Hydrosilicate formation

We believe that these phenomena can be explained byassuminga skin of silica on the metal surface. The fact that such a skin develops is readily acceptable for the hydrosilicates, since actually the prereduction state shows this skin in the form of Si206 layers on top of the octahedral layers containing the Ni++ ions. That, however, even co-precipitation catalysts such as 5421 in which no hydrosilicate formation could be observed by thermal analysis show inaccessibility comes somewhat as a surprise (Fco/Fr = 0.8 46% Ni removable). 

We observe again that the formation of hydrosilicates in the co-precipitation catalysts is deleterious for the activity, especially when montmoril-lonite is formed. The interaction causing the decrease in activity proves active even before a definite indication of hydrosilicate formation can be obtained, and it is interesting to note that partial deactivation occurs with a co-precipitation catalyst (8241), but not with a mixture type catalyst (8242). This fact appears to confirm the suggestion (see I, 5) that even a layer of silica on the Ni(OH)2 crystals during co-precipitation is harmful. 

To produce Fe(oxide)/Si02 particles one could use a solution of Fe(II), in which case air has to be excluded to prevent its oxidation to Fe(III). Iron (II) starts to react markedly above pH = 4.8 (urea, 90°C). In this case, the reaction is not limited to Eqn. 9.14, but a bulk hydrosilicate is formed. Upon performing an injection experiment at 45°C, it is observed that the reaction with the support is less extensive at that temperature. As the slightly higher pH at the injection point brings about the formation of a less reactive iron species, attack of the support is less marked than in the urea case even at 90°C. The structures obtained in the three different experiments are indicated in Fig. 9.11. The different extents of hydrosilicate formation are reflected in the temperature-programmed reduction (TPR) experiments, as can be seen in Fig. 9.11. Previous air-drying partially oxidizes the Fe(II). As interaction with silica stabilizes Fe(II), the supported Fe samples show a separate reduction step to Fe(II), which is not displayed by bulk Fe oxide. The iron hydrosilicate obtained in urea precipitation at 90°C is fairly stable and is reduced only above 650°C. The Fe(II) precipitated at 45°C is more... 

Hydrosilicate formation is also in evidence in the Cu(II)-Si02 system. Via precipitation from a homogeneous solution one can obtain highly dispersed copper oxide on silica (cf. above, Fig. 9.10, where it should be noted that the Cu case is more complicated than the Mn one in that intermediate precipitation of basic salts can occur). Reaction to copper hydrosilicate is evident from temperature-programmed reduction. As shown in Fig. 9.12 the freshly dried catalyst exhibits reduction in two peaks, one due to Cu(II) (hydr)oxide and the other, at higher temperature, to Cu(II) hydrosilicate. Reoxidation of the metallic copper particles leads to Cu(II) oxide, and subsequent reduction proceeds therefore in one step. The water resulting from the reduction of the oxide does not produce significant amounts of copper hydrosilicate, in contrast to what usually happens in the case of nickel. 

The Formation of SiSi Bonds from Silanes in the Presence of a Hydrosilation... 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

Recently, this work has been extended and further developed by Brown-Wensley into a preparative method for the synthesis of disilanes. The results of competitive reactions with several silanes allow insight into the reaction kinetics, in particular the relative rates of disilane formation versus hydrosilation (Table 5a, b) [61]. 

Table 5b. Relative rates of disilane formation and hydrosilation process in the presence of various catalysts...

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

The formation of cyclohexasilanes and the zirconium catalyzed hydrosilation of poly(phenylsilylene), referred to above, both suggest that slow cleavage of polymer chains may occur in the presence of the catalysts. Such cleavage may also play an... 

A random distribution of D s and H s was almost achieved in this example. Such a result required many exchange steps before hydrosilation i.e., these exchanges were much faster than the formation of the alkylsilane. The alkylsilane left the complex as the last, irreversible, relatively slow step in hydrosilation. 

Dicobaltoctacarbonyl at concentrations about 10-3 M was an excellent catalyst (lib) for hydrosilation of 1-hexene by common silanes, including (MeO)3SiH, Et3SiH, and PhCl2SiH, to give exclusively n-hexyl silanes. The only observable side reaction was formation of hexene-2 and hexene-... 

