Unsymmetrical Tetraorganosilanes

At least one organic residue of the compound R —Si—R may be different from the 

Fritz has studied [69-72] a great number of unsymmetrical tetraorganosilanes containing alternating carbon and silicon atoms ( carbosilane chemistry ). 

A reaction between two symmetrical tetraorganosilanes can also be used for the preparation of some unsymmetrical tetraorganosilanes. Heating a mixture of tetraethyl and tetrapropylsilane to ISC C for 5h in the presence of aluminimn chloride as catalyst gives three mixed species in statistical proportions [73] (Eq. 3.6)  

The preparation of a large number of unsymmetrical tetraorganosilanes includes reaction of certain Si-functional organosilanes with organometallic compounds and the hydrosilation of unsaturated hydrocarbons. Table 3.1 presents typical examples of the preparation of unsymmetrical tetraorganosilanes from Si-functional organosilanes and organometallic compounds. 

The hydrosilation of alkenes and alkynes with hydrotriorganosilanes is an essential method for the preparation of unsymmetrical tetraorganosilanes, e. g. Eq. 3.7  

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Some pathways lead to the preparation of unsymmetrical tetraorganosilanes. One of these is the thermal decomposition of tetramethylsilane, which has been studied extensively [69]. Both linear and cyclic compounds (about 45 of them) have been trapped below 100°C, (266 Pa = 2 mm Hg) with bis(trimethylsilyl)methane. Most of these, including the main product, contain alternating carbon and silicon atoms. The primary dissociation involves homolysis of the carbon-silicon bond. The methyl radical then extracts hydrogen to provide the precursor for bis(trimethylsilyl)methane and some cyclic products (Scheme 3.1) ... 

Table 3.1 Unsymmetrical tetraorganosilanes from Si-functional organosilanes and organomet-allic compounds.

With substituents like 9-methylfluorene and diphenylmethane, Si-C bonds can be activated for a cleavage under mild conditions. In contrast to the 9-methylfluorenyl-substituted silanes 7a and 7b, diphenylmethyl-substituted tetraorganosilanes of types 10a, 10b and roc-20 have proven to be valuable precursors for the synthesis of silyllithium reagents like 11a, 11b and rac-21 (Eq. 5). Therefore they correspond well to the silyl anion synthons B. Furthermore the bis(diphenylmethyl)-substituted silane 15 allows a sequential synthesis of unsymmetrical trisilanes and thus is a valuable silyl dianion synthon D (Eq. 6). 

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