Preparation hydrosilicate formation

Alternatively, monomers required for the synthesis of a/r-poly(carbosilane/ siloxanes) have been prepared by formation of Si-C bonds by Pt catalyzed hydrosilation reactions. For example, Pt catalyzed hydrosilation of the C-C double bond of the Diels-Alder adduct of cyclopentadiene and maleic anhydride with sym-tetramethyldisiloxane yields a difunctional monomer 5,5 -bis(l,l,3,3-tetramethyl-l,3-disiloxanediyl)norborane-2,3-dicarboxylic anhydride, see Figure 2. Reaction of this with diamines such as 4,4 -diaminobiphenyl yields a thermally stable poIy[imide-a/r(carbosilane/siloxane)] [3]. 

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]. 

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. 

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. 

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]. 

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. 

The reaction is surprisingly clean under these vigorous hydrosilation conditions, with no evidence for the formation of l,2-bis[tris(dimethyl-amino)]ethane (32). Thus, very satisfactory processes have been developed for both Tris and vinyl-Tris that do not involve chlorosilane intermediates. Both monomers are suitable intermediates for the preparation of polysilazane preceramic polymers that could be converted thermally to silicon nitride and mixtures of silicon nitride and silicon carbide. 

The band that in the pure zeolite appears at about 797 cm and that corresponds to symmetrical stretch of Si04 tetrahedra, diminishes with DP time indicating a probably dissolution process of the siliceous framework during the DP process. This also occurs with the band at about 1086 cm . This behaviour supports the formation of Ni hydrosilicates at the expense of the siliceous framework in the DP prepared samples. The... 

In the DP samples, after two hours deposition-precipitation, there is formation of a clear shoulder with DP time at about 1005 cm, which according to the literature [7], points to the presence of 1 1 nickel phyllosilicate. This is more clearly seen in the subtraction spectra. It appears that in the case of the cationic competitive exchange-prepared sample the formation of hydrosilicate species is only incipient. In fact, these... 

From the above results one can conclude that the DP preparation method leads to Ni/H(3 catalysts with better Ni dispersion than those prepared by cationic competitive exchange. This result seems to be due to the formation of a stronger support metal interaction (formation of Ni hydrosilicates) in the case of the DP method. The better deposition of Ni achieved by the DP method is clearly reflected in a superior activity in the naphthalene hydrogenation reaction. 

Silanes normally reduce aldehydes or ketones under catalytic conditions to the silyl ethers. However, with certain catalysts such as nickel sulfide,Co2(CO)8, or (Ph3P)3RhCl, carbonyl compounds react with silanes to yield an equilibrium mixture of enol silyl ethers (Scheme 17). In a similar vein, the silyl-hydroformylation reaction of cycloalkenes with CO and silanes may be a practical way to prepare enol silyl ethers. An example is the preparation of compound (49). Catalytic 1,4-hydrosilation of a, -un-saturated ketones or aldehydes gives the corresponding enol silyl ethers. The reaction is similar to the reductive silylation referred to previously, but the reaction conditions are neutral and milder. The formation of the enol silyl ether (50) is outlined below. ... 

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