Silarylene has been an important structural motif in silicone material synthesis. The related research has been mostly focused on silphenylene, but the utility of its siloxane copolymers was limited due to the existing multi-step synthetic methods to obtain the silphenylene monomers. We disclose one-step synthetic method to access non-phenyl-based silarylene–siloxane copolymers by C–H silyl polymerization: the protocol directly utilizes commercial heteroarenes as monomer with no need of pre-functionalization. The optimal catalytic systems were identified by a series screening of Rh and phosphorous-based ligands, and the corresponding preformed complexes efficiently catalyzed the polymerization reactions to afford various silarylene–siloxane copolymers: the use of the well-defined catalysts is important in controlling silane redistribution side reactions, so as to improve overall yield and efficiency. This C–H silyl polymerization protocol can be further utilized for material curing process by the corresponding cross-linking polymerization with multifunctional siloxane monomers.

Various aryl and heteroaryl monomers for polysiloxane materials are finally accessible by intermolecular dehydrogenative C-H silylation between commercial (hetero)arenes and the industry-relevant triethoxysilane. The development of well-defined rhodium catalysts enables the silylation of triethoxysilane, which is known for poor reactivity in this silylation and prone to undergo the redistribution side reaction. For the silylation of electronically unactivated arenes, portionwise addition of the silane is necessary to ensure a high efficiency. Mechanistic investigation including computational study led to the isolation of two important catalytic intermediates and their dynamic interconversion, which provide mechanistic insight into the importance of portionwise addition and the intrinsic difference between arenes and heteroarenes in the silylation. In addition to their monomer roles, (hetero)aryl triethoxysilanes can be broadly utilized as versatile intermediates or coupling agents in chemical synthesis.

Step- and atom-economical catalytic C-H silylations were developed to prepare heteroaryl alkoxysilane monomers for silicone materials. The silylations are operable in the presence of 1-3 mol % of a well-defined Rh complex to afford the desired aryl monomers in up to 95% yield. The preformed complexes are able to suppress silane redistribution side reactions so as to improve overall efficiency of less reactive HSiMe(OEt)2. The resulting monomers will broaden the scope of aryl substituents that is currently limited to only phenyl groups, so that various polysiloxanes bearing nonphenyl aryl substituents can be readily prepared with modulating physical and chemical properties.

Conventional techniques for the removal of silicone oils from small siloxane molecules (either cyclic species, such as D4 or linear siloxanes, known as L2-L6) include stripping and adsorption. Membrane separations could serve as low-energy alternatives or complements to these industrial separation techniques to reduce the overall process energy demand since they do not involve a phase change. This paper presents polymeric membranes as materials for purifying D4 (octamethylcyclotetrasiloxane) rich feed of 90% D4 from a silicone fluid mixture. The transport properties (diffusion, sorption, and permeation) are studied experimentally to understand the driving force and mechanism for the separation related to the properties of the penetrant molecules, such as molecular size. These parameters are utilized in the solution-diffusion model to predict the permeation rates and separation performance of PDMS membranes for the siloxane-silicone fluid mixtures that are rich in the small siloxane. The results presented here suggest that PDMS membranes may serve as an industrially scalable low-cost alternative to small siloxane purification.

Volatile methylsiloxanes (VMS) are man-made, nonbiodegradable chemicals produced at a megaton-per-year scale, which leads to concern over their potential for environmental persistence, long-range transport, and bioaccumulation. We used directed evolution to engineer a variant of bacterial cytochrome P450BM3 to break silicon-carbon bonds in linear and cyclic VMS. To accomplish silicon-carbon bond cleavage, the enzyme catalyzes two tandem oxidations of a siloxane methyl group, which is followed by putative [1,2]-Brook rearrangement and hydrolysis. Discovery of this so-called siloxane oxidase opens possibilities for the eventual biodegradation of VMS.

Efficient protocols for intermolecular C-H silylations of unactivated arenes and heteroarenes with HMe2SiOEt are disclosed. The silylations are catalysed by a Rh-complex (0.5 mol%) derived from commercially available [Rh(coe)2Cl]2 and (S,S)-Ph-BPE in the presence of cyclohexene at 100 °C, furnishing desired arylethoxydimethylsilanes up to 99% yield. The regioselectivity is mainly affected by the steric bulk of the substituents in arenes and by electronic effects as an ancillary factor. Mechanistic study revealed that the mono-hydrido dimeric Rh-complex, [Rh2(Ph-BPE)2(μ-H)(μ-Cl)], is an active catalytic intermediate, which further suppresses the formation of redistribution byproducts in the silylation. Preliminary results show that the current protocol can be extended to double C-H silylations affording bis-silylated arenes and is applicable to the silylation of HMeSi(OEt)2 to deliver the corresponding (aryl)SiMe(OEt)2.

