Silicon hydrogen insertion

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 reaction is catalysed by many transition-metal complexes, and a mechanism for the hydrosilylation of an alkene under transition-metal catalysis is depicted in Figure Si5.7. Initial coordination of the alkene to the metal is followed by cis addition of the silicon-hydrogen bond. A hydride migratory insertion and elimination of the product silane complete the cycle. 

The most studied reaction of silylene is the insertion reaction into silicon-hydrogen bonds. It was first postulated on the basis that higher silanes were generally formed in pyrolysis of silane, disilane, and trisilane (74, 81, 98). If silylene is the major intermediate, the higher silanes are formed by insertion reactions of the type... 

Copper complexes derived from bis(-2,6-dichlorophenyle-dene)-( 15,25)-1,2-diaminocyclohexane (11) catalyze various reactions such as Diels-Alder reaction, aziridination (eq 20), cyclopropanation, and silyl enol ether addition to pyruvate esters. Although the scope of these reactions may be sometimes limited, enantioselectivities are generally high. The same complex (with copper(I) salts) catalyzes the asymmetric insertion of silicon- hydrogen bond into carbenoids. ... 

Attempts to initiate a desirable second insertion of PhN=C=X into the remaining silicon-hydrogen bond in compounds 6a or 6b failed. However, if a 10-fold excess of PhN=C=X (X = S, O) is used, trimers of phenylisothiocyanate (8a) or phenylisocyanate (8b) are formed. The latter can also be obtained in much better yields by direct reaction of 1 with a 10-fold excess of PhN=C=X. 

The formation of heterobutadiene 9 can be explained by an insertion-elimination reaction Insertion of sulfur into the silicon-hydrogen bond yields in the first step (2-Me2NCH2C6H4)(CH=CH2)Si(SH)2 as intermediate. This species eliminates H2S, producing the 1-thia-2-sila-1,3-diene 9, which is one of the rare examples possessing a silicon-sulfiir double-bond unit... 

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. 

Adam, W. Mello, R. Curci, R. Oxygen atom insertion into silicon-hydrogen bonds with assistance by dioxiranes. A stereospecific and direct rearrangement of silanes to silanols. Angew. Chem. 1990, 102, 916. 

Various transient trifluoromethylearbenes are known to insert into silicon-hydrogen bonds. [60]... 

The loss of ethylene may be synchronous with the oxidative addition of the hydrosilane to give the platinum(II) species, in which the metal atom inserts into the silicon-hydrogen bond in a three center process. 

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. 

Copper complexes are also able to catal) e the insertion of a carbene into a silicon-hydrogen bond. Associated with C2-symmetric Schiff bases, CuOTf allows the formation of chiral... 

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