Hydrosilyl anions

The authors suggested a mechanism via the intermediacy of a reactive pentacoordi-nated hydrosilyl anion,50c which is formed by the addition of hydride (H-) on the silanes, for the redistribution reactions. 

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

A 1990 patent reported the quantitative cydization of 1,5-hexadiene 66 to 67 with Cp 2LuCH3 (Cp = C5Mc5 anion) in the presence of PhSiH3 (Scheme 14) [39]. This result was followed by reports that Cp 2NdCH(SiMe3)2 reductively cyclized both 66 and 1,6-heptadiene (68) under similar conditions [40] and that Cp 2SmCH(SiMe3)2 also served as a precatalyst for the cydization of 1,5-hexadiene [41]. In the case of the neodymium and samarium catalysts competitive alkene hydrosilylation was observed. 

An alternative disconnection of homopropargylic alcohols substrates for intramolecular hydrosilylation is the opening of an epoxide with an alkynyl anion. This strategy was employed in a total synthesis of the macrolide RK-397 (Scheme 20). Epoxide ring opening serves to establish homopropargylic alcohol C with the appropriate stereochemistry. A hydrosilylation/oxidation protocol affords the diol E after liberation of the terminal alkyne. The... 

The synthesis and characterization of a series of dendrigraft polymers based on polybutadiene segments was reported by Hempenius et al. [15], The synthesis begins with a linear-poly(butadiene) (PB) core obtained by the sec-butyllithium-initiated anionic polymerization of 1,3-butadiene in n-hexane, to give a microstructure containing approximately 6% 1,2-units (Scheme 3). The pendant vinyl moities are converted into electrophilic grafting sites by hydrosilylation with... 

Hydrosilylation of hex-l-ene and ethylene with SiH4 occurs in the presence of LiAlH4 as a catalyst. A reaction mechanism involving SiH4 and the alkyl anion, generated by the initial interaction of the olefin with LiAlH4, has been proposed.62... 

Polyphenylsilane, (PhSiH) , can be derivatized by free-radical hydrosilylation in the presence of a radical initiator. Alkenes, ketones and aldehydes react readily, to replace up to 93% of the Si-H bonds. This route can be employed to make polysilanes with hydrophilic groups, such as hydroxy, amino and carboxylic acid functions.43 Dialkylamino substituted polysilanes, made by the anionic polymerization of masked disilenes (see equation (17)), when treated with acetyl chloride give chloro-substituted poly silanes. The chlorine can then be displaced by other nucleophiles.27... 

Besides the hydrosilylation, another type of reaction was developed using a strong base to form an enolate anion which is attacked by the added silylating agent (Scheme 25)121-12 6. ... 

The reactions leading to stereoselective formation of the C — Si bond fall into two main categories hydrosilylation of alkenes or dienes (cf. Houben-Weyl, Vol. 13/5, p 51) and the related addition of silyl anions to activated C-C double bonds (cf. Houben-Weyl, Vol. 13/5, p 53). Also of interest is the stereoselective silylation ofcarbanions (cf. Houben-Weyl, Vol. 13/5, p 39). 

Other Applications. In the reactions discussed so far, methylenebis(oxazolines) were found to be superior to bioxazo-lines of type (4). However, there are some enantioselective metal-catalyzed processes for which the bioxazolines (4) are better suited than neutral or anionic methylenebis(oxazolines). Two examples, the Ir-catalyzed transfer hydrogenation of aryl alkyl ketones and the Rh-catalyzed hydrosilylation of acetophenone, are given in eq 10 and eq 11. 

The trichlorosilyl anion [CljSi /BujNH ] (see also Section V-2) is a reductive silylating agent and is useful for some transformations (Scheme 34) (57a-g). A similar species prepared from Cl3SiH or Cl2RSiH (R = Me, Ph) with TMEDA undergoes /8-hydrosilylation of acrylates in the presence of a copper salt [Eq. (57)] (55). 

Reactions of silylcuprates provide additional examples of a silyl anion based mechanism (Scheme 3). The reagent (19), prepared from silyllithium and copper(I) cyanide, reacts with a C=C bond to give, after aqueous work-up, c/j-hydrosilylated products (20). Conjugate addition of (21) to a,P-unsaturated... 

The Pt-catalyzed hydrosilylation of trimethyl silane and alkenols or alkenyl-polyethers lead to nonionic silane surfactants, whereas the addition of allylglycidyl ether to trimethylsilane results in a precursor for ionic derivatives. The epoxy group is highly reactive towards nucleophilic agents and can be easily transformed into quaternary ammonium, betaine, or sulfonate complexes. Additionally, cation-anion complexes can be formed by the transformation of two equivalents of epoxy silane with one equivalent of trialkyl ammonium hydrogen sulfite. The reaction of hydroxyalkyltrimethylsilane... 

Systems containing a transition-metal anion, with a quaternary onium ion as the cation (e. g., [R4N]3 [IrCl6] ), should possess an advantage [198-200]. The bifunctional catalytic properties of such systems were demonstrated in the hydrosilylation of PhC=CH and subsequent PTC reaction with dichlorocarbene... 

Ruckenstein and Zhang reported that the anionic copolymerization of 4-(vinylphenyl)-l-butene with styrene in toluene/THF at —40 °C gives well-defined polymers,133 since under these experimental conditions the vinylic double bond is selectively polymerized. The copolymers were subjected to hydrosilylation for the introduction of Si—Cl groups at the olefinic double bonds. These groups were used as grafting sites for the linking of PSLi, PILi, and PMMALi living chains in order to synthesize PS- -... 

Block—graft copolymers, having a PS—PB diblock as a backbone and PS, PI, PB and PS-b-PI branches, were prepared by anionic polymerization and hydro-silylation reactions.156 The diblock copolymer backbone was prepared by sequential addition of styrene and butadiene. The polymerization of butadiene took place in the presence of dipiperidinoethane, resulting in high 1,2-content. The backbone was then subjected to hydrosilylation in order to incorporate the desired amount of SiCl groups on the PB block. These groups were then used as branching sites, where PSLi, PILi,... 

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