Designing Cross-Coupling Reactions using Aryl(trialkyl)silanes

Organo(trialkyl)silanes have several advantages, including high stability, low toxicity, good solubility, easy handling, and ready availability compared with heteroatom-substituted silanes. However, methods for the cross-coupling of organo(trialkyl)silanes are limited, most probably because of their exceeding robustness. Thus, a practical method for the cross-coupling of organo(trialkyl)silanes has been a long-standing challenging research target. This article discusses how aryl(trialkyl)silanes can be used in cross-coupling reactions. A pioneering example is Cu^II catalytic conditions with the use of electron-accepting aryl- or heteroaryl(triethyl)silanes and aryl iodides. The reaction forms biaryls or teraryls. This design concept can be extended to Pd/Cu^II-catalyzed cross-coupling polymerization reactions between such silanes and aryl bromides or chlorides and to Cu^I-catalyzed alkylation using alkyl halides.     

Stereoselective Synthesis of (Z)- and (E)-Allylic Silanes by Copper-Mediated Substitution Reactions of Allylic Carbamates with Grignard Reagents

Both (Z)- and (E)-allylic silanes were prepared with high stereoselectivity by the copper-mediated substitution of allylic carbamates by organometallic reagents. The reaction of alkylmagnesium reagents with (E)-allylic carbamates provides (Z)-allylic silanes, whereas both alkylmagnesium and alkyllithium reagents react with (Z)-allylic carbamates to afford (E)-allylic silanes. Because Grignard reagents are often more facile to prepare than alkyllithium species, these reagents are the optimal nucleophiles for the synthesis of both (Z)- and (E)-allylic silanes. This method also allows readily available nonracemic allylic carbamates to be converted to chiral, nonracemic (Z)- and (E)-allylic silanes with high stereoselectivity.     

Regio‐ and Enantioselective Cobalt‐Catalyzed Sequential Hydrosilylation/Hydrogenation of Terminal Alkynes

A highly regio‐ and enantioselective cobalt‐catalyzed sequential hydrosilylation/hydrogenation of alkynes was developed to afford chiral silanes. This one‐pot method is operationally simple and atom economic. It makes use of relatively simple and readily available starting materials, namely alkynes, silanes, and hydrogen gas, to construct more valuable chiral silanes. Primary mechanistic studies demonstrated that highly regioselective hydrosilylation of alkynes with silanes occurred as a first step, and the subsequent cobalt‐catalyzed asymmetric hydrogenation of the resulting vinylsilanes showed good enantioselectivity.     

Wege zu Si-funktionalisierten Organostannylsilanen

Summary Silicon substituted organostannyl silanes are formed by the reaction of stannylallyl silanes with triflic acid and from stannyl-hydrogen silanes and CHBr₃. The structures have been proved by NMR-investigations.

Herrn Prof. Dr.E. Hengge zum 65. Geburtstag gewidmet     

Headgroup effect on silane structures at buried polymer/silane and polymer/polymer interfaces and their relations to adhesion

Sum frequency generation (SFG) vibrational spectroscopy was used to study the effect of silane headgroups on the molecular interactions that occur between poly(ethylene terephthalate) (PET) and various epoxy silanes at the PET/silane and PET/silicone interfaces. Three different silanes were investigated: (3-glycidoxypropyl) trimethoxysilane (γ-GPS), (3-glycidoxypropyl) methyl-dimethoxysilane (γ-GPMS), and (3-glycidoxypropyl) dimethyl-methoxysilane (γ-GPDMS). These silanes share the same backbone and epoxy end group but have different headgroups. SFG was used to examine the interfaces between PET and each of these silanes, as well as silanes mixed with methylvinylsiloxanol (MVS). We also examined the interfaces between PET and uncured or cured silicone with silanes or silane-MVS mixtures. Silanes with different headgroups were found to exhibit a variety of methoxy group interfacial segregation and ordering behaviors at various interfaces. The effect of MVS was also dependent upon silane headgroup choice, and the interfacial molecular structures of silane methoxy headgroups were found to differ at PET/silane and PET/silicone interfaces. Epoxy silanes have been widely used as adhesion promoters for polymer adhesives; therefore, the molecular structures probed using SFG were correlated to adhesion testing results to understand the molecular mechanisms of silicone-polymer adhesion. Our results demonstrated that silane methoxy headgroups play important roles in adhesion at the PET/silicone interfaces. The presence of MVS can change interfacial methoxy segregation and ordering, leading to different adhesion strengths.     

