Olefin insertions metal-silicon bonds

A few studies of isolated metal-silyl complexes and ttie computational study of rhodium-sUyl complexes illustrate the insertion of olefins into metal-silicon bonds. Wrighton studied the photochemical reaction of iron-silyl complexes witti ettiylene (Scheme 9.13). Photolysis of Cp FefCOl fSiMej) in the presence of ettiylene forms Cp Fe(CO)(CjHJ(SiMej). This complex appears to insert ethylene, but ttie 16-electron insertion product is unstable and forms the corresponding vinylsilane and iron hydride complexes as products. Photolysis of Cp Fe(CO)j(SiMe3) in the presence of ethylene and CO forms ttie p-silylaDcyl complex containing two CO ligands. 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

Several observations led to the proposal that some of the catalysts containing metals other than platinum do not react by the Chalk-Harrod mechanism. First, carbon-silicon bond-forming reductive elimination occurs with a sufficiently small number of complexes to suggest that formation of the C-Si bond by insertion of olefin into the metal-silicon bond could be faster than formation of the C-Si by reductive elimination. Second, the formation of vinylsilane as side products - or as the major products in some reactions of silanes with alkenes cannot be explained by the Chalk-Harrod mechanism. Instead, insertion of olefin into the M-Si bond, followed by p-hydrogen elimination from the resulting p-silylalkyl complex, would lead to vinylsilane products. This sequence is shown in Equation 16.39. Third, computational studies have indicated that the barrier for insertion of ethylene into the Rh-Si bond of the intermediate generated from a model of Wilkinson s catalyst is much lower than the barrier for reductive elimination to form a C-Si bond from the alkylrhodium-silyl complex. ... 

The latter reaction was revealed as a side-reaction of the hydrosilylation occurring especially in the presence of Fe and Co-triad complexes and this made the basis for an alternative to the Chalk and Harrod concept of the hydrosilylation known as the Seitz and Wrighton mechanism [8,15]. The key step of this mechanism involves insertion of an alkene into a metal-silicon bond (equation 2). Concurrent insertion of olefin into the M-H and M-Si bonds can potentially lead to a complex containing a-alkyl and a-silylalkyl ligands. Competitive P-H transfer from the two ligands to the metal is a decisive step for alternative hydrosilylation and dehydrogenative silylation [16]... 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

The mechanisms of all metal-catalyzed hydrosilations are thought to be very similar. The pathway probably involves an adduct composed of the silane, the alkene, and the metal. Transfer of the silicon to the carbon is believed to occur after the 7r-bonded olefin rearranges to a a complex. Whereas the mechanism displayed in the following scheme involves olefin insertion into Pt-H, equally possible is insertion into Pt-Si followed by reductive elimination of the alkyl silane. 

Insertions of Olefins and Acetylenes into Metal-Silicon and Metal-Boron Bonds... 

The mechanism of hydrosilylation involves a sequence of elementary reactions described in the earlier chapters of the book. The most commonly cited mechanism for hydrosilylation was first described by Chalk and Harrod and involves oxidative addition of the silane, insertion of an olefin into the metal-hydride bond, and reductive elimination to form the silicon-carbon bond in the organosilane product. More recently, a related but distinct mechanism involving insertion of the olefin into the silyl group has been recognized, and this mechanism is often called the modified Chalk-Harrod mechanism. Before these steps are described, some of the mechanistic issues regarding the specific systems of Speier s catalyst and Karstedt s catalyst are described briefly. 

This Chalk-Harrod mechanism includes the insertion of an olefin into a hydrogen-metal bond (step iii). However, it is also conceivable that an olefin can insert into a silicon-metal bond, and this type of mechanism was found to be operative when Fe(CO)5, M3(CO)12 (M = Fe, Ru, Os) and R3SiCo(CO)4 were used as catalysts for the photocatalyzed hydrosilylation of alkenes104. 

On the basis of the fact that (R)-BMPP coordinated to the metal center can induce asymmetric addition of methyldichlorosilane across the carbon-carbon double bond of 2-substituted propenes to afford an enantiomeric excess of (R)-2-substituted propylmethyldichlorosilanes, the following processes should be involved in these reactions (a) insertion of the metal center into the silicon-hydrogen bond (oxidative addition of the hydrosilane) (b) addition of the resulting hydridometal moiety to the coordinated olefin preferentially from its re face (in a cis manner) to convert the olefin into an alkyl-metal species and (c) transfer of the silyl group from the metal center to the alkyl carbon to form the product. Since process (b) most likely involves diastereomeric transition states or intermediates, the overall asymmetric bias onto the R configuration at the chiral carbon would have already been determined prior to process (c). A schematic view of such a process is given in Scheme 1. 

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