Hydrosilylation general scheme

Figure 30 General scheme of hydrosilylation reactions using Ts[OSiMe2H]8.

Figure 5. General schemes for the Chalk-Harrod (right) and the modified-Chalk-Harrod (left) mechanisms for the metal catalyzed hydrosilylation of olefins.

Intramolecular hydrosilylation, leading to cyclic product, competes with polyaddition according to the general scheme (Scheme 34). 

Scheme 5.1 General scheme for alkenes and alkynes hydrosilylation.

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

This problem may be generally solved by catalyzed hydrometallation, which proceeds as shown in Scheme 1. Here the actual hydrometallating species is a transition metal hydride, but only catalytic amounts are needed. The following survey of such methods is brief more details on the two most important systems, hydroalumination and hydrosilylation, may be found in Volume 8, Chapters 3.11 and 3.12 respectively. 

Entirely different in concept are several useful methods based on alkynes. Hydrosilylation of alkynes, generally catalyzed by chloroplatinic acid and hydrostannation, generally in a radical chain process, give vinylsilanes and vinylstannanes (Scheme 88). Catalyzed hydrosilylation is stereoselectively syn, but radical chain hydrostannylation is stereoselectively anti only when under kinetic control. Under thermodynamic control, as when there is a small excess of tin hydride present, the reaction is often stereoselectively syn. Alternatively, the silyl or stannyl groups can be introduced using the corresponding cuprate... 

Hexachloroplatinic acid in isopropanol (Speier s catalyst) is the most commonly used hydrosilylation catalyst because of its broad applicability and effectiveness at very low concentrations. A generally accepted mechanism for homogeneous platinum catalysed hydrosilylation involves reduction of Pt(IV) to Pt(II) by the silane, ligand substitution by the alkene, oxidative addition of SiH to the metal, rearrangement to a a-bonded complex and finally reductive elimination of the product and recycle of the catalyst 206 208. The basic mechanism is summarised in Scheme 2. 

Silanes bearing Si—H bonds are of basic interest in catalytic reactions. Transition metals or transition metal complexes, respectively, are able to catalyze hydrosilylation which is a useful approach to polysilanes. The major drawback of this method is the relatively low molecular weight of the polysilanes generated. The reaction pathway, as well as the reaction products, depend on the nature of the educts and particularly on the catalyst which is used. Numerous transition metals can be effective catalysts either as metal or in compounds. For a general overview some key reactions of transition metal catalysts are given in Scheme 5. The hydrosilylmetal species (a), which is... 

Condensation polymers containing aromatic units, synthesized by means of hydrosilylation, are also good candidates for membrane materials of high permeability. They can be synthesized in four different ways which generally consist in two types of reactions — intermolecular hydrosilylation and polyaddition (Schemes 2 and 3). 

In general, radical hydrosilylation of alkenes cannot be conducted with tri-alkylsilanes, which is due to a rather strong Si—H bond in the latter. However, the hydrosilylation of carbon-carbon multiple bonds with modified silanes such as tris(trimethylsilyl)silane has been successfully used in radical hydrosilylation (16). The reversible addition of tris(trimethylsilyl)silyl [(TMSlsSi] radical to the C=C bonds is due to the ability of this radical to isomerize alkenes. The hydrosilylation of monosubstituted and gem-disubstituted olefins are efficient processes and have been shown to proceed with high regioselectivity for both electron-rich and electron-poor olefins (140). Walton and Studer presented the results of the radical hydrosilylation with silylated cyclohexadienes as radical initiators (141). The bisvinylic methylene group acts as the hydrogen donor in these reactions. H-transfer leads to a cyclohexadienyl radical (2) that subsequently rearranges to provide er -butyldimethylsilyl radical and arene (3) (see Scheme 20) (141). 

Asymmetric Hydrosilylation of C=C Bond. Successful asymmetric hy-drosilylation of terminal olefins requires the catalyst ensuring both Markovnikov selectivity (unusual for platinum and rhodium) and satisfactory level of asymmetric induction. Asymmetric hydrosilylation followed by Tamao-Fleming oxidation (known to proceed with retention of configuration at the stereogenic carbon) has become one of the most useful, general methods for the preparation of optically active alcohols from alkenes (Scheme 27). 

Transition metal-free hydrosilylation of carbonyl compounds can be realized with the use of Brpnsted or Lewis acids as well as Lewis bases. Alkali or ammonium fiuorides (CsF, KF, TBAF, and TSAF) are highly effective catalysts for the reduction of aldehydes, ketones, esters, and carboxylic acids with H2SiPh2 or PMHS. Lithium methoxide promotes reduction of esters and ketones with trimethoxysilane. A generally accepted mechanism of Lewis base-catalyzed hydrosilylation of carbonyl compovmds involves the coordination of the nucleophile to the silicon atom to give a more reactive pentacoordinate species that is attacked by the carbonyl compound giving hexacoordinate silicon intermediates (or transition states), in which the hydride transfer takes place (Scheme 30) (235). 

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