Reductions, Reductive Alkylations, and Intramolecular Cyclizations of Acyl Silanes with Samarium Diiodide or Tributyltin Hydride

A series of acyl silanes including aliphatic-, aromatic-, and bis-acyl silanes, as well as the acyl silanes bearing other substituents such as a bromine atom and alkenyl, succinimide, and carbonyl groups, were prepared, and their reactions with samarium diiodide or tributylstannane were studied. The reactions of acyl silanes occurred in various manners such as reductions, reductive alkylations, intramolecular radical cyclizations, pinacol couplings, aldol reactions, and Tishchenko reactions, depending on the nature of substrates and reaction conditions. Acyl silanes were generally reduced to give the corresponding alpha-silyl alcohols without transfer of silyl groups. Intramolecular radical cyclizations of 5-hexenoyl silanes and 1-silyl-1,5-pentanedione were realized to give alpha-silyl cyclopentanols and 1,2-cyclopentanediol derivatives, respectively. On treatment with samarium diiodide in tetrahydrofuran, 1-(trimethylsilyl)-1,6-hexanedione underwent a pinacol coupling reaction in the presence of t-BuOH, whereas it underwent a Tishchenko reaction in the presence of MeOH. The Tishchenko reaction of 1-silyl-1,5-pentanedione gave a delta-silyl-delta-lactone. On treating with samarium diiodide, 1-(trimethylsilyl)-1,5-hexanedione and 1,5-bis(trimethylsilyl)-1,6-hexanedione, underwent, respectively, intramolecular aldol reactions.     

Adsorption of organosilanes at a Zn-terminated ZnO (0001) surface: molecular dynamics study

Four different organosilanes (octyltrihydroxysilane, butyltrihydroxysilane, aminopropyltrihydroxysilane, and thiolpropyltrihydroxysilane) adsorbed at a reconstructed Zn-terminated polar ZnO (0001) surface are studied via constant temperature (298 K) molecular dynamics simulations. Both single adsorbed silane molecules as well as adsorbed silane layers are modeled, and the energy, distance, orientation, and alignment of these adsorbates are analyzed. The adsorbed silane molecules exhibit behavior depending on the chemical nature of their tail (nonpolar or polar) as well as on the silane concentration at the solid surface (single adsorption or silane layer). In contrast to the O-terminated ZnO surface studied previously, now adsorption can only occur at the vacancies of this reconstructed crystal surface, thus leading to an arched structure of the liquid phase near the crystal surface. Nevertheless, both nonpolar and polar single adsorbed silanes show a similar orientation and alignment at the surface (orthogonal in the former, parallel in the latter case) as for the O-terminated ZnO surface, although the interaction energy with the surface is considerably increased for nonpolar silanes while it is nearly unaffected for the polar ones. For adsorbed silanes within silane layers, the difference to single adsorbed silanes depends on the polarity of the tail: nonpolar silanes again show an orthogonal alignment, while polar silanes exhibit two different orientations at the solid surface-a head and a tail down configuration. This leads to two completely different but nevertheless stable orientations of these silanes at the Zn-terminated ZnO surface.     

Enantio-, Regio- and Chemoselective Copper-Catalyzed 1,2-Hydroborylation of Acylsilanes

Enantioselective synthesis of synthetically significant (α-hydroxyallyl)silanes, (α-hydroxyaryl)silanes, and (α-hydroxyalkyl)silanes is reported. The present copper-catalyzed 1,2-selective hydroborylation of acylsilanes affords the aforementioned products in high yields and with high enantiomeric excesses. This robust and scalable additive-free catalytic system relies on the use of low copper(II) acetate and diphosphine ligand loadings at room temperature in the presence of a commercially available and bench-stable hydride source.     

Gold-catalyzed intramolecular allylation of silyl alkynes induced by silane alcoholysis

The activation of alkynyl allyl silanes with a cationic gold catalyst in the presence of alcohols provides vinyl silanes that contain the allyl group at the beta-position and the alkoxysilyl group in cis-orientation. The bond reorganization process is most consistent with the involvement of a carbocationic intermediate, which undergoes a nucleophilic attack by an alcohol selectively at the silicon center. The cis-vinyl silyl ether products can be further elaborated by ring-closing and cross metathesis to form more substituted 1,4-dienyl silanes.     

