Reductive lithiation of methyl substituted diarylmethylsilanes: application to silanediol peptide precursors

Reductive lithiation of methyl-substituted diarylmethylsilanes using lithium naphthalenide represents a practical method for the preparation of the corresponding silyl lithium reagents. Their addition to chiral sulfinimines affords versatile precursors to silanols and silanediols. The replacement of the currently used diphenylsilane motif by a more labile diarylsilane moiety allows the selective hydrolysis of one or two aryl groups by treatment with TFA.     

The Rearrangement of α-Acetylenic Alcohols to α, β-Unsaturated Carbonyl Compounds by Silylvanadate Catalysts

New catalytic systems have been found which effect the isomerization of α-acetylenic alcohols to the corresponding α, β-unsaturated carbonyl compounds with high efficiency. These are combinations of silylvanadates with silanols or silanediols.     

Selective Manganese‐Catalyzed Oxidation of Hydrosilanes to Silanols under Neutral Reaction Conditions

The first manganese‐catalyzed oxidation of organosilanes to silanols with H₂O₂ under neutral reaction conditions has been accomplished. A variety of organosilanes with alkyl, aryl, alknyl, and heterocyclic substituents were tolerated, as well as sterically hindered organosilanes. The oxidation appears to proceed by a concerted process involving a manganese hydroperoxide species. Featuring mild reaction conditions, fast oxidation, and no waste byproducts, the protocol allows a low‐cost, eco‐benign synthesis of both silanols and silanediols.     

Palladium-catalyzed cross-coupling of silanols, silanediols, and silanetriols promoted by silver(I) oxide

Palladium-catalyzed cross-coupling of aryl- or alkenylsilanols, silanediols, and silanetriols with a variety of iodoarenes by the catalysis of palladium(0) and in the presence of silver(I) oxide furnished the coupling products in good to excellent yields. The reactions of silanediols or silanetriols under similar conditions proceeded much faster than those of silanols to afford the corresponding coupling products in excellent yields within shorter reaction periods (5-12 h). The measurement of the X-ray diffraction (XRD) pattern of the silver residue after the reaction revealed that silver(I) oxide was converted to silver(I) iodide.     

Si-O and P-O motifs in inorganic rings and clusters

This article presents a brief overview of different kinds of silanols and the important reactions of N-bonded silanetriols. The synthesis and characterization of silanediols containing Si-N and Si-C bonds, (2,4,6-Me₃C₆H₂)N(SiMe₃)Si(Me)(OH)₂ (1e) is described. 1e shows a polymeric crinkled tape type of structure due to extensive intermolecular hydrogen bonding. We also describe the formation of a neutral copper(II) complex derived from the reaction of CuCl₂ with (O)P(3,5-Me₂Pz)₃ where the ligand undergoes a facile P-N bond cleavage and functions as an N₂O type of tridentate ligand.     

Silanediols: A New Class of Potent Protease Inhibitors

Transition state analogues of the peptide hydrolysis intermediate can take the form of complex silanediols such as 1 , which inhibits angiotensin-converting enzyme (ACE) at nanomolar concentrations. In contrast, earlier investigation of enzyme inhibition with simple silanediols and silanetriols showed them to be inactive.     

Comparison of D,L-Mandelic Acid Resolution on Zeolite and Silica Supported Pirkle-Type Chiral Stationary Phases

 Zeolite A, a material of crystalline character, and Hypersil silica have been used as support for the preparation of chiral stationary phases. On the amorphous silica support surface the silanol groups are randomly dispersed. The crystalline zeolite secondary building units consisting primarily of SiO₄, AlO₄ ⁻ tetrahedra determine the regularity of surface silanol groups. Owing to the crystal lattice structure, the location of silanols is well determined and hence the dispersion of chiral selector molecules chemically bonded onto the zeolite surface silanol groups is fundamentally arranged. Amides of DNB-L-Leu, DNB-L-Phe, B-L-Leu chiral selector molecules were anchored onto the zeolite silanols and B-L-Leu onto the silica support silanols. Lipophilic buffer in RP conditions has dynamically modified the residual silanols of each support. The enantioseparation of ion paired D,L-mandelic acid from aqueous solution on the zeolite and silica supported chiral stationary phases prove a superior enantioseparation on the zeolite supported phases. Revision February 18, 2000.     

