SiH-Containing Carbosilanes

Similar results were obtained by analysing the products obtained in the reaction of (H2Si—0112)3 with MeMgCl in ether or THF. In no case was the molecular skeleton split. Even using a large excess of MeMgCl in ether caused only one SiH group to be methylated, while in the presence of THF, methylation was considerably enhanced, as is illustrated in Table 38 [94]. 

One observes from the NMR spectra of the reaction mixtures, that the multiply methylated 1,3,5-trisilacyclohexanes are cis-trans mixtures. This is also indicated during gas chromatographic separation by the largely differing melting points of the isomers observed in the product cooling traps [31]. As a result, by addition of excess 

Mole equiv. of MeMgCl Solvent Total yield of compounds shown 18 Et20 57% 0.5 THF 80% 0.8 THF (reflux) 90% 

Compounds formed (H2Si-CH2)3 Number of Me groups 0 Percentage amount % 87 Percentage amount % 1 Percentage amount % 

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The product is a limpid colorless liquid with an odor typical of SiH-containing carbosilanes, with bp. 83 ° (extrapolated from the vapor pressure measurement). It can be stored successfully in a sealed glass tube, but in contact with traces of H2O or air, polymerization occurs. While pure (11381)2 l2 remains unchanged over a period of years, in the presence of Al l3 at 20 ° it degrades over a period of months the starting material is completely consumed. Thirty percent of the 8i in (11381)2 l2 is converted to 8iH. are present are... 

Reduction of partly C-chlorinated SiCl-containing carbosilanes enables preparation of the corresponding SiH-containing carbosilanes with a partly chlorinated bridging C atom ... 

Linking of Linear SiH-Containing Carbosilane Units over HC = CH... 

The bromination of SiH4 P5] and SiH-containing alkylsilanes is known [96] and can be used in the quantitative determination of Si—H groups present [97]. Partial bromination of carbosilanes, whereby Si—H groups are still contained in the compounds, is of interest in a number of aspects. Partial chlorination with CI2 could be achieved by careful manipulation of the reaction. Reaction with Br2 offers more favorable possibilities [64]. 

The Si- and C-chlorinated derivatives differ from SiH-containing C-chlorinated compounds only through the Si—Cl group. Because the Si—Cl bond has a large electron density available for shielding of the Si atom, due to the increased C-chlorination a reduced polarization and reactivity of the Si—Cl bond is observed. Therefore the different behavior of SiH- and CCl-containing carbosilanes, as opposed to SiCl-and CCl-containing carbosilanes, can be understood. 

Once the basic synthetic methodology for carbosilane dendrimers was established by these early reports, many variations of carbosilane dendrimers were synthesized and described in the literature. A primary modification has been the use of different core molecules. Syntheses involving different core molecules are summarized in Table 1. Core molecules containing SiH groups were first reacted with a chlorovinylsilane in a hydrosi-lylation reaction in order to provide reactive sites for a nucleophilic substitution step. Note also that in some cases, the final dendrimers are terminated with groups different from the nucleophilic reagent. 

Figures 5 and 6 show the mass spectra of linear and cyclic carbosilanes containing SiH groups. The most striking difference between carbosilanes with SiH and SiCH3 groups is that, as the hydride content increases, the mass spectrum becomes richer in lines. With methylated compounds, ions are formed almost exclusively by cleavage of Si—C bonds (mass lines single, apart from isotope effects, with a minimum separation of 14 mass units). On the other hand, ions are formed from molecules with Si—H bonds by...

The PMR spectra of carbosilanes containing SiH groups are more complicated than those of the SiH-free compounds. Tables XIII and XIV... 

Coupling Constants (Cycles/Sec) of Linear Carbosilanes Containing SiH Groups... 

Spectra of cyclic carbosilanes containing SiH groups are much more complicated and cannot be explained yet in detail. Compounds which have been examined are shown with their NMR parameters in Table XV. 

With carbosilanes containing no SiH groups it is not always possible to determine the structure unambiguously from the PMR spectra because of the lack of characteristic spin-spin couplings. In such cases a solution of the structural problem may be possible by using nuclear resonance to follow the course of a chemical reaction of the compound in question. The small amount of substance required is a distinct advantage. For example,... 

If instead the reaction is conducted in the presence of MeLi (Table 41), methyla-tion of the SiH groups is accompagnied by attack on the molecular skeleton. Apart from methylated silanes including a large amount of SiMc4, carbosilanes containing more than two Si atoms were also produced, which comprised not only those compounds formed due to simple chain extension but also cyclic fully methylated 1,3,5-trisilacyclohexane. Even on addition of 12 mole equivalents of MeLi, the reaction ceases as soon as all products are Cl-free. Under no circumstances does methylation of bridging C atoms occur as is observed in reactions of perchlorinated carbosilanes with MeLi [122]. 

This occurs via reactive SiH groups that exist in 2,2-dichloro-l,3-disilapropane or its corresponding methylated products, for instance, producing the next higher carbosilane series containing three Si atoms. 

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