Tetraorganosilanes

One organic group is readily transferred from tetraorganosilanes (and some other silanes) to palladium ). Tetramethylsilane, lithium tetrachloropalladate(II) and styrene at 120 C in acetonitrile solution form 1 -phenyl- 1-propene in 65% yield along with ca. 1.5% 2-phenyl-1-propene.33 Trimethylphenylsil-ane transfers phenyl and with styrene under the above conditions gives fra/w-stilbene in 94% yield.33 Similar vinyl substitution reactions have been achieved with potassium ( )-alkenyl pentafluorosili-cates.34... 

Finally, hydroxyorganosilanes can be obtained by breaking up the Si— C bond in tetraorganosilanes with sulfuric or hydrochloric acid ... 

Tetravalent silicon compounds are able to form adducts with Lewis bases, depending on the acceptor strength of the silicon atom. Tetraorganosilanes show only very little... 

The oxidation potentials of benzylsilanes are also less positive than those of the corresponding aromatic hydrocarbons and tetraorganosilanes owing to the a-n interaction. The preparative electrochemical oxidation of benzylsilanes results in the cleavage of the C-Si bond and the introduction of a nucleophile on the carbon [110]. The following example demonstrates that the benzylic C-Si bond is cleaved selectively without affecting the aromatic C-Si bond ... 

The preparative electrochemical oxidation of a-heteroatom-substituted tetraorganosilanes gives rise to cleavage of the C-Si bond and the introduction of a nucleophile such as methanol on the carbon [Eq. (27)]. Various reactions of this type have been reported for compounds containing oxygen [117-119], nitrogen [121], sulfur [116,119,120,122,123], and selenium [123]. 

The dynamic coordination is also effective for the activation of a-heteroatom-sub-stituted tetraorganosilanes [143]. The oxidation potentials of the 2-pyridylethyl (2-Pye)-substituted compounds are less positive than those of the corresponding parent compounds. The decrease of the oxidation potential can be explained in terms of the coordination of the pyridyl group to silicon in the cation radical intermediate (Table 8). 

Symmetrical tetraorganosilanes are synthesized by the reaction of silicon(IV) chloride with excess Grignard reagents or alkyllithiums (eq (45)) [41]. Similarly the combinations of suitable haloorganosilanes and organometallic compounds afford various unsymmetricai tetraorganosilanes (eq (46)) [42]. 

A second method that exploits silica has been developed by Corriu and coworkers. In this case, silica is reacted with catechol and the resultant catecholate complex is converted to an organosilane by reaction with a Grignard reagent159. Triorganosilanes are formed directly by reaction with an alkyl Grignard and tetraorganosilanes are produced from allyl, phenyl or alkynyl Grignards. An example is shown in equation 14. 

The organic groups need not necessarily be all alike. In the other class are the tetraorganosilanes... 

The transmetalation of organosilanes, SiR4, with transition metal complexes is uncommon due to the stable and non-polar Si-C bond [67], Although Si-C bond activation of tetraorganosilanes promoted by transition metal complexes is known, the intermolecular (Eq. 5.18) [68-71] and intramolecular (Scheme 5.10) [72-78] reactions are classified into oxidative addihon more appropriately than transmetalation. 

Syntheses of Silyllithium Reagents Starting from Tetraorganosilanes... 

Keywords lithium, Si-C cleavage, silyl anions, silyllithium compounds, tetraorganosilanes... 

Unfortunately this reaction is rather special and does not allow further functionalization at the silicon center. Therefore we decided to search for alternative starting materials with activated Si-C bonds, which can be cleaved under attractive conditions without any additives like HMPA. These precursors should furthermore allow a variety of functional groups at the silicon center and thus be useful building blocks for the field of synthetic chemistry. In general these tetraorganosilane precursors correspond to silyl anion and silyl dianion synthons of type B and D in Eqs. 3 and 4. 

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). 

C-CN bond activation by silylrhodium species can be applied to a catalytic cycle for silylative decyanation of nitriles, when rhodium intermediates active in C-CN activation are generated from disilanes instead of hydrosilanes (Scheme 14). The reaction proceeds with a range of nitriles, including aryl, alkenyl, and alkyl cyanides, to give the corresponding tetraorganosilanes (Scheme 15) [53, 57], although the yields of tetra-alkylsilanes are modest. 

Aromatic and cyclic hydrocarbons, chloroform, carbon tetrachloride, diethyl ether and tetrahydrofuran are used as solvents for the tetraorganosilanes. Their solubility in ethanol and other alcohols is moderate. 

Tetraorganosilanes with peripheral functional groups or multiple bonds can be obtained, such as alkylhalides, alcohols, olefins, etc. These functional groups decrease the stability of the silicon-carbon bond, in some cases significantly. 

C) is sensitive to hydrolysis. Organolithium compounds are common starting materials for the preparation of symmetrical tetraorganosilanes. They react equally well with monosilane (tetrahydrosilane) and tetrahalosilanes (Eq. 3.3). 

Figure 3.1 shows an apparatus for the preparation of symmetrical tetraorganosilanes from organolithium and monosilane. 

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