Bis(2-thienyl)silanes: new, versatile precursors to arylsilanediols

Silanediols have been shown to be effective bioisosteres for the hydrated carbonyl group. Current methods for the formation of silanediols place a number of constraints on how and where this functionality may be used. A range of arylsilanes that would allow both the formation of arylsilanediols and that are also compatible with multi-step synthetic routes, have been investigated as possible precursors to silanediols. Through this study bis(2-furyl)silanes and, in particular, bis(2-thienyl)silanes have been identified as practical precursors to arylsilanediols.     

Microstructure and optical properties of PMMA/gel silica glass composites

FT-Raman and near infrared (NIR) spectroscopies have been used to investigate PMMA/gel silica composites prepared by the post-doping method. The changes of the vibrational features of silanols on the silica surface and ester carbonyls from PMMA indicate that the two phases in the organic-inorganic hybrids interact with each other through hydrogen-bonding. Unlike unmodified gel silica, the as-prepared composites are hydrophobic due to the elimination of the accessible adsorbing sites provided by the surface silanols. Residual water inherited from the preparation process is not evenly distributed in the composites but locally enriched on the silica surface by hydrogen-bonding to limited surface silanols. The conversion efficiency of MMA-to-PMMA depends not only on the polymerisation conditions applied, but also on the initial structure of the sol-gel substrate used. The composites show high optical transmittance in the near UV and visible region due to a reduction of scattering from an originally porous structure. However, their application in the near IR is restricted due to the combined vibrational energy absorptions by different silanol and water species, and in particular by C-H functional groups from organic modifiers.     

Zur Synthese von Siloxanen auf der Basis von Silyltriflat-Derivaten

Synthesis of Siloxanes on the Basis of Silyltriflates The reaction of silyltriflates with silanols leads to the formation of siloxanes. This method is of importance if the synthesis of silyltriflates is more simply and selectively than the preparation of the corresponding chlorosilanes. This is observed in the case of functional substituted silyltriflates or triflate derivatives of oligosilanes. The synthetic potential of these compounds for the preparation of siloxanes is shown on selected examples.     

A robust method for the Hiyama-type coupling of arylsiloxanes and disiloxanes with aryl halides

Palladium catalysed Hiyama-type coupling of aryl disiloxanes or aryl silanols with aryl halides in the presence of stoichiometric silver(I) oxide and catalytic TBAF allows the rapid preparation of biaryls in moderate to high yield under mild thermal or microwave irradiation conditions.     

Residual silanols at reversed-phase silica in HPLC--a contribution for a better understanding

As porous silica gel is the most common adsorbent and support for bonded stationary phase synthesis, residual silanol groups are a recurring problem in the field of liquid chromatography and other separation techniques. Residual silanols most often have a negative effect on the separation process by causing peak tailing. Therefore, there was an attempt to remove residual silanols during stationary-bonded phase synthesis. The type and surface concentration of residual silanols were measured using different instrumental techniques such as NMR and infrared spectroscopy, calorimetry, and various chromatographic methods. Residual silanols exhibit acidic characteristic and they can ionize depending on the environment. Thus, they posses cation-exchange properties and cause the zeta potential of silica particle in liquid environment. Presented review discusses the influence of the residual silanol groups on the solvation process and retention of polar compounds. The novel methodology of residual silanols determination is presented as well as the influence of the silanols on the zeta potential of the stationary-bonded phases in chromatographic conditions.     

