Carbon-silicon bond formation mechanism

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... 

The corresponding inter-and intra-raolecular additions of allyl-silanes to iminium salts give rise after desilylation to synthetically useful biradical intermediates. Evidence for the existence of two mechanisms for such reactions, differing only in the timing of carbon-silicon bond cleavage and carbon-carbon bond formation, has been described. Analogous electron transfer... 

The Pd-catalyzed reaction of silacyclopropanes wifh terminal or electron-deficient alkynes gives siloles along wifh silacyclopentenes (Scheme 10.255) [685]. The mechanism for fhe formation of fhese products might involve oxidative addition of fhe carbon-silicon bond to a Pd(0) complex generated in situ. 

A second new class of MAO mechanism-based inactivators, (aminoalkyl)tri-methylsilanes, have been reported by Silverman and Banik (114). The idea for this class of MAO inactivators is based on the known activation of the carbon-silicon bond toward homolytic cleavage reaction when the silicon atom is /3 to a radical cation (115, 116). The aminomethyl-, aminoethyl-, and (amino-propyl)trimethylsilanes are all pseudo-first-order time-dependent inactivators of beef liver MAO that reduce the flavin cofactor during the inactivation reaction. Since denaturation of the inactivated enzyme allows flavin leoxidation, covalent bond formation might be to an amino acid residue (114). The stabilities of the enzyme adducts from the (aminoalkyl)trimethylsilanes were found to be differ-... 

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. ... 

In this case, no product arising from the reaction of the silicon-carbon double-bonded intermediate with methanol can be observed at all. However, on prolonged irradiation of the solution two products, 1,1-dimethyl-2,3-benzo-5-trimethylsilyl-l-silacycIopentene (48) and 1-methoxy-dimethylsilyl-l-trimethylsilyl-2-phenylethane are obtained in 17 and 7% yield, in addition to the (Z)- and (E)-isomers (15 and 12% yield). The formation of the latter compound can best be understood by the transient formation of a silacyclopropane followed by reaction with methanol (98). The mechanism for the production of 48 in the prolonged irradiation of PhCH=CHSiMe2SiMe3 is not fully understood but is tentatively given in Scheme 16. 

The possible reaction mechanism for the formation of 31 is shown in Scheme 15. Insertion of alkyne 14 into silazirconacyclopropane 3 gives silazirconacyclopentene 22. Then, insertion of carbon monoxide into the carbon-zirconium bond in silazirconacyclopentene 22 gives silazirconacyclohexenone 34, whose carbonyl oxygen would coordinate to zirconium metal. Then the zirconium carbon bond migrates onto silicon to afford oxazirconacyclohexene 36 via 35 [26]. Deuterolysis of 36 would afford 31-D2, which has two deuteriums. 

The silicon-carbon bonds of silacyclobutanes are readily activated by Pd and Pt complexes [691]. This notable characteristic has been used for carbon-carbon bond formation using silacyclobutanes [692-695]. The Pd-catalyzed reaction of 191a with alkynes affords silacyclohexenes and allylvinylsilanes (Scheme 10.262) [693]. A plausible mechanism for formation of the cyclic product involves three steps - oxidative addition of 191 a to a Pd(0) species, alkyne insertion into the Si-Pd bond, and reduc-... 

Reaction with ei,fi-Unsaturated Sulfoxides. The reaction of TMSI with a, -unsaturated sulfoxides in chloroform at ambient temperature is a mild, efficient, and general method for the preparation of carbonyl compounds (eq 63). The proposed reaction mechanism is shown in eq 63. Formation of a strong oxygen-silicon bond is followed by reduction of the sulfur function and oxidation of iodide to iodine, the latter precipitating in chloroform. The trimethylsiloxy anion attacks the unsaturated carbon linked to the sulfur function, which leaves the substrate, allowing the formation of the sUyl enol ether species. Finally, hydrolysis converts the silyl enol ether into the carbonyl compound. ... 

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