The three-step procedure described for the preparation of the illustrated crotylsilanes is initiated with the hydrosilation of rac-3-butyn-2-ol. This procedure is significantly improved with respect to the positional selectivity of the hydrosilation resulting in exclusive formation of the racemic (E)-vinylsilane, and as a result the present procedure is much more amenable to scale-up than those previously described in the literature.8 The enzymatic resolution of the racemic secondary allylic alcohol (vinylsilane) has also been reported using commercially available lipase extracts. The use of a Johnson ortho ester Claisen rearrangement affords the (E)-crotylsilanes 4 in nearly enantiomerically pure form. 

Jander, W., and J. Wurhrer (1938). Hydrothermal reactions (I) Formation of magnesium hydrosilicates. Zeit. Anorg u allgem Chemie 235 273-294. Jefferson, D. A., L. G. Hallinson, J. L. Hutchison, and J. M. Thomas (1976). Structural irregularities in nephrite jade an electron microscope study. Mater. Res. Bull. 11 1557-1562. 

Silacyclohex-2-enes are conveniently made through the formation of l-trichlorosilyl-5-chloro-l-pentene by hydrosilation, followed by CyClization using magnesium (Scheme 230) (65JOM(4)284). The Diels-Alder addition of silenes to 1,3-dienes provides a useful route to the isomeric silacyclohex-3-enes (Scheme 231) (79CR529). 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

Hydrosilation of 1,5- and 1,6-dienes, catalyzed by a chiral neodymium complex, leads to intramolecular bond formation and results in the formation of silylated methylcy-clopentanes (equation 147)538. This reaction is in stark contrast to the reaction catalyzed by group VHI complexes, which gives a range of silanes but no carbocyclic compounds539,540. 

It was as early as 1946 that de Lange and Yisser (1) published their finding that the precipitate formed on adding alkali hydroxide to a suspension of diatomaceous earth in a nickel salt solution is not merely a mixture of nickel hydroxide and diatomaceous earth, but that a reaction has taken place between the hitherto supposed inert carrier and the nickel compound. They ascertained the occurrence of such a reaction from the very great increase in accessible surface and the more difficult reduction of the reaction-product as compared with that of nickel hydroxide, and, on the basis of their X-ray work, suggested the formation of nickel hydrosilicates having a layer structure. Further, they assumed that after reduction... 

In the first place, we learned more about the formation of nickel hydrosilicates under certain circumstances from an investigation of Van Eijk van Voorthuysen and Franzen (2). These investigators made a number of preparations by combining boiling dilute solutions of nickel nitrate and alkali silicate in various proportions. In order to find out to what extent co-precipitation is required for the formation of hydrosilicate structures, acid was added to a nickel hydroxide suspension in a silicate solution by which silica is precipitated, or conversely, alkali was added to a suspension of silica gel in a nickel nitrate solution. Some of the preparations were subjected to a hydrothermal treatment at 250° C. for 50 hrs. with a sufficient quantity of water for developing the best possible structure. 

It was further found by Van Eijk van Voorthuysen and Franzen ( ) that the SiOa-content and the occurrence of the hydrosilicates in the various preparations have a definite influence on their reducibility. This can be clearly seen from Fig. 3, where the degree of reduction (as determined by chemical analysis) is plotted as a function of the reduction temperature. From the reduction curves of 8272 (0.0% Si02), 8227 (6.1% Si02), 8201 (27.4% Si02), and 8281 (41.0% Si02), it is apparent that the reducibility of a compound decreases greatly with increasing quantities of silica. Moreover, the formation and improvement of hydrosilicate structures is invariably accompanied by a decreased reducibility. 

It has been observed that rapid isomerization accompanies the cobalt carbonyl-catalyzed hydrosilation of olefins (18). The reaction of equimolar amounts of a trisubstituted silane and dicobalt octacarbonyl has been shown to result in the formation of cobalt hydrocarbonyl (cf. Section IV). A very effective isomerization catalyst may be prepared by treatment of a solution of Co2(CO)8 in olefin ( 0.01 M) with a silicon hydride in sufficient quantity to slightly exceed the cobalt carbonyl concentration. 