Evaluation of high-performance liquid chromatography column retentivity using macromolecular probes. II. Silanophilic interactivity traced by highly polar polymers

A new method of HPLC column retentivity testing utilizes polymeric probes instead of conventional sets of low molar mass substances. The procedure allows at least semiquantitative, separate and independent evaluation of adsorption and partition properties of column packings. In this present work, the method is applied for comparison of the polar interactivities of selected silica gel C18 HPLC columns. It is shown that free silanols which remained on the surface of the end-capped silica C18 column packings are accessible for interaction with highly polar macromolecules. High molar mass polymeric test probes are adsorbed on the surface silanols and their retention volumes increase. As result, deviations from regular size-exclusion chromatographic (SEC) behavior are observed. The extent of retention volume changes depends on both the nature of polymer probes and on column packing type. Adsorption of macromolecules can be suppressed by addition of a highly polar substance to the mobile phase. The amount of polar additive which is needed to attain regular SEC elution of the polymer probe depends on the column packing type and can be used as a characteristic of silanophilic column interactivity. Courses of dependences of retention volumes on sizes of macromolecules indicate the presence of "U-turn" adsorption which allows two and more silanols situated among C18 groups to be occupied simultaneously with the same macromolecule.     

Fourier-Transform Infrared Spectroscopy Studies of the Interaction of Functionalized Siloxane Polymers with Porous Silica

Adsorption behavior of the siloxane polymer, cyanopropylmethyl-phenyl-methyl-siloxane, in contact with a mesoporous silica has been investigated by Fourier-transform infrared spectroscopy (FT-IR) using a series of physically modified materials with different coating levels up to 20 w/w%. The nitrile stretch bands, which are observed in the 2150-2350 cm(1) range, exhibited complex profiles due to a variety of interactions of the cyano groups within the polymer and with the surface silanols. These complex bands have been resolved into multiple components using a curve-fitting procedure. Three major spectral components have been found and assigned to the nitrile stretch in the bulk polymer, interfacial polymer, and polymer attached to surface silanols via hydrogen bonding. The latter spectral component is dominating for the samples with low coating levels. Its relative contribution decreased with the increasing amount of the polymer on the silica surface due to increasing contribution of the pure-polymer spectral component. Copyright 2000 Academic Press.     

Monitoring free silanols on reversed-phase supports with peptide standards

Summary A series of synthetic peptide HPLC standards has been designed for use in monitoring free silanols on silica-based reversed-phase supports. The four standards (+ 1 to + 4 net charge) showed great versatility in monitoring silanol activity over a pH range of 2.0 to 7.0. We have developed a silanol monitoring system based on the sensitivity of the standards to ionic interactions with reversed-phase packings at pH 7.0. This monitoring procedure involves chromatographing the peptides at pH 7.0 with a combined acetonitrile and sodium perchlorate linear AB gradient and is generally applicable to reversed-phase packings exhibiting a wide range of silanol concentrations. These standards not only demonstrate the presence and extent of free silanols, they also allow the researcher to determine what changes must be made to the mobile phase composition to minimize ionic interactions. In addition, the standards, in conjunction with the double gradient monitoring system at pH 7.0, will aid researchers and manufacturers in the improvement and development of reversed-phase matrices.     

On the nature of the Brønsted acidic groups on native and functionalized mesoporous siliceous SBA-15 as studied by benzylamine adsorption from solution

The concentration and Br?nsted acidity of surface silanol groups on mesoporous silica (SBA-15) has been studied by following the adsorption of benzylamine, BA, from water as a function of pH. The adsorbed amount of BA from water was compared to the maximum amount of BA that could be adsorbed from cyclohexane. Furthermore, the surface concentration and acidity of carboxylic acid functions on surface-functionalized SBA-15 was also studied, which allowed the relative surface concentration of remaining silanols to be obtained. Two types of silanols can be identified, where about 1/5 of the silanols have a pKa 相似文献   

DFT study of the adsorption of microsolvated glycine on a hydrophilic amorphous silica surface

