Hydrosilylation reactions bonds

In recent years the solid-phase hydrosilylation reaction was successfully employed for synthesis of hydrolytically stable surface chemical compounds with Si-C bonds. Of special interest is application of this method for attachment of functional olefins, in particular of acrolein and some chiral ligands. Such matrices can be used for subsequent immobilization of a wide range of amine-containing organic reagents and in chiral chromatography. 

Chen et al. utUized a direct chemical reaction with a given solution (wet treatment) to modify the surface of the silicone rubber. The presence of a layer of PEO on a biomaterial surface is accompanied by reductions in protein adsorption, and cell and bacterial adhesion. In order to obtain a PEO layer on top of the silicone rabber surface, the surface was firstly modihed by incorporating an Si-H bond using (MeHSiO) , and followed by PEO grafting to the surface using a platinum-catalyzed hydrosilylation reaction. These PEO-modified surfaces were demonstrated by fibrinogen adsorption both from buffer and plasma, as well as albumin adsorption from buffer. Reductions in protein adsorption of as much as 90% were noted on these surfaces. 

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). 

Employing ketones or aldehydes as starting materials, the corresponding silylethers are obtained. Thereafter, the oxidation or hydrolysis of the obtained silylethers gives the corresponding alcohols (Scheme 17). In most cases, a hydride (silyl) metal complex H-M-Si (M = transition-metal), which is generated by an oxidative addition of H-Si bond to the low-valent metal center, is a key intermediate in the hydrosilylation reaction. 

Bis(imino)pyridine iron complex 5 acts as a catalyst not only for hydrogenation (see 2.1) but also for hydrosilylation of multiple bonds [27]. The results are summarized in Table 10. The reaction rate for hydrosilylations is slower than that for the corresponding hydrogenation however, the trend of reaction rates is similar in each reaction. In case of tra s-2-hexene, the terminal addition product hexyl (phenyl)silane was obtained predominantly. This result clearly shows that an isomerization reaction takes place and the subsequent hydrosilylation reaction dehvers the corresponding product. Reaction of 1-hexene with H2SiPh2 also produced the hydrosilylated product in this system (eq. 1 in Scheme 18). However, the reaction rate for H2SiPh2 was slower than that for H3SiPh. In addition, reaction of diphenylacetylene as an atkyne with phenylsilane afforded the monoaddition product due to steric repulsion (eq. 2 in Scheme 18). 

Transition-metal chemistry is currently one of the most rapidly developing research areas. The record of investigation for compounds with metal silicon bonds is closely comparable to that for silicones it was in 1941 when Hein discovered the first metal silicon complex, followed by Wilkinson in 1956. A milestone in the development of this chemistry was Speier s discovery of the catalytic activity of nobel metal complexes in hydrosilylation reactions in 1977. Hydrosilylation is widely used in modem organic syntheses as well as in the preparation of organo functionalized silicones. Detailed investigations of the reaction mechanisms of various catalysts continue to be subject of intense research efforts. 

Based on the extraordinary selectivity in hydrosilylation reactions when an alkyne competes with other groups for a silicon-bonded active hydrogen, further derivatisation can be carried out. The hydrosilylation of 2-methyl-3-butynol, which works very well with polymeric siloxanes, gives hydroxyal-kenylsilicon compounds - a l-silylalkenyl/2-silylalkenyl mixture from cis-addition across the triple bond. Elimination of water from the tert. alcohol produced, catalyzed by traces of a strong acid, results in isoprenylic siloxanes in more than 90 % overall yield (Eq. 8). 

Iridium complexes are known to be generally less active in hydrosilylation reactions when compared to rhodium derivatives, although iridium-based catalysts with bonded chiral carbene ligands have been used successfully in the synthesis of chiral alcohols and amines via hydrosilylation/protodesilylation of ketones [46-52] and imines [53-55], The iridium-catalyzed reaction of acetophenone derivatives with organosubstituted silanes often gives two products (Equation 14.3) ... 

Table III gives details of the use of supported catalysts in hydrosilylation reactions. There are, in principle, three ways in which the supported catalyst could act (i) the reaction could take place at the metal site which itself remains bonded to the surface throughout the reaction (ii) the catalytically active species or its precursor may be abstracted from the support into the solution by a reversible process so that the catalysis is effectively a homogeneous process (iii) a possible mode of action essentially the same as the second except that the abstraction is irreversible. The fact that most of the catalysts can be reused several times and, indeed, that their catalytic activity increases slightly after the...

The first report of a densely-packed organic monolayer bound directly to silicon through Si—C bonds was based upon the hydrosilylation reaction on the H Si(l 11)... 

Figure 5.8. STM images of the Si(100)—(2 x 1)—H surface after hydrosilylation reaction using both styrene and allyl mercaptan, from Hossain, Kato and Kawai. The image shown in (a) contains two allyl mercaptan lines. Image (b) shows the system after a new dangling bond is generated using the STM tip. After the surface is exposed to styrene molecules, a styrene line forms which joins the two allyl mercaptan lines (c). The allyl mercaptan lines form across the Si dimer rows, while the styrene line forms along the dimer row. Figure reproduced with permission from Ref. [143]. Copyright 2005 American Chemical Society.

Hydrosilylation of divinyl ether has been applied for the synthesis of silacyclopentane 12 using Speier s catalyst (Scheme l)13. One of the two carbon-carbon double bonds was hydrosilylated first with a dialkyl(ethoxy)silane, giving silylethyl vinyl ether 10 in 53-59% yield, which was reduced with LiAlELr to hydrosilane 11. The intramolecular hydrosilylation of 11 affords silacyclopentane 12 in moderate yields (Scheme 1). The reaction with HSiEt2(OEt) gives 12a exclusively in 45% yield, while silacyclohexane 13b is formed as the minor product when HSiMe2(OEt) is used as the hydrosilane (12b/13b = 2.3/1 50% total yield)13. Other intramolecular hydrosilylation reactions useful in organic syntheses will be discussed in the section n.C. (vide infra). 

Another method for preparing polysiloxane networks is to use the hydrosilylation reaction an SiH group is added to a double bond e.g., an allyl group with the help of a catalyst (platinum catalyst) ... 

As Weber has shown, polymers bearing reactive Si-H bonds can be formed by ring-opening polymerization of 1-silacyclobutane <1992PB281>. These polymers can be further converted into Si-functionally-substituted polycarbo-silanes by Pt-catalyzed hydrosilylation reaction of Si-H functionality with alkenes (Scheme 18) <1993MM563>. 

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