Silicon—carbon bonds double bonded

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. 

Since allylsilane can be considered as a very soft nucleophile because of the involvement of o-7i conjugation between the -electrons of the double bond and the o-electrons in the carbon-silicon bond, the addition of allylsilanes to ,/3-unsaturated enone moiety occurs exclusively via 1,4-addition19,20. A typical Sakurai-Hosomi reaction is illustrated in the reaction of allylsilane 66 with enone 67 in the presence of TiCU to give 68 which is used for the synthesis of a cyclic enediyne (equation 45)108. A similar reaction has been used for the synthesis of ewf-herbasolide 70 from enone 69 (equation 46)109. Prenylsilane undergoes 1,4-addition with squaric acid chloride 71 followed by dechlorosilylation to give 72 as the predominant product (equation 47). Interestingly, other simple allylsilanes react in a 1,2-addition fashion to yield 73110. 

Si4.6 whereas a silicon-based approach proved to be successful and gave the required exocyclic double bond in (3-Gorgonene, a non-isoprenoid sesquiterpene isolated from Pseudopteragorgia americana. The success of the a-silyl carbanion here is attributed to the lengthy carbon-silicon bond which makes the carbanion sterically less demanding. 

In the examples of electrophilic substitution of vinylsilanes described rove, collapse of the carbon-silicon bond to form a carbon-carbon 7t bond faster than nucleophilic attack on the carbocation by the anionic counterion l the electrophile. This is not always the case. For example, on treatment of -l-trimethylsilylpropene with bromine followed by aqueous ethanol, (Z)--bromopropene is formed almost exclusively (Figure Si5.10). This is pnsistent with anti addition of bromine to the double bond followed by anti limination of trimethylsilyl bromide. 

The hydrosilation reaction is the addition of a silane, RsSi-H to a C=C double bond, to form new carbon-silicon bonds. In the absence of a catalyst, this reaction occurs only with a very large (50-60 kcal/mol) barrier (46). 

Three types of products have been observed in intermolecular acylations of homoallylic silanes, the major one being cyclopropylmethyl ketones, along with minor amounts of 3-butenyl ketones and -chlo-ro ketones. It is likely that all derive from the carbenium ion formed by acylation of the double bond, which then undergoes cyclodesilylation or hydride transfer followed by 3-elimination (Scheme 14). The former leads to the cyclopropane, which can ring open to give the chloro products. The latter pathway gives the butenyl ketone, and is supported by location of substituent positions on methylated substrates. However, the direct acylation of the carbon-silicon bond should not necessarily be excluded in consideration of more general cases. Titanium tetrachloride seems the preferred catalyst in these cyclodesilyl-ations, and low temperatures minimize the formation of the chloro by-products. Intramolecular versions... 

Silicon-containing epoxides with hydrolytically stable carbon-silicon bonds were first prepared by Pleuddeman by the addition of hydrogen functional silanes to epoxy compounds containing double bonds (7,8). We have employed this reaction extensively to prepare several different difunctional epoxy monomers as shown in Table I. An example of this reaction is given in equation 1 for the preparation of difunctional monomer HI. 

As the bulk of the cyclizations reported started with an electrophilic attack at the double bond, we want to finish this section with at least one further radical induced and one nucleophilic cydization. In radical cyclizations the halomethyl-dimethylsilyl group has become quite popular as radical starter and generally one makes use of H2O2 oxidations to finally cleave the carbon-silicon bond [169]. 

The carbon-silicon bond to saturated alkyl groups is not very reactive. 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. 

Apeloig and Kami (13) have also studied the effects of substituents on the reactivity of silenes by the frontier molecular orbital (FMO) approach. They have concluded that, concerning electronic factors, the polarity of the carbon-silicon double bond, and thus the coefficients of the frontier orbitals, play a more important role than the energies of these orbitals in controlling the reactivity of silenes. 

Like carbon, silicon is able to form covalent compounds. Unlike carbon, silicon is not able to form double or triple bonds. Hence silicon is able to form compound by condensation reaction. 

Elemental silicon is relatively stable in most substances at ordinary temperatures. Silicon shows similarity with other elements of its group, especially with germanium in many chemical properties. It forms tetravalent compounds with tetrahedral geometry almost exclusively. However, only in silicon monoxide, SiO, is its valence +2. Also, unlike carbon, silicon does not form unsaturated double or triple bond compounds. Silicon dissolves in germanium... 

Ito and co-workers have also used the Pd(OAc)2/t-alkyl isocyanide catalyst to affect the double silylation of carbon-carbon multiple bonds in an intramolecular system to yield silacarbocycles.59 Alkenes or alkynes that are tethered to a disilanyl group through a carbon chain, an ether linkage, or an amine functionality undergo intramolecular addition of the disilane moiety to the multiple bond. Activation of the disilane by the presence of electron-withdrawing groups on silicon is not necessary for the reaction to... 

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