Group carbon-silicon bonds

The view has also existed in the past that the carbon-silicon bond should be similar in behaviour to the carbon-carbon bond and would have a similar average bond energy. There is some measure of truth in the assumption about average bond energy but because silicon is more electropositive than carbon the C—Si bond will be polar and its properties will be very dependent on the nature of groups attached to the carbon and silicon groups. For example, the CH3—Si group is particularly resistant to oxidation but H13—Si is not. 

The silyl group directs electrophiles to the substituted position. That is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted position is relatively electron-rich. The ability of silicon substituents to stabilize carboca-tion character at )9-carbon atoms (see Section 6.10, p. 393) also promotes ipso substitution. The silicon substituent is easily removed from the c-complex by reaction with a nucleophile. The desilylation step probably occurs through a pentavalent silicon species ... 

Alkylsilanes are not very nucleophilic because there are no high-energy electrons in the sp3-sp3 carbon-silicon bond. Most of the valuable synthetic procedures based on organosilanes involve either alkenyl or allylic silicon substituents. The dominant reactivity pattern involves attack by an electrophilic carbon intermediate at the double bond that is followed by desilylation. Attack on alkenylsilanes takes place at the a-carbon and results in overall replacement of the silicon substituent by the electrophile. Attack on allylic groups is at the y-carbon and results in loss of the silicon substituent and an allylic shift of the double bond. 

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

The carbon-silicon bond is quite strong ( 75 kcal/mol), and trimethylsilyl groups are stable under many of the reaction conditions which are typically used in organic synthesis. Thus, much of the repertoire of synthetic organic chemistry can be used for elaboration of organosilanes.46 47 48... 

Organic silane derivatives with hydrolyzable groups on silicon (—OR, —Cl, —0C(0)R, etc.) are usually derived from a chlorosilane. Before or after formation of a carbon-silicon bond by hydrosilylation, the chlorosilanes are commonly converted to alkoxysilanes ... 

Oxidative cleavage of the carbon-silicon bond has been the subject of a recent and comprehensive review119, in which emphasis has been placed on the compatibility of the oxidation conditions with various functional groups, with the inclusion of very useful compatibility tables. 

Substitution of the carbon-silicon bond in allyl- and vinylsilanes by an electrophile has served as a powerful tool in organic synthesis. Electrophiles ranging from proton, carbon and main group heteroatoms to certain transition metal species have been employed. A... 

As described in Section II, Lewis acid catalyzed desilylative carbon-carbon bond formation with an electrophile has been shown to be very versatile in organic synthesis. Occasionally, depending on the nature of the substrates (e.g. the presence of appropriate functional groups), the carbon-silicon bond may remain intact. For example, treatment of 132 with a Lewis acid affords a mixture of cyclization products 133-135 (equation 113). The isolation of 133 indicates that the carbocation intermediate thus formed is trapped by the oxygen nucleophile before elimination of the silyl moiety occurs204. 

The fact that oxidation of carbon-silicon bond is possible in the presence of a silyl ether increases the attractiveness of siletanes as masked hydroxyl groups. 

Organosilicon. Pertaining to a compound of silicon with an organic group or groups, in which the two are united by a carbon-silicon bond or bonds (as distinguished from ester). 

Organosiloxane. A siloxane in which organic groups are linked directly to one or more of the silicon atoms by carbon-silicon bonds, as in hexamethyl-disiloxane, (CH3)3Si—0—Si(CH3)3. 

Oxidation of Si—C bonds. The carbon-silicon bond of organoalkoxysilanes is cleaved by 30% H202 or m-chloroperbenzoic acid in the presence of a base or under neutral conditions in the presence of KHF2 (equation I). Only one alkoxy group on silicon... 

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