Density functional theory (DFT) periodic ab initio molecular dynamics calculations are used to study the adsorption of gaseous and microsolvated glycine on a hydroxylated, hydrophilic silica surface. The silica model is presented and the interaction of water with surface silanols is studied. The heat of interaction of water is higher with the associated silanols (be they terminal or geminal ones) studied here than with isolated silanols presented in past works. Glycine is stabilized in a parallel mode on the hydroxylated surface. Terminal silanols do not allow the stabilization of the zwitterionic form, whereas geminal silanols do. Molecular dynamics (MD) first-principle calculations show that microsolvated zwitterion glycine directly binds through the carboxylate function to a surface silanol rather than through water molecules. The adsorption mode, whether with or without additional water molecules, is parallel to the surface. The ammonium function does not interact directly with the silanol groups but rather through water molecules. Thus, the carboxylate and ammonium functions exhibit two different reactivities towards silanols. The calculated free energies, taking into account the chemical potentials of water and glycine in the gas phase, suggest the existence of a thermodynamic domain in which the glycine is present in the gas phase as well as strongly adsorbed on specific sites of the surface.     

氢氟酸后处理对Silicalite-1催化环己酮肟气相Beckmann重排反应性能的影响

 研究了氢氟酸后处理对 silicalite-1 催化环己酮肟气相Beckmann重排反应性能的影响. 结果表明,经适当浓度的氢氟酸溶液处理后,催化剂的选择性和稳定性都明显改善. 其中, silicalite-1 原粉先经硝酸铵预处理后再进行氢氟酸后处理所得到的催化剂催化性能最好,反应53 h后环己酮肟的转化率仍保持在96%左右,己内酰胺的选择性高达96.1%. XRD, FT-IR和 29Si MAS NMR的结果表明,较高的具有氢键相互作用的硅羟基与孤立硅羟基的比例值对环己酮肟气相Beckmann重排反应有利,同时, silicalite-1 表面硅原子的排布方式对该反应也有重要影响.     

Determination of the thermodynamic contribution to peak asymmetry of basic solutes in reversed-phase liquid chromatography

To this day packing materials manufacturers are still trying to develop reversed-phase stationary phases that have silica more completely reacted with bonding ligands to afford more homogeneous particle surfaces. Incomplete bonding causes inhomogeneous effects that are readily observed when separating basic solutes because of the acidic silanols that are unreacted. However, it is still not understood exactly what types of silanol sites are unreacted or if metal impurities are contributing to the resulting peak asymmetry observed. A method is presented which utilizes (1) the frontal analysis method of chromatography to obtain adsorption/partition isotherms, (2) a heterogeneous Langmuir distribution model for the resulting isotherm, (3) an expectation-maximization numerical procedure to solve the mathematical problem to yield the most probable distribution of adsorption parameters, and (4) the equilibrium-dispersive model of chromatography incorporating the fitted isotherm model to check the validity of the sorption model with experimental observations. Correlation of packing materials characterization parameters with results obtained by this procedure will indicate what type of silanols or other sites are responsible for observed tailing behavior. Developers and manufacturers will then be able to more efficiently target their synthetic designs for "base-deactivated" reversed-phase silicas.     

Reversed-phase liquid chromatography without organic solvent for determination of tricyclic antidepressants

The chromatographic behavior of seven tricyclic antidepressants (amitryptiline, clomipramine, doxepin, imipramine, maprotiline, nortryptiline, and trimipramine) was examined with micellar mobile phases containing the nonionic surfactant Brij-35. Acetonitrile-water mixtures were also used for comparison purposes. Tricyclic antidepressants are moderately polar basic drugs, which are positively charged in the usual working pH. This gives rise to a strong association with the alkyl chains and residual ionized silanols in silica-based stationary phases, which is translated in a high consumption of organic solvent to get appropriate retention times. Brij-35 modifies the surface of the stationary phases creating a neutral bilayer that masks silanols and reduces the polarity. Consequently, the retention times are decreased. A simple chromatographic procedure for the control of tricyclic antidepressants in pharmaceutical formulations was developed, using 0.02 M Brij-35 at pH 3 and UV detection. Satisfactory recoveries were achieved, with intra- and inter-day relative standard deviations usually below 1 and 2%, respectively. The preparation of the samples was simple and only required solubilization and filtration steps previous to injection. The proposed procedure has the advantage of not using an organic solvent in the mobile phase, and the biodegradable character of Brij-35. This makes an example of "green" liquid chromatographic analysis.