically highly active but unselective. No evidence was found for the earlier reported selectivity to campholenic aldehyde with zeolite catalysts.

Inter-and Intramolecular [4 + 2] Cycloadditions of Nitroalkenes with Olefins. 2-Nitrostyrenes

Abstract: Aromatic nitroalkenes 9-12 underwent Lewis acid catalyzed cycloadditions with various cyclic alkenes to afford high yields of nitronates 25-30 with exclusive anti selectivity. Hammett studies helped to further delineate the role of the Lewis acid. Reaction of nitroalkenes 8 and 10 with various cyclic dienes in the presence of a Lewis acid demonstrated the ability of nitroalkenes to behave as dienes in cycloadditions. The major products were the syn diastereomers which arise from an endo-folded transition structure

Immobilized lewis acid catalysts

Abstract: Immobilized Lewis Acid catalyst comprising polymer having at least one Lewis Acid immobilized within the structure therein, said polymer having monomer units represented by the structural formula: --[A]a--[B]b--[C]c-- wherein a represents about 1 to about 99 mole %; b represents about 0 to about 50 mole %; c represents about 1 to about 99 mole %; a + b + c is preferably about 100 %; A is (α); B is (β); C is selected from the group consisting of (I), (II), and (III) combinations thereof, wherein D is OH, halide, OR4, NH2, NHR3, OM', or OM''; E is the residue of the reaction of at least one Lewis Acid with the D substituent of monomer unit B; R1 represents proton, C?1?-C24 alkyl group, or C3-C24 cycloalkyl; R?2? represents C?1?-C24 alkylene group, C3-C24 cycloalkylene, C6-C18 arylene, or C7-C30 alkylarylene; R?3? represents C?1?-C24 alkyl, C3-C24 cycloalkyl, C1-C24 aryl, or C7-C30 alkylaryl; R?4? represents C?1?-C24 alkyl, C3-C24 cycloalkyl, C1-C24 aryl, or C7-C30 alkylaryl; M' represents alkali metal; M'' represents alkaline-earth metal. Also disclosed are polymerization and alkylation processes utilizing the immobilized Lewis Acid catalysts. Another aspect of the present invention is a method of manufacturing immobilized Lewis Acid catalysts.

The fluoro(hydrogen cyanide)xenon(II) cation. Preparation of HC.tplbond.NXeF+AsF6-: a multinuclear magnetic resonance and Raman spectroscopic study

Abstract: The XeF + or Xe 2 F 3 + cations react with HC≡N in anhydrous HF solvent to give the Lewis acid-base adduct cation HC≡NXeF + . The HC≡NXeF + cation has been characterized in HF and BrF 5 solvents by 129 Xe, 19 F, 14,15 N, 13 C, and 1 H NMR spectroscopy and has been characterized as its AsF 6 - salt in the solid state by low-temperature Raman spectroscopy.

The mechanism of the zirconium-catalyzed carbomagnesiation reaction. Efficient and selective catalytic carbomagnesiation with higher alkyls of magnesium

Abstract: In zirconium-catalyzed carbomagnesation of alkenes, the dramatic enhancement in reactivity and selectivity induced by an internal Lewis base is such that higher order alkylmagnesium halides, normally significantly less reactive than EtMgCl, may be used efficiently. In reactions with n-BuMgCl, two secondary carbon stereogenic centers are formed with excellent levels of stereocontrol (4,>95%). Experimental data are presented which demonstrate that the observed levels of selectivity and reactivity are not simply the result of regioselective insertion of the alkene into the zirconacyclopropane

The role of Lewis acid-base processes in ligand-exchange chromatography of benzoic acid derivatives on zirconium oxide.

Abstract: Porous microparticulate zirconium oxide shows very different selectivities and pH dependencies for the separation of benzoic acid derivatives than do conventional bonded-phase anion-exchange supports This results from a very significant ligand-exchange contribution to the retention of hard Lewis bases on the surface of transition-metal oxide supports We have found that the capacity factors of a wide variety of derivatives of benzoic acid are closely correlated with their Bronsted acidities The eluent pH is also a critical factor in determining the magnitude of the capacity factor, but it does not have much influence on chromatographic selectivity The differential selectivity of this phase in comparison to conventional polymeric and bonded-phase anion exchangers can be attributed to complexation and steric effects which profoundly alter the elution patterns of certain solutes