Si-H addition

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. 

Wilkinson s catalyst brings about the hydrosilylation of a range of terminal alkenes (1-octene, trimethylvinylsilane) by 2-dimethylsilylpyridine with good regioselectivity for the anti-Markovnikoff product. Both 3-dimethylsilylpyridine and dimethylphenylsilane are less reactive sources of Si-H. In contrast, these two substrates are far more reactive than 2-dimethylsilylpyridine for the hydrosilylation of alkynes by [Pt(CH2 = CHSiMe2)20]/PR3 (R = Ph, Bu ). This difference was explained to be due to the operation of the two different pathways for Si-H addition—the standard Chalk-Harrod pathway with platinum and the modified Chalk-Harrod pathway with rhodium.108... 

Silicones can be prepared in such a way that they contain only one percent or less of vinyl groups and silicon hydrides, which undergo a catalytic hydrosilylation reaction to give the desired cross-linking (Figure 18.3). Vinyl silanes are made by Si-H addition to acetylene, and thus two hydrosilylations are involved. 

Catalysts 1, 3 and 5 employed in the modification of a polyhydrosiloxane chain via rhodium mediated Si-H addition to a C-C double bond appeared to be very efficient in giving a high conversion of Si-H bonds. With 1, the catalytic activity of the catalyst portion initially used remained unchanged even up to 10 cycles this makes these catalysts suitable for use in the commercial synthesis of modified polysiloxanes. 

Aromatic and aliphatic nitriles react with trimethylsilane by Si-H addition across the C-N triple bond. A variety of functional groups can be tolerated, although reaction rates appear to be affected by electronic and steric characteristics of the substrates. For a,/3-unsaturated nitriles, consecutive 1,2- and 1,4-addition of two equivalents of HSiMe3 occurs to selectively yield Z- or / A A-disilylenamines. 

It is gratifying that later molecular orbital calculations (3a) and structure correlations (49) for the addition of C—H a bonds to transition metals gave similar results. The results obtained by structure correlation appear more straightforward for the Si—H addition, because they are derived from a single type of complex and the correction of the silicon radius is more precise than the very crude correction of the metal radius, which was applied in the C—H case. 

The picture we have at present may be incomplete or biased because it is based only on available compounds, which are limited in number. As more and more complexes of this kind are found, it may be that there is a smooth reaction profile for both C—H and Si—H addition, corresponding to a gradual change of the physical parameters. 

More recently, using the very efficient homogeneous diplatinum catalyst [Pt(SiR3XjuHXC6Hn)3P] 2, the hydrosilylation of but-l-yne and phenylace-tylene was shown to afford /rans-vinylsilanes 119 as the major products corresponding to cis Si-H addition (192) (eq. [40]). Products corresponding to nonterminal addition (120) were formed in minor amounts. 

The addition of reagents X-Y to carbon-carbon ir-bonds may also proceed via a concerted mechanism in which each new a-bond is formed simultaneously on the same face of the ir-bond. The stereochemistry of such reactions is necessarily syn. For example, the reaction of potassium permanganate, which is purple, with an alkene such as cyclohexene proceeds via si/H-addition of permanganate ion across the ir-bond to give 39, which is colorless. Subsequent decomposition of 39 gives a ds-l,2-diol and manganese dioxide, the brown precipitate that is observed as the other product of the reaction (Eq. 10.19). This decoloration of potassium permanganate by alkenes forms the basis of the Baeyer qualitative test for the presence of carbon-carbon ir-bonds (Sec. 25.8B). 

Other catalytic hydroelementations, E-H (E = P, B, Si, H) addition to C C multiple bonds, are efficiently mediated by organolanthanide complexes, and many are highly diastere-oselective. Hydrophosphination/cyclization appears to follow the catalytic patiiway similar to hydroamination/cyclization, whereas hydroboration, hydrosilylation, and hydrogenation proceed via different catalytic cycles, in which a lanthanide-hydride is the intermediate that is generated from protonolysis of the precatalyst. The proposed catalytic cycle of hydrosilylation of alkene using Cp LnCH(TMS)2 is presented in Scheme 5. 

The synthesis of cyclic carbosilanes and related compounds through Si—H addition has already been reviewed [60]. Si—H addition catalysed by H2PtCl5 can be used in cyclization, as shown by the following example [71] ... 

The catalytic Si—H addition of 1,3,5-trisilacyclohexanes to monosilylethynes discussed in this section occurs in polycyclic carbosilane synthesis. The scheme below illustrates the principles of synthesis, whereby only the 1,1-disilylolefme is shown as product, since the 1,2-disilylolefine does not lead to ring formation. 

Ren W, Zhou E, Fang B, Zi G, Fang D-C, Walter MD. Si-H addition followed by C-H bond activation induced by a terminal thorium imido metallocene a combined experimental and computational study. Chem Sci. 2014 5 3165-3172. 

The results of the extensive ah initio calculations performed by Sakaki s group using MO/MP2-MP4(SDQ) and CCD methods, for Pt-catalyzed Si-H addition to ethylene, can be summarized as follows (1) Oxidative addition of Si-H to Pt is facile as is ethylene coordination. (2) The rate determining step for a Chalk-Harrod pathway is the coordinated-ethylene-assisted cis-trans isomerization of the Si-Pt-H complex, with a barrier of ca. 22-26 kcal/mol. The isomerization is proposed to occur through a distorted square-pyramidal intermediate via the Berry pseudo-rotation process. (3) For a modified Chalk-Harrod mechanism, the... 

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