Catalytic hydrosilation reactions

The addition of Si-H bonds to organic unsaturations such as olefins, acetylenes, and ketones is known as the hydrosilation reaction. A majority of the hydrosilation reactions are catalyzed by soluble transition-metal complexes and are known as homogeneous hydrosilation reactions [43]. Catalytic hydrosilation reactions are known to be very complex reactions. However, a few generalizations have been drawn about the mechanism of such reactions (Figure 15.4). Homogeneous olefin hydrosilation are presumed to start with an obligatory oxidative addition of the Si-H bond in a cis fashion to the catalytic metal [44]. This process is followed... 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

It was found, however, in the course of the present study that the catalytic activity of the commercial Pt/C catalyst for the present hydrosilation reaction was not reproducible from one lot to another. Hence, we have developed an alternative process to produce la and l a, i.e. the direct end-capping reaction of mono- and bifunctionally... 

Previous work (48) with homogeneous analogues showed that Si-H oxidative additions yield cis products. A cis geometry of hydride and silyl may be allowed in catalytic hydrosilation. Because the industral homogeneous hydrosilation catalyst (62) is h PtClg we tested the activity of our surface generated reagent for the reaction of Equation 24. A suspension of the catalyst was irradiated in 1-... 

Crabtree and coworkers used the disappearance of color to monitor the activity of 12 potential catalysts for hydrosilation reactions [6], The group used alkenes and imines with ferrocenyl and pyrimidyl substituents at either end as colored reactants. On hydrosilation the color is bleached, because conjugation between the electron donor and acceptor groups is reduced. The bleaching is easily observed by eye and can be recorded quantitatively by means of a digital camera (Figure 5.4.1). By use of this method, the known Wilkinson catalyst was identified in a proof-of-concept experiment. A palladacycle, [ Pd (o-tolylfi PQ H4 OAc 2, usually used in Heck reactions, was also found to be catalytically active. 

Comparison of Catalytic Dehydrocondensation/Hydrosilation Reactions of CH2=C(Me)CH2OH in the Presence of Rh and TI-Based Catalysts... 

Experiments with trichlorosilane-d, Cl3SiD, were most instructive about side reactions that can take place in the hypothetical catalytic olefin PtH(—S ) species during hydrosilation. Although trichlorosilane-d and methyldichlorosilane showed no exchange of deuterium and hydrogen at 100°C during many hours in the absence of a catalyst, traces... 

Recently, various rhodium carbene complexes were investigated as catalysts for hydrosilation of olefins, acetylenes, and dienes to see whether carbene ligands modify catalytic activity. All reactions were... 

It is anticipated that many of the catalytic Cp2Zr(II) reactions that might have been considered to proceed via oxidative addition and reductive elimination, such as hydrosila-tion [224] and hydrogenation [225], may actually proceed via a couple of o-bond metatheses, i. e. transmetallation and p-H abstraction, as exemplified by the two contrasting mechanisms for the hydrosilation of alkenes (Scheme 1.70). 

In the Pt(0)-catalyzed hydrosilation, the addition of a catalytic amount of Na(0) appears to be necessary to achieve high levels of regioselectivity (cf., 1,1- vs 1,2-disubstituted vinylsilane). Without the inclusion of Na(0), the reaction yields a minor regioisomer, ( )-2-(dimethylphenylsilyl)-1 -buten-3-ol, 2b, in a ratio of 10-15 1. The checkers found that the reaction was considerably faster and higher-yielding with 60 mg of the catalyst, bis(T(-divinyltetramethyldisiloxane)tri-tert-butyIphosphine-platinum(O). 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

A second puzzling observation was the inverse dependence of hydrosilation rates on the concentration of substrate. In a rate study in which the concentration of acetophenone was varied, rate suppression was observed as [acetophenone] increased. Since raising [acetophenone] is expected to shift the equilibrium for adduct formation towards the adduct, this phenomenon argues against direct involvement of the adduct in the catalytic cycle for hydrosilation. Finally, it was observed that, in a competition experiment between benzaldehyde and ethylbenzoate, benzaldehyde was hydrosilated almost exclusively at a comparable rate to the non-competitive hydrosilation of the aldehyde. This observation shows that the relative basicity of the substrate does play an important role in the reaction, determining the chemoselectivity of the reaction. 

Unsaturated ligands such as alkenes often undergo insertion into M-H bonds to give alkyls or vinyls (equation 23). This is an important step in catalytic alkene isomerization, hydrogenation, hydroformylation, hydrocyanation, and perhaps also hydrosilation, although in this last case, insertion into the M-Si bond may be preferred. This is simply the reverse of the -elimination reaction discussed above. 

The selectivity of hydride insertion into an unsymmetrical unsaturated species plays a role in determining the selectivity of catalytic reactions the outcome is a complex function of electronic and steric effects and cannot readily be predicted other than by analogy with similar systems. In hydrogenation, the direction of insertion is usually not an important issue because an H is added to each end of the multiple bond, but in hydroformylation and hydrocyanation, where HX (X = CHO or CN) adds to the unsatnrate, the direction of the initial insertion can be inferred. In hydrosilation, an Si-H bond is added, but there is some doubt as to whether the first insertion occurs into the M-H or M-Si bonds. Among non-H ligands, the M-Si bond is the closest analog to M-H in polarity and reactivity. 

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