Silane treatment in polypropylene composites: Adsorption and coupling

The study of the effect of eight silane coupling agents on mechanical properties of PP/CaCO₃ composites showed that the application of certain compounds leads to the reactive coupling of the components. The adsorption of the silanes on the surface of the filler was studied by a dissolution method, while the structure of the adsorbed layer was determined by FTIR spectroscopy and GPC measurements. Model reactions and an FTIR study were carried out to reveal possible coupling reactions of PP and the active silanes. The results showed that amino-functional silane coupling agents adsorb on the surface of the filler, polycondensation taking place and a polysiloxane layer of high molecular weight forms as a result. The model reactions indicated that during processing, PP oxidizes even in the presence of stabilizers and oxygen-containing functional groups form on the polymer chains. These easily react with the amino functionality of the silane coupling agents strongly bonding the polymer to the treated filler.     

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.     

Vinyl tris(trimethylsilyl)silanes: substrates for Hiyama coupling

The oxidative treatment of vinyl tris(trimethylsilyl)silanes with hydrogen peroxide in aqueous sodium hydroxide in tetrahydrofuran generates reactive silanol or siloxane species that undergo Pd-catalyzed cross-couplings with aryl, heterocyclic, and alkenyl halides in the presence of Pd(PPh₃)₄ and tetrabutylammonium fluoride. Hydrogen peroxide and base are necessary for the coupling to occur while activation of the silanes with fluoride is not required. The conjugated and unconjugated tris(trimethylsilyl)silanes serve as good cross-coupling substrates. The (E)-silanes undergo coupling with retention of stereochemistry while coupling of (Z)-silanes occurred with lower stereoselectivity to produce an E/Z mixture of products.     

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.     

Stereocontrolled Synthesis of Vinyl Boronates and Vinyl Silanes via Atom‐Economical Ruthenium‐Catalyzed Alkene–Alkyne Coupling

The synthesis of vinyl boronates and vinyl silanes was achieved by employing a Ru‐catalyzed alkene–alkyne coupling reaction of allyl boronates or allyl silanes with various alkynes. The double bond geometry in the generated vinyl boronates can be remotely controlled by the juxtaposing boron‐ and silicon groups on the alkyne substrate. The synthetic utility of the coupling products has been demonstrated in a variety of synthetic transformations, including iterative cross‐coupling reactions, and a Chan‐Lam‐type allyloxylation followed by a Claisen rearrangement. A sequential one‐pot alkene‐alkyne‐coupling/allylation‐sequence with an aldehyde to deliver a highly complex α‐silyl‐β‐hydroxy olefin with a handle for further functionalization was also realized.     

Enantioselective Construction of α‐Chiral Silanes by Nickel‐Catalyzed C(sp3)−C(sp3) Cross‐Coupling

An enantioselective C(sp³)?C(sp³) cross‐coupling of racemic α‐silylated alkyl iodides and alkylzinc reagents is reported. The reaction is catalyzed by NiCl₂/(S,S)‐Bn‐Pybox and yields α‐chiral silanes with high enantiocontrol. The catalyst system does not promote the cross‐coupling of the corresponding carbon analogue, corroborating the stabilizing effect of the silyl group on the alkyl radical intermediate (α‐silicon effect). Both coupling partners can be, but do not need to be, functionalized, and hence, even α‐chiral silanes with no functional group in direct proximity of the asymmetrically substituted carbon atom become accessible. This distinguishes the new method from established approaches for the synthesis of α‐chiral silanes.     

Investigation of titanium-catalysed dehydrogenative coupling and hydrosilylation of phenylhydrogenosilanes in a one-pot process

Titanium-catalysed dehydrocondensation and hydrosilylation of primary, secondary and tertiary phenylsilanes have been investigated in a one-pot process with Cp₂Ti(OPh)₂ as catalyst by NMR studies. Only primary and secondary silanes were found to undergo simultaneous dehydrogenative coupling and hydrosilylation reactions to produce the functional polysilanes.     

Highly stereoselective synthesis, structure, and application of (E)-9-[2-(silyl)ethenyl]-9H-carbazoles

(E)-N-(Silyl)vinylcarbazole has been easily prepared via a new catalytic route, silylative coupling (SC) of vinylcarbazole with vinyltrisubstituted silanes catalyzed by [RuH(Cl)(CO)(PCy3)2]. X-ray structures of two silylvinylcarbazoles as first N-vinylcarbazole derivatives have been resolved. The Pd-catalyzed Hiyama coupling reaction (also as the tandem reaction with SC) of synthesized (E)-N-(triethoxysilyl)vinylcarbazole with iodobenzene has been performed to afford (E)-N-(phenylvinyl)carbazole with high yield and stereoselectivity.     