氨基硅烷偶联剂对蒙脱石的修饰改性研究

研究了氨基硅烷偶联剂对蒙脱石的修饰改性,并和长链烷基硅烷偶联剂作对比.通过改性前后蒙脱石的傅立叶红外光谱(FT-IR),广角X射线衍射(WAXD),热失重分析(TGA)研究发现,在冰醋酸的处理下,氨基硅烷偶联剂不但能够对蒙脱石进行表面偶联修饰而且能够以插层剂的形式进入蒙脱石的层间.初步的浸润/分散性实验结果表明:氨基硅烷插层/表面修饰改性的蒙脱石在弱极性乙醇溶剂中的分散性能明显提高.     

硅烷化学改性环氧涂层的耐蚀性能

利用"活性"与"非活性"硅烷化学改性环氧涂层以提高其耐蚀性."活性"硅烷指含有可以与环氧树脂的环氧端基发生开环反应官能团的硅烷,通常为氨基硅烷;"非活性"硅烷指不与环氧端基发生反应,但在有机锡催化剂存在下可与环氧树脂骨架上的羟基发生缩合反应的硅烷.红外光谱显示,两类硅烷均可成功接枝在环氧树脂上.电化学阻抗谱(EIS)和加速腐蚀试验(Machu试验)测试表明,经硅烷化学改性后的环氧涂层均能提高其耐蚀性能.     

Ligand-Regulated Regiodivergent Hydrosilylation of Isoprene under Iron Catalysis

A method for the regiodivergent and stereoselective hydrosilylation of the basic industrial feedstock isoprene with unactivated silanes has been developed using earth-abundant iron catalysts. The manipulation of regioselectivity relies on fine modification of the coordination geometry of the iron center. While a bidentate pyridine imine ligand promoted the formation of allylic silanes through 4,1-addition, selectivity for the 3,4-adduct homoallylic silanes was observed with a tridentate nitrogen ligand. Experimental studies and analysis were carried out to elucidate the reaction mechanism and the factors enabling manipulation of the regioselectivity. This study contributes to the art of regioselectivity control in alkene hydrofunctionalization.     

Improved photoprotection of wood by chemical modification with silanes: NMR and ESR studies

Silylation of wood specimens was performed with the aim of providing weathering protection, in particular photoprotection from the detrimental effect of sunlight. Primer formulations containing various silanes, were prepared and applied to wood specimens. ²⁹Si NMR, ¹³C‐ and ²⁹Si‐CP/MAS NMR spectroscopy were employed to investigate silane oligomer formation in primers, the extent of reaction occurring between wood and silanes, and the type of polysiloxane network formed inside the wood structure. Furthermore, the weathering performance of silylated wood was tested by exposing specimens to UV radiation, and subsequently measuring the photo‐induced free radicals using electron spin resonance spectroscopy. Remarkable differences of individual silanes have been observed regarding polymerization patterns and reactivity with wood. Aminosilanes oligomerized most readily in primer formulations to produce siloxanes whereas γ‐epoxysilane was least reactive. α‐Silanes and di‐functional silanes showed higher reactivity with wood than γ‐silanes and tri‐functional silanes. α‐Amino‐ and α‐methacryloxysilanes provided long chain and highly cross‐linked polysiloxane networks, while γ‐epoxysilane formed a modestly cross‐linked structure. Best photoprotection was achieved with α‐aminosilanes; however, wood specimens turned yellow during silylation. Nevertheless, silylation of wood was considered successful in providing superior photostability using α‐ and γ‐methacryloxysilanes and γ‐epoxysilane. Approximately, 35% less free radicals were generated compared with no treatment, and negligible color change was observed. Copyright © 2012 John Wiley & Sons, Ltd.     

Copper-catalyzed allylation of carbonyl derivatives using allyl(2-pyridyl)silanes

[reaction: see text] We have developed an efficient copper-catalyzed allylation of carbonyl derivatives using allyl(2-pyridyl)silanes, in which the strong directing effect of the 2-pyridyl group was observed. A useful synthesis and allylation of substituted allyl(2-pyridyl)silanes is also described.     

