Hydrosilation of alkenes

Platinum(O) phosphine complexes undergo a variety of oxidative addition reactions with compounds containing Group 14 elements. These reactions are of widespread interest because similar processes are probably involved in the catalysis by platinum complexes of reactions such as the hydrosilation of alkenes and the disilylation of dienes and alkenes. 

Figure 6.6. Hydrosilation of alkenes under fluorous biphasic conditions.[34]...

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). 

Scheme 1.70. Dichotomous mechanisms suggested for Zr-cataly2ed hydrosilation of alkenes.

The C—Si bond formed by the hydrosilation of alkene is a stable bond. Although it is difficult to convert the C—Si bond to other functional groups, it can be converted to alcohols by oxidation with MCPBA or H2O2. This reaction enhances the usefulness of hydrosilylation of alkenes [219], Combination of intramolecular hydrosilylation of allylic or homoallylic alcohols and the oxidation offers regio- and stereoselective preparation of diols [220], Internal alkenes are difficult to hydrosilylate without isomerization to terminal alkenes. However, intramolecular hydrosilation of internal alkenes can be carried out without isomerization. Intramolecular hydrosilylation of the silyl ether 572 of the homoallylic alcohol 571 afforded 573 regio- and stereoselectively, and the Prelog-Djerassi lactone 574 was prepared by applying this method. 

A recent investigation of the hydrosilation of alkynes by Et3SiH in the presence of the bridged derivative (PhCH2)Me2SiPt[P(cyclo-hexyl)3](p.-H) 2, analogous to (LVIII), has shown that the rate increases as the 7r-acceptor power of the alkyne increases, consistent with the formation of Si-Pt-alkyne complexes as intermediates 416). This complements earlier kinetic studies on the hydrosilation of alkenes catalyzed by Co, Rh, and Pt compounds, in which similar Si-metal-alkene intermediates were postulated 85, 134). 

Colorimetric and Fluorescent Screening Hydrosilation of Alkenes and Imines... 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

Numerons examples are known in which hydrosilation reactions are nsed to prepare monomers for polymerization experiments. The reaction itself can also be used for a polymerization process. This can be accomphshed as the hydrosilation of alkenes either using a single-sonrce precnrsor (equation 17) or a two-component mixture (equation 18). ... 

Despite its few binary compounds, the element gives rise to many complexes. Table 1 indicates the diversity of complexes formed. Several low oxidation state complexes of the element are important homogeneous catalysts, particularly in the oligomerization, isomerization. Hydrogenation, Hydro-formylation, Hydroboration, or Hydrosilation of alkenes. lodorhodium complexes catalyze the industrial-scale carbony-lation of methanol to acetic acid. 

Oxidative addition of the H—Si bond is the most studied of the group IVB elements with the exception of carbon because of its relation to the industrially important catalytic hydrosilation of alkenes, alkynes and ketones. Compounds containing the Si—M bond are stable. While they are also synthesized by routes other than oxidative addition of the H—Si bond T H—Si, H—Ge and H—Sn add to transition metals ... 

Chlorosilanes with longer alkyl-groups are produced by hydrosilation of alkenes... 

The hydrosilation of alkenes and alkynes is catalyzed by transition metal complexes, including several platinum species, and oxidative addition of the Si—H bond is a fundamental step in the process. 

Additions of Si-H bonds to alkynes occur under similar conditions and with the same catalysts as hydrosilation of alkenes. Free radical addition to terminal alkynes gives cis products by a stereospecific terminal trans addition . Supported platinum catalysts give trans products by a terminal cis addition . Chloroplatinic acid catalyzed additions to terminal alkynes give mixtures of trans-1-alkenylsilanes and trans-2-alkenylsilanes in a ratio ranging from 1 1 to 1 5 depending on the substituents on silicon . Addition of SiH2Cl2 to CH2=CHC(CH3)3 gives trans-1-alkenyl- and bis(trans-l-alkenyl)silane products, but no (2-alkenyl)silane °. Internal alkynes react more slowly than terminal alkynes, and even reactions catalyzed by chloroplatinic acid require heat. 

The hydrosilylation reaction (often termed hydrosilation) of alkenes is... 

Two different mechanisms can be envisioned for these reactions that differ in the insertion step. Since both processes are very similar we will only discuss the silation procesess which have been more extensively studied. The so called Chalk-Harrod mechanism (Scheme 6.62, a) was first proposed for the hydrosilation of alkenes, and involves insertion of the alkene into the M-H bond formed by oxidative addition of the silane to the metal, followed by reductive elimination of a silyl alkyl. A competing route (the modified Chalk-Harrod mechanism. Scheme 6.62, b) derives from insertion of the alkene into the M-SiRs bond to... 

B.i.a. Palladium-Catalyzed Hydrosilation ofAlkenes. Aside from the above-mentioned study by Takahashi and co-workers and an isolated example of Pd-catalyzed hydrosilation of styrene with HSiCls by Kiso, Yamamoto, Tamao, and Kumada, the first systematic study of Pd-catalyzed hydrosilation of alkenes most probably was that by Tsuji and co-workers reported in 1974. In this study, the scope of Pd-catalyzed hydrosilation with respect to (i) substrate structural type, (ii) silanes, and (iii) catalysts including phosphines was delineated for the first time. Some representative results obtained by using HSiCl3 and Pd(PPh3)4 are shown in Scheme 4, which indicate that not... 

The hydrosilation of alkenes and alkynes with hydrotriorganosilanes is an essential method for the preparation of unsymmetrical tetraorganosilanes, e. g. Eq. 3.7 ... 

The chloro dimer (1) has also been employed in Pd-catalyzed asymmetric hydrosilation of alkenes (eq 13). ... 

The use of CpFe-containing complexes with polysilane ligands to deoligomerise silan has been demonstrated. The insertion of ethylene into the CpFe-SiMe3 bond has also been examined for implications for transition metal catalysed hydrosilation of alkenes. Useful CpFe substrates such as [Cp Fe(CO)(PMe3)(THF)]PFs have also been described. 

Unlike the asymmetric hydrosilation of alkenes and acetylenes where Pd catalysts seem to dominate activity, for enantioselective hydrosilation of ketones a preponderance of Rh-based catalysts have been used, although a few other metals including Pd have been examined and some ligand templates such as binaphthyl are in common. 

The resulting chlorine end groups can easily be converted into other interesting end groups such as epoxide, ionomer, acid, silane, alkenes and hydroxy (Scheme 45). a,o)-Dialkene telechelics have been converted into diepoxides and used as precursors for dialdehydes (equation (44), Scheme 45). Alternatively, sulfonation of a,o)-dialkenic PIB with acetyl sulfate yielded the corresponding sulfonate ( ionomer ) quantitatively (equation (45), Scheme 45). " Hydrosilations of alkenic telechelics in the presence of suitable catalysts yield telechelics with reactive Si-Cl groups. ... 

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