Aryl(triethyl)silanes for Biaryl and Teraryl Synthesis by Copper(II)‐Catalyzed Cross‐Coupling Reaction

Aryl(triethyl)silanes are found to undergo cross‐coupling with iodoarenes in the presence of catalytic amounts of CuBr₂ and Ph‐Davephos, as well as cesium fluoride as a stoichiometric base. Because the silicon reagents are readily accessible through catalytic C?H silylation of aromatic substrates, the net transformation allows coupling of aromatic hydrocarbons with iodoarenes via triethylsilylation.     

Adsorption and characterization of molecular adhesion promoter monolayers on iron surfaces under UHV conditions

To investigate the chemical modification of metal surfaces by silanes and mercaptans used as molecular adhesion promoters between metal surfaces and polymer films, the adsorption of chlorosilanes and n-propylmercaptan has been examined on iron surfaces under ultra high vacuum conditions (UHV). The adsorption of silanes directly from the vapour phase has been impossible in UHV, however, a simultaneous condensation of water and silane leads to a stable silane layer. The hydrolysis reaction is rate determining. Stable mercaptan monolayers have been obtained only on oxygen covered iron surfaces. On pure iron the mercaptan molecules have been cracked, so that methyl groups as well as sulphur atoms could be found. The characterization of the surface layers has been performed by XPS and AES.     

Solution-based assembly of conductive gold film on flexible polymer substrates

Conductive films of gold were assembled on flexible polymer substrates such as Kapton and polyethylene using a solution-based process. The polymer substrates were modified by using argon plasma and subsequent coupling of silanes with amino- or mercapto- terminal groups. These modified surfaces were examined by X-ray photoelectron spectroscopy and contact angle measurements. Colloidal gold was assembled onto the silane-modified surface from solution. The gold particles are attached to the surface by covalent interactions with the thiol or amine group. Formation of a conductive film is achieved by increasing the coverage of gold by using a "seeding" method to increase the size of the attached gold particles. Field emission scanning electron microscopy was used to follow the growth of the film. The surface resistance of the films, measured using a four-point probe, was about 1 Omega/sq.     

Interaction of Silane Coupling Agents with CaCO3

A study of eight silane coupling agents showed very different effect of these compounds on the mechanical properties of PP/CaCO₃composites. The application of aminofunctional silane coupling agents resulted in the reactive coupling of the two inactive components leading to increased strength and decreased deformability. A detailed study of the interaction between CaCO₃and the various coupling agents was carried out in order to find an explanation for the strong coupling effect. The amount of coupling agent creating a monolayer coverage was determined by a dissolution method for each coupling agent. The obtained values changed between 0.3 and 1.0 wt% calculated for the CaCO₃. An attempt was made to determine the orientation of the adsorbed molecules to the filler surface. Most of the coupling agents are oriented perpendicularly to the surface with the exception of a methacryl functional silane compound. Possible interactions between hydrolyzed or condensed silane coupling agents and the filler were studied by Fourier transform infrared spectroscopy using transmitting (FTIR-TS) and diffuse reflectance (DRIFT) modes, as well as gel permeation chromatography (GPC). The results showed that bulky organofunctional groups form a caged, polycyclic, low-molecular-weight structure on the surface, while silanes with smaller groups tend to condense into open, ladder type, high-molecular-weight polysiloxane chains. Polymer/filler adhesion, however, depends primarily on the chemical character of the organofunctional group. Aminofunctional silane coupling agents adhere well to the filler surface and react also with the polymer. In the case of similar functionality the size of the organofunctional group determines the strength of the adhesion.     

Stereoselective Alcohol Silylation by Dehydrogenative Si–O Coupling: Scope,Limitations, and Mechanism of the Cu–H‐Catalyzed Non‐Enzymatic Kinetic Resolution with Silicon‐Stereogenic Silanes

Ligand‐stabilized copper(I)–hydride catalyzes the dehydrogenative Si–O coupling of alcohols and silanes—a process that was found to proceed without racemization at the silicon atom if asymmetrically substituted. The present investigation starts from this pivotal observation since silicon‐stereogenic silanes are thereby suitable for the reagent‐controlled kinetic resolution of racemic alcohols, in which asymmetry at the silicon atom enables discrimination of enantiomeric alcohols. In this full account, we summarize our efforts to systematically examine this unusual strategy of diastereoselective alcohol silylation. Ligand (sufficient reactivity with moderately electron‐rich monophosphines), silane (reasonable diastereocontrol with cyclic silanes having a distinct substitution pattern) as well as substrate identification (chelating donor as a requirement) are introductorily described. With these basic data at hand, the substrate scope was defined employing enantiomerically enriched tert‐butyl‐substituted 1‐silatetraline and highly reactive 1‐silaindane. The synthetic part is complemented by the determination of the stereochemical course at the silicon atom in the Si–O coupling step followed by its quantum‐chemical analysis thus providing a solid mechanistic picture of this remarkable transformation.     