Polyhedral oligomeric silsesquioxane (POSS) cage structures were exclusively generated as the reaction products of methyltrichlorosilane, CH3SiCl3 (or MeSiCl3 where Me denotes a CH3-group), with a supported Re/CeO2 catalyst at 500 degrees C. The cage structures are generated by the exchange of oxygen for chlorine in the ceria support under a H2 reducing atmosphere. Regeneration of the support can occur by passing air over the catalyst at 500 degrees C, and then the reaction can be repeated. This presents a novel way of generating cage structures exclusively via a gas-phase reaction. Gas chromatography-mass spectrometry (GCMS) of the reaction products indicates that the cage structures that are formed are Me6-T6, Me8-T8, and Me10-T10. The reaction proceeds at high conversion and high yield toward these POSS cages. When dimethyldichlorosilane ((CH3)2SiCl2 or Me2SiCl2) is used as the reactant, mostly cyclic siloxanes D4-D6 are produced. Hydrido-chlorosilane, methyldichlorosilane (CH3SiHCl2 or MeHSiCl2), generates cage structures through an additional Si-H-O ligand exchange reaction that is facilitated by the oxygen vacancies on the ceria support. When Si-H ligands are present, rearrangement reactions are also observed under these conditions and constitute a supplementary process for the formation of the POSS cage structures. Trichlorosilane (HSiCl3) does not form cage structures, but it does undergo rearrangement. It also undergoes Si-H-O ligand exchange with oxygen vacancies on the ceria support to form cristobalite (crystalline SiO2) on the support.

Significant inroads have been made using biocatalysts to perform new-to-nature reactions with high selectivity and efficiency. Meanwhile, advances in organosilicon chemistry have led to rich sets of reactions holding great synthetic value. Merging biocatalysis and silicon chemistry could yield new methods for the preparation of valuable organosilicon molecules as well as the degradation and valorization of undesired ones. Despite silicon's importance in the biosphere for its role in plant and diatom construction, it is not known to be incorporated into any primary or secondary metabolites. Enzymes have been found that act on silicon-containing molecules, but only a few are known to act directly on silicon centers. Protein engineering and evolution has and could continue to enable enzymes to catalyze useful organosilicon transformations, complementing and expanding upon current synthetic methods. The role of silicon in biology and the enzymes that act on silicon-containing molecules are reviewed to set the stage for a discussion of where biocatalysis and organosilicon chemistry may intersect.

The direct reaction of methyl chloride with magnesium and palladium infused silica substrates to synthesize methyl chlorosilanes is reported. First, high energy ball milling on solid Mg-SiO2 mixtures produces elemental silicon and MgO. When PdCl2 is infused into the mixture, after additional ball milling and high-temperature reduction under H2 , dipalladium silicide (Pd2 Si) is produced. The silicon of the Pd2 Si readily reacts with MeCl under Müller-Rochow reaction conditions, to produce methyl chlorosilanes at yield ratios analogous to those of the traditional process. The dominant product is Me2 SiCl2 (selectivity > 30%), followed by MeSiCl3 and Me3 SiCl, with minor amounts of the remaining chlorosilanes. Silicon conversion exceeds 20% for most of the substrates. The elemental palladium, which remains within the Pd-Mg-SiO2 contact mass is re-converted to Pd2 Si at the next H2 /high-temperature treatment and reacts again with MeCl to repeat the methyl chlorosilane production. In principle, the resulting cycle of the mechanochemically induced formation of Pd2 Si followed by the reaction with MeCl can be repeated until the starting SiO2 converts completely to methyl chlorosilanes.

Compared to the biological world’s rich chemistry for functionalizing carbon, enzymatic transformations of the heavier homologue silicon are rare. We report that a wild-type cytochrome P450 monooxygenase (P450BM3 from Bacillus megaterium, CYP102A1) has promiscuous activity for oxidation of hydrosilanes to make silanols. Directed evolution enhanced this non-native activity and created a highly efficient catalyst for selective silane oxidation under mild conditions with oxygen as terminal oxidant. The evolved enzyme does not touch C–H bonds also present in the silane substrates, nor does this biotransformation lead to disiloxane formation, a common problem in silanol syntheses. Computational studies reveal that catalysis proceeds through hydrogen atom abstraction followed by radical rebound, as observed in the P450’s native C–H hydroxylation mechanism. Enzymatic silane oxidation now extends Nature’s already impressive catalytic repertoire.

