Silylmetallics reactions with

Apart from problems of silicon-carbon bond cleavage, a second difficulty limiting the utility of this synthesis is that of obtaining the appropriate silylcarbinol. Whereas a-hydroxysilanes can be isolated from the reaction of silylmetallic reagents with aliphatic aldehydes 33,34), the initial adducts from reaction with aromatic aldehydes such as benzaldehyde rearrange to the isomeric silyl ethers too rapidly for any silylcarbinol to be isolated 33, 35). The mechanism of this reaction, summarized below, has been investigated in detail 30). 

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

Recently, another type of catalytic cycle for the hydrosilylation has been reported, which does not involve the oxidative addition of a hydrosilane to a low-valent metal. Instead, it involves bond metathesis step to release the hydrosilylation product from the catalyst (Scheme 2). In the cycle C, alkylmetal intermediate generated by hydrometallation of alkene undergoes the metathesis with hydrosilane to give the hydrosilylation product and to regenerate the metal hydride. This catalytic cycle is proposed for the reaction catalyzed by lanthanide or a group 3 metal.20 In the hydrosilylation with a trialkylsilane and a cationic palladium complex, the catalytic cycle involves silylmetallation of an alkene and metathesis between the resulting /3-silylalkyl intermediate and hydrosilane (cycle D).21... 

The reaction of 1-propargyltetrahydroisoquinoline 76 with PhMe2SiH, catalyzed by Rh(acac)(CO)2 at 60°C and 50 atm. CO furnished the silylcyclocarbonylation (SiCCa) product 77 in 65% yield (Eq. 22) [49]. The SiCCa reaction has been applied to the syntheses of enantiopure indolizidine skeletons, where the aminolysis of the acyl-[Rh] bond took place instead of reductive elimination after silylmetallation of the alkyne moiety [49]. 

When trialkylsilanes are employed, vinylsilane is often produced as the main product. This oxidative hydrosilylation can be carried out with [Me3SiCo(CO)4], [Cp 2Rh2Cl4] (equation 29), [ Ir(OMe)(COD))2], [Cp Fe(CO)2SiMe3], or Na[HRu3(CO)i ] catalysts. This unusual reaction is understood in terms of silylmetallation rather than the ordinary hydrometallation mechanism. ... 

The formation of silylmetallics by cleavage of Si—Si bonds with alkali metals is described in 5.5.42.1. Because Si—Si bonds can be formed by condensation of halosilanes with alkali metals, a direct route to silyl metallics should be possible by forming and reacting the disilane (or polysilane) with a metal without isolating the intermediate Si—Si-bonded species. However, this approach contains the same inherent restrictions as those described in 5.5.4.2.1 i.e., there must be one aryl group bonded to Si in the starting halosilane (see Table 1). Reaction of PhjSiCl does not occur with Na, but... 

Reaction of hydrosilanes with alkali metals is seldom employed for generating silylmetallic reagents, except for HjSiM (M = Na, K, Pb, Cs). The silylmetallics of K, Rb and Cs may be prepared and isolated from 1,2-dimethoxyethane (DME) in 7-28 days The reaction of SiH4 and potassium in hexamethylphosphoramide (HMPA) (a carcinogen) gives a high yield of KSiHj, and Na and SiH. react in DME. 

Reaction of hydrosilanes with alkali metals is seldom employed for generating silylmetallic reagents, except for HjSiM (M = Na, K, Pb, The silylmetallics of K,... 

The synthesis of disilanes via silylmetallic reagents has been the subject of several recent reviews (4, 5, 14). Since the silylmetallic reagents are prepared by the reaction of metals with triorganohalosilanes ( direct preparation) or hexaorganodisilanes, the reagents are chiefly used for the synthesis of unsymmetrical rather than symmetrical disilanes. 

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