Nickel‐Catalyzed Decarboxylative C–Si Bond Formation: A Regioselective Cross‐Coupling Between Trialkyl Silanes and α,β‐Unsaturated Carboxylic Acids

This report presents the first example of nickel‐catalyzed mild decarboxylative cross‐coupling reaction for the regioselective formation of C–Si bond. An easily accessible and significantly stable Ni (dmg)₂ owes the role of key promoter. This reaction is highly functional group tolerant and offers α,β‐unsaturated silanes in synthetically useful yields. The reaction gives access to the successful utilization of otherwise difficult trialkyl silanes as coupling partners and operates at a moderate temperature, which is beneficial to deal with highly volatile silanes.     

Reaction of sodium trimethylsilylacetylide with alkoxychlorosilanes and methylalkoxychlorosilanes

Conclusions Sodium trimethylsilylacetylide reacts with alkoxychlorosilanes and methylalkoxychlorosilanes to give alkoxy(trimethylsilylethynyl)silanes and methylalkoxy(trimethylsilylethynyl)silanes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1674–1675, July, 1973.     

Asymmetric Synthesis of Silicon-Stereogenic Silanes by Copper-Catalyzed Desymmetrizing Protoboration of Vinylsilanes

The catalytic asymmetric creation of silanes with silicon stereocenters is a long-sought but underdeveloped topic, and only a handful of examples have been reported. Moreover, the construction of chiral silanes containing (more than) two stereocenters is a more arduous task and remains unexploited. We herein report an unprecedented copper-catalyzed desymmetrizing protoboration of divinyl-substituted silanes with bis(pinacolato)diboron (B₂pin₂). This method enables the facile preparation of an array of enantiomerically enriched boronate-substituted organosilanes bearing contiguous silicon and carbon stereocenters with exclusive regioselectivity and generally excellent diastereo- and enantioselectivity.     

In situ silane synthesis and bonding of monomeric stationary phases for high-performance liquid chromtography

Summary A two-step, in situ synthesis and bonding reaction of monomeric alkyl chemically bonded stationary phases is described. Silane ligands are first synthesized from the corresponding terminal olefin and dimethylhydrochlorosilane using a hexachloroplatinic acid catalyst. The platinum may then be removed if desired by adsorption to activated carbon. Bonding to silica gel is then carried out in the normal manner directly, without further purification. The resulting bonded phase is equivelant both chemically and chromatographically to phases produced in the normal manner using vacuum-distilled or otherwise purified silanes. This approach provides a simple protocol for the synthesis of novel silanes not commericially available, or of silanes not sufficiently volatile for purification by vacuum distillation.     

Two-dimensional Monte Carlo simulations of ionic and nonionic silane self-assembly on hydrophilic surfaces

Aqueous chemistries have recently been shown to be useful for the deposition of hydrophobic films of nonionic and cationic silanes on hydrophilic substrates for the prevention of stiction in MEMS. The Monte Carlo method is used to simulate in two dimensions the self-assembly of silane films on a hydrophilic surface. We investigate the impact of charged group in cationic silane on the overall structure of the films. We characterize the film structure with spatial pair correlations at each molecular layer of the deposited films. The simulations reveal long-range correlations for the film of cationic silanes. Based on our two-dimensional simulations, we report an average "most probable" structure for the films of nonionic and cationic silanes.     

Asymmetric Synthesis of Silicon‐Stereogenic Silanes by Copper‐Catalyzed Desymmetrizing Protoboration of Vinylsilanes

The catalytic asymmetric creation of silanes with silicon stereocenters is a long‐sought but underdeveloped topic, and only a handful of examples have been reported. Moreover, the construction of chiral silanes containing (more than) two stereocenters is a more arduous task and remains unexploited. We herein report an unprecedented copper‐catalyzed desymmetrizing protoboration of divinyl‐substituted silanes with bis(pinacolato)diboron (B₂pin₂). This method enables the facile preparation of an array of enantiomerically enriched boronate‐substituted organosilanes bearing contiguous silicon and carbon stereocenters with exclusive regioselectivity and generally excellent diastereo‐ and enantioselectivity.