Carbon-silicon bonds, reductive elimination

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

Transition-metal catalyzed metathesis of carbon-halogen bonds with Si-Si bonds provides useful access to organosilicon compounds. Most of the reaction may involve initial oxidative addition of the carbon-halogen bond onto the transition-metal followed by activation of the Si-Si bond to give (organosilyl)(orga-no)palladium(II) complex, which undergoes reductive elimination of the carbon-silicon bond. 

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 origin of the persistent radicals which are produced in low yields is of some interest. Simple photochemical silicon-carbon bond cleavage lacks precedent and is not consistent with the failure to observe any alkane or 1-alkene in the photolysis mixture. While the latter could not be expected to survive in the irradiated solution, the former would. A possible route involving photochemical 1,1-reductive elimination is described below. 

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. 

This mechanism consists of several steps (1) oxidative addition of hydrosilane to the rhodium(I) complex (2) and (3) coordination and insertion of the ketone into the rhodium-silicon bond to form a diastereomeric a-silyloxyalkylrhodium intermediate (4) reductive elimination of alkoxysilane as a primary product and (5) hydrolysis of the alkoxysilane yielding an optically active alcohol. Hydrosilylation of prochiral ketones by prochirally disubstituted silanes leads to asymmetry on the silicon atom as well as on the carbon atom and, in the presence of chiral rhodium complexes, results in optically active monohydrosilanes (eq. (5)) [2] ... 

A variety of catalytic bis-silylation reactions, i.e., addition of Si-Si bonds across multiple bonds, have been reported. Generally the reaction mechanism can be presented as follows (1) formation of bis(organosilyl) transition-metal complexes through activation of Si-Si bonds, (2) insertion of unsaturated organic molecules into the silicon-transition-metal bonds, and (3) reductive elimination of the silicon-element (mostly carbon) bonds giving bis-silylation products. The final step regenerates the active low-valent transition-metal complexes. Not only appropriate choice of transition metal, but also choice of suitable ligand on the transition metal is crucially important for the success of the bis-silylation reaction. In addition, substituents on the silicon atoms of disilane are also of importance. 

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. 

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. 

An application of the synthetic concept to carbon analogs is not possible. LiSPh-elimination and/or LiSPh-elimination after previous H,Li exchange occurred for alkyllithium-intermediates formed by reductive cleavage of C-S bonds. Reaction of this type have been observed for similar compounds [7]. The silicon atom obviously plays a central role in the reactivity of the presented structure types. 

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