A recent investigation of the hydrosilation of alkynes by Et3SiH in the presence of the bridged derivative (PhCH2)Me2SiPt[P(cyclo-hexyl)3](p.-H) 2, analogous to (LVIII), has shown that the rate increases as the 7r-acceptor power of the alkyne increases, consistent with the formation of Si-Pt-alkyne complexes as intermediates 416). This complements earlier kinetic studies on the hydrosilation of alkenes catalyzed by Co, Rh, and Pt compounds, in which similar Si-metal-alkene intermediates were postulated 85, 134). 

First, it was observed that substrates of lower basicity were hydrosilated by Ph3SiH much more rapidly than substrates of higher basicity. Thus, for the substrates benzaldehyde, acetophenone and ethylbenzoate, observed turnover numbers were 19, 45 and 637 h 1, respectively, while the measured equilibrium constants for adduct formation of these substrates with B(C6F5)3 were 2.1 X 104, 1.1 X 103 and 1.9 X 102. A similar inverse correlation between turnover number and equilibrium constant was observed for a series of para-substituted acetophenone derivatives, where much faster hydrosilation rates were observed for substrates with strongly electron withdrawing groups in the para position. Clearly, if activation of the substrate via adduct formation is important in the hydrosilation reaction, the opposite correlation between TON and Keq should be observed. 

A second puzzling observation was the inverse dependence of hydrosilation rates on the concentration of substrate. In a rate study in which the concentration of acetophenone was varied, rate suppression was observed as [acetophenone] increased. Since raising [acetophenone] is expected to shift the equilibrium for adduct formation towards the adduct, this phenomenon argues against direct involvement of the adduct in the catalytic cycle for hydrosilation. Finally, it was observed that, in a competition experiment between benzaldehyde and ethylbenzoate, benzaldehyde was hydrosilated almost exclusively at a comparable rate to the non-competitive hydrosilation of the aldehyde. This observation shows that the relative basicity of the substrate does play an important role in the reaction, determining the chemoselectivity of the reaction. 

After etching, bonding of an organic moiety to the etched inner wall for the stationary phase takes place using the silanization/hydrosilation process adapted for the capillary format [18-20], This approach for attaching a variety of organic moieties to silica involves first silanizing the surface by reaction with triethoxysilane (TES) to create a hydride intermediate [21] as shown in equation 7.1. 

The OTCEC capillaries described in this chapter have been fabricated in a manner so that the major problems associated with packed capillaries are not present. The open tubular approach greatly reduces the likelihood of bubble formation so that pressurization of the system is not necessary. The other major problem, strong adsorption of basic compounds on the typical support material, is eliminated through the modification scheme, silanization/ hydrosilation, that removes silanols and replaces them with hydride groups. This type of separation medium also eliminates the need for any additives in the mobile phase to suppress adsorption of basic compounds, a technique that is often used in packed capillaries as the only means to elute such analytes. Therefore, the bulk of the applications developed to date have centered on the elution characteristics of compounds and separation of mixtures that are difficult to obtain in the packed capillary format. The major exception is the resolution of optical isomers that often can be done equally as well or often better with packed capillaries. The main objective of the chiral separations is to illustrate the presence of... 

For some systems, however, the influence of the temperature on the phase composition can be predicted based on chemical considerations. For instance, the composition of bismuth molebdatc catalysts is believed to be determined by the nature of the molybdate anion present in solution [28] which is dependent on the solution temperature. For Ni/SiCh catalysts the differences between catalysts prepared at high or low temperatures are explained by the formation of nickel hydrosilicate at high temperatures, while at low temperature the main precipitate is nickel hydroxide [29]. 

On the basis of the data obtained, the authors conclude that mechanical activation of the mentioned mixtures leads either to partial interaction (in the case of anhydrous oxides) or to profound interaction (in the case of hydrated oxides). In the second case, the reaction completes with the formation of X-ray amorphous calcium hydrosilicates belonging to tobermorite group and being crystallized under heating in the form of vollastonite p-CaSi03. 

The preparahon of calcium hydrosilicates with Ca/Si = 2 by means of mechanical treatment was inveshgated in [29]. It is known that it is very difficult to obtain hydrosihcates characterized by the ratio Ca/Si = 2 by any method including hydrothermal synthesis. The mixture under activation was 2CaO+Si02 (sihca gel) + 2-fold excess of water (with respect to theoretical amount). Activation resulted in the formation of X-ray amorphous hydrosilicate differed from that prepared by hydrothermal synthesis it was a mixture of monomers and dimers. 

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