Well-Defined,Molecular Bismuth Compounds: Catalysts in Photochemically Induced Radical Dehydrocoupling Reactions

A series of diorgano(bismuth)chalcogenides, [Bi(di-aryl)EPh], has been synthesised and fully characterised (E=S, Se, Te). These molecular bismuth complexes have been exploited in homogeneous photochemically-induced radical catalysis, using the coupling of silanes with TEMPO as a model reaction (TEMPO=(tetramethyl-piperidin-1-yl)-oxyl). Their catalytic properties are complementary or superior to those of known catalysts for these coupling reactions. Catalytically competent intermediates of the reaction have been identified. Applied analytical techniques include NMR, UV/Vis, and EPR spectroscopy, mass spectrometry, single-crystal X-ray diffraction analysis, and (TD)-DFT calculations.     

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.     

Catalytic enantioselective dehydrogenative Si–O coupling of oxime ether-functionalized alcohols

Asymmetric silylation of alcohols is an unusual but effective approach to their kinetic resolution, and the Cu–H-catalyzed dehydrogenative Si–O coupling is particularly noteworthy as dihydrogen is formed as the sole by-product. Our laboratory had previously reported on diastereoselective (with silicon-stereogenic silanes) and enantioselective (with achiral silanes) Si–O couplings of azine donor-functionalized alcohols. The limitation, that is, the requirement of a nitrogen donor atom, prompted us to seek equally useful donor groups. Oxime ethers were identified as a suitable alternative, and we describe herein the preparation of a series of oxime ether-functionalized alcohols. To assess different substitution patterns in their kinetic resolution, these were tested in the reagent-controlled Si–O coupling using a silicon-stereogenic silane. The optimal substituents at the oxime carbon atom (dr=85:15 in the diastereoselective coupling) were then chosen for the related catalyst-controlled Si–O coupling but selectivity factors were only moderate.     

Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles

The synthesis, spectroscopic characterization, and fluorescence quenching efficiency of polymers and copolymers containing tetraphenylsilole or tetraphenylgermole with Si-Si, Ge-Ge, and Si-Ge backbones are reported. Poly(tetraphenyl)germole, 2, was synthesized from the reduction of dichloro(tetraphenyl)germole with 2 equivs of Li. Silole-germole alternating copolymer 3 was synthesized by coupling dilithium salts of tetraphenylsilole dianion with dichloro(tetraphenyl)germole. Other tetraphenylmetallole-silane copolymers, 4-12, were synthesized through the Wurtz-type coupling of the dilithium salts of the tetraphenylmetallole dianion and corresponding dichloro(dialkyl)silanes. The molecular weights (M(w)) of these metallole-silane copolymers are in the range of 4000 approximately 6000. Detection of nitroaromatic molecules, such as nitrobenzene (NB), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and picric acid (PA), has been explored. A linear Stern-Volmer relationship was observed for the first three analytes, but not for picric acid. Fluorescence spectra of polymetalloles or metallole-silane copolymers obtained in either toluene solutions or thin polymer films displayed no shift in the maximum of the emission wavelength. This suggests that the polymetalloles or metallole-silanes exhibit neither pi-stacking of polymer chains nor excimer formation. Fluorescence lifetimes of polymetalloles and metallole-silanes were measured both in the presence and absence of TNT, and tau(o)/tau is invariant. This requires that photoluminescence quenching occurs by a static mechanism.     

Palladium/Copper Dual Catalysis for the Cross‐Coupling of Aryl(trialkyl)silanes with Aryl Bromides

Whereas aryl(trialkyl)silanes are considered to be ideal organometallic reagents for cross‐coupling reactions owing to their stability, low toxicity, solubility, and easy accessibility, they are generally inert under typical cross‐coupling conditions. Disclosed herein is a palladium/copper catalytic system that enables the cross‐coupling of trimethyl, triethyl, tert‐butyldimethyl, and triisopropyl aryl silanes with aryl bromides. This process is applicable to the sequential C?H and C?Si bond arylation of thiophenes and the synthesis of poly(thiophene–fluorene)s.     

Synthesis of organosilicon polymers containing donor-acceptor type π-conjugated units and their applications to dye-sensitized solar cells

Novel organosilicon polymers having donor-acceptor type π-conjugated units in the backbone were prepared by Stille-coupling reactions of bis(tributylstannylthienyl)silanes with diiodoquinoxialine, benzothiadiazole, and benzothiaselenazole. Dehaloganative copolymerization of di(bromothienyl)silanes and dibromoquinoxaline was also studied, which gave the corresponding random copolymer. Applications of these polymers to dye-sensitized solar cells (DSSCs) were examined and it was found that the polymer containing benzoselenadiazole units as the acceptor exhibited the best performance as the sensitizer among the present polymers.     

Catalysis by titanocene-functionalized polymer-supported dendrimers

A series of variously-functionalized first-, second-, and third-generation dendrimers have been prepared and linked via a biphenyl core to a bis-styryl moiety suitable for use as a crosslinker in polymerization. Attachment of titanocene moieties to the first-generation system and copolymerization with styrene affords polymeric disks that exhibit catalytic properties superior to comparable solution-phase systems in a multicomponent coupling of chlorosilanes with Grignards to give bis-allylic silanes.