We disclose the direct synthesis of methylmethox-ysilanes from pentacopper silicide, Cu5Si, and dimethyl carbonate, DMC, affording high levels of dimethylsilyl products without the use of halide catalysts. When Cu5Si powder (99.5%) was reacted with DMC at 350 degrees C, Me2Si(OMe)(2) was the major silane product at >70% selectivity. In contrast, when a high purity Cu5Si (99.99%) was used, the reaction afforded mainly permethoxylated silanes. ICP-OES identified several impurity elements in the low purity Cu5Si. By synthesizing materials that contained the individual impurity elements and reacting them with DMC, tin was revealed as being a crucial promoter. XPS revealed that tin segregates to the surface under reaction conditions, and elevated tin levels have a significant impact on DMC reactivity with the surface. XPS also suggests that tin is in the zero oxidation state at the surface, which provides some insight to its role in the direct synthesis of methylmethoxysilanes.

A synthetic process for new functional metallosiloxanes containing M-OEt groups (where M is metal) has been developed. Compounds of iron, aluminum and zirconium were obtained by the interaction of a corresponding metal chloride with a mixture of sodium organoalkoxysilanolate and sodium ethylate in a ratio that makes it possible to preserve and adjust the number of functional groups at the metal atoms. A process of partial hydrolysis of functional metallosiloxanes and a possibility of using the products obtained as a binder in compositions based on silicone rubber have been investigated. It has been shown that such compounds can be used as cross-linking agents in silicone rubber compositions.

The present study relates to the development of a new generation of copper aluminate-type spinel catalysts for the production of dimethyldichlorosilane ((CH3)(2)SiCl2) from silicon tetrachloride (SiCl4) in a two-step reaction process. The first step is the reaction of SiCl4 with H-2 over the spinel catalysts at high temperature (similar to 650 degrees C) to produce a copper-silicon-rich solid (copper silicide) by silicon deposition. In the second step, the silicon component of this solid reacts with CH3Cl at 300 degrees C to form (CH3)(2)SiCl2. Copper aluminates (CuAl2O4) were found to exhibit superior activity over other conventional copper catalysts and held better particle integrity in laboratory-scale fixed bed and fluid bed reactors. Their activity was maintained even at a 240 degrees C reaction temperature with CH3Cl, producing high selectivity toward (CH3)(2)SiCl2. Maximum production of (CH3)(2)SiCl2 was achieved right from the beginning of the reaction with CH3Cl without requiring any "activation period". In contrast, the conventional Cu-supported catalysts required "an activation period" of several initial cycles to obtain maximum steady state activity. The high rate of methylchlorosilanes production on spinel catalysts is attributed to the smaller particle sizes of copper and the in situ formation of a reactive copper silicide (Cu3.17Si) phase as a result of the reaction with SiCl4/H-2. The copper aluminate catalyst was tested in 45 experiments in time on stream and showed stable selectivity toward (CH3)(2)SiCl2 in a laboratory-scale fixed bed reactor. It demonstrated superior and acceptable attrition resistance and copper retention, whereas conventional supported copper catalysts did not retain the copper during piloting in a fluidized process. Overall, we developed a low cost, efficient, and scalable process to produce dimethyldichlorosilane ((CH3)(2)SiCl2).

The present work shows a new one-stage mechanochemical method for the direct synthesis of alkoxysilanes.

New ethoxy-functional metallosiloxanes with a high activity in alkoxysilane polycondensation reactions have been synthesized with the use of Rebrov salts. A possibility of using the obtained methyldiethoxysiloxy derivatives of metallosiloxanes as binders for the preparation of homogeneous polydimethylsiloxane-containing compositions by realizing the concept of "liquid filling" has been demonstrated. The properties of the resulting compositions have been studied. (C) 2018 Elsevier B.V. All rights reserved.

This paper details a method to chlorinate tetraalkyl orthosilicates in the presence of a catalyst using SOCl2 as the chloride source/deoxygenating agent. Several inexpensive catalysts were screened, and it was found that soluble chloride salts performed better than Lewis base catalysts. The optimized reaction employed a widely used and commercially available soluble chloride salt catalyst (e.g., NBu4Cl, 0.4 equiv), 16 equiv of SOCl2, and afforded quantitative yield of SiCl4 after 3 h. As the bulk of the orthosilicate substrate increased, the yield of SiCl4 decreased. A reaction mechanism has been proposed.

This report describes a method to synthesize SiCl4 from alkyl orthosilicates and gaseous HCl. Reacting tetramethyl orthosilicate with HCl gas at 0 degrees C in the presence of a catalytic amount of hexamethyl phosphoramide (10 mol %) and four equivalents of acetonitrile afforded an 86% yield of SiCl4 after 6 h. Exchange between HCl and alkoxy groups on silicon during the reaction generates methanol. The methanol then reacts with acetonitrile (in the presence of HCl) to form an imidate, thus removing it from the reaction mixture. Other Lewis acid and base catalysts were also observed to accelerate the reaction. A kinetics experiment using Si-29 NMR to monitor reaction intermediates in situ was conducted to determine the rates of chloride/alkoxide exchange on silicon.