Alkenes, hydrosilylation

The addition of silanes across alkenes has been investigated both experimentally145 and theoretically.146 The effect of optically active thiol catalysts to catalyse radical hydrosilylation (polarity reversal catalysis) has been studied. The use of 2,3,4,6-tetra-O-acetyl-thio-jS-D-glucopyranose as the chiral thiol (used to reduce the intermediate carbon-based radicals) furnished the hydrosilylated alkenes in low to moderate enantiomeric excesses.145 In addition to this work a theoretical study on the reactions of SiH3 with ethene and propene has been undertaken using PMP2(6-31G ) and QCISD(T)(6-31G ) methods. Results indicated that the alkene-addition pathway is favoured over the alternative possible mode of reaction (H-abstraction). This is contrary to that previously suggested for the reaction of SiF with propene.146... 

Silyl radical addition onto the alkene leads to a )8-silylalkyl radical, which upon reduction with 1 affords the hydrosilylated alkene and the chain carrying radical 2. 

Scheme 20 Hydrosilylation of an alkene by hydrogen-terminated Si(l 11) surface.

Scheme 21 Proposed mechanism for the initiation of the light-induced hydrosilylation of an alkene.

Figure 3 Schematic representation of light-promoted hydrosilylation of an alkene through masking procedure.

Organosilicon compounds are widely used in our daily life as oil, grease, rubbers, cosmetics, medicinal chemicals, etc. However, these compounds are not naturally occurring substances but artificially produced ones (for reviews of organosilicon chemistry, see [59-64]). Hydrosilylation reactions catalyzed by a transition-metal catalyst are one of the most powerful tools for the synthesis of organosilicon compounds. Reaction of an unsaturated C-C bond such as alkynes or alkenes with hydrosilane affords a vinyl- or alkylsilane, respectively (Scheme 16). 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Complexes of the type 48-53 (Scheme 2.7) have been targeted as pre-catalysts for the hydrosilylation of alkenes [44]. For example, in the hydrosilylation of 1-octene with (Me3SiO)2Si(Me)H, which was studied in detail as a model reaction, the activity of complexes 48-49 with alkyl substituted NHC ligands, is inferior to that of the Karstedt s system. However, selectivity and conversions are dramatically improved due to the suppression of side-product formation. In this reaction... 

To give some new examples of the remarkable catal5dic activity of SMA, we report here some recent results obtained on the sUyl-carbocyclization of alkynes, promoted by arene-solvated Rh atoms, and on the hydrosilylation of functionalized alkenes, catalyzed by arene-solvated Pt atoms. 

There is much interest in the hydrosilylation of functionalized alkenes as a powerful synthetic tool [14]. Mesitylene-stabilized Pt nanoaggregates, obtained by reaction of Pt vapor and mesitylene, which have already been reported to be very active homogeneous catalysts in the hydrosilylation... 

In 1993, Linford firstly reported a quite useful method to prepare monolayers of alkyl chains by thermal hydros-ilylation of hydrogen-terminated silicon surfaces [25]. Alkyl chains are covalently bound to Si surface by Si-C bonds. This thermal hydrosilylation could be attributed to a free-radical process with 1-alkene. First, a diacyl peroxide initiator was used to produce free radicals. However, at higher temperature, only hydrogen-terminated silicon and a neat solution of 1-alkene or 1-alkyne can form Si-C linkages [26]. Furthermore, lately it is found that such Si-C covalent links can be observed even in dilute solutions of 1-alkenes [27]. In that case, the density of monolayer packing strongly depends on the reaction temperature. 

Examples of asymmetric hydrosilylation of alkenes performed in the presence of chiral sulfur-containing ligands are much rarer in literature than... 

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