A-silyl carbanion

SynttMSH ol alkenes from a silyl carbanions and cartMnyl compounds In cases where separation ol sitylaloohol diastereomers (e g 4] can be achieved, pure Z or E oleflns can be Isolaled... 

A carbon-substituted five-membered ring, 24, was also successfully ring-opened via a silyl carbanion intermediate after initiator attack on a silicon adjacent to carbon to give a sequence-ordered C/Si... 

These P elimination reactions have been used in an olefine synthesis called the Peterson olefination reaction which is analogous (and sometimes superior) to the Wittig reaction. The Peterson olefination reaction involves the addition of an a-silyl carbanion to an aldehyde or ketone to give P-hydroxysilane, followed by P-elimination to give the olefine. 

Alkenes from a-silyl carbanion and carbonyl compounds. Also known as sila-Wittig reaction. 

The carbomagnesiation of vinyisiianes is a powerful method for the generation of synthetically useful a-silyl carbanions. However, simple vinyisiianes such as trimethyl(vinyl) silane do not undergo carbometalation with Grignard reagents (Scheme 59) °. In early days, only limited success had been achieved by using perfluorovinylsilanes °°. 

Another common a-silyl anion is produced by die halogen exchange from a methyl (but not odier group) attached to silicon. Odier a-silyl carbanions can be generated by other processes. Such anions lack the resonance stabilization of an ester group seen in the previous example. They are consequently less stable and must be generated under carefully controlled conditions. They are good nucleophiles and add effectively to aldehydes and ketones. 

Generally, the direct metallation of unactivated alkyl groups is not a synthetically useful reaction. However, under certain circumstances unactivated a-silylcarbanion formation has been reported. Treatment of orf/zo-silylated benzamides 149 with LDA gave an a-silyl carbanion 150, stabilized by a complex-induced proximity effect, which then underwent... 

The stability of an a-silyl carbanion is responsible for the unproved synthetic utility of the Stork annulation over other annulations195,196. These reactions involve the Michael addition of an enolate ion to an enone, and in the absence of a a-silyl substituent suffer drawbacks due to the reversibility of the Michael reaction. However, the addition of enolate ions to a-trimethylsilylvinyl ketones is not reversible, owing to a-silicon stabilization of the canonical form 152 shown in equation 122. 

The stereochemical outcome of the Peterson reaction between unsymmetrically substituted a-silyl carbanions and aldehydes or unsymmetrical ketones is determined by the relative rates of formation of the threo and erythro /3-oxidosilancs. Often the rates are similar, to give a product alkene E Z ratio of 1 1, although some workers report a predominance of cis olefins in the reactions of aldehydes. 

In 1968 Peterson published his results about the reaction of a-silylated carbanions with carbonyl compounds to achieve /3-hydroxyalkyl-silanes. He found that instantaneous elimination affords unsaturated hydrocarbons ( Peterson olefmation )180 Thus, the lithiated trimethyl-benzyl-silane (274) plus benzophenone (135) give via lithium l,l,2,-triphenyl-2-TMS-ethanolate (275) 1,1,2-triphenyl-ethene (276)180 ... 

The mechanism involves the preferential attack of the hydride on the less hindered silicon with formation of a pentacoordinated anionic species which collapses to give an a-silyl carbanion intermediate and SiH4 gas. The a-silyl carbanion may then take up a hydrogen from the hydrogen source (e.g., silane or solvent) to yield 7 or may lose... 

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. 

The variety of methods available for the formation of a-silyl carbanions can be divided into four categories. 

Anions of a-silyl phosphonates of type (153) also undergo additions to carbonyl compounds. The corresponding addition products, 3-silyl alkoxides, can react with ketones to yield the product of the Peterson alkenation or the Wittig reaction. In practice only the Peterson product (154) is obtained, indicating that loss of OSiMes is faster than elimination of C PPhs (Scheme 68). 72 If the a-silyl carbanion is adjacent to a chlorine atom (155), an internal displacement reaction follows the initial formation of the -silyl alkoxide, and epoxides (156) are formed (Scheme 69). 74... 

As a general mle, unless an anion-stabilizing group, such as phenyl, or a heteroatom such as sulfur is present, the alkylsilane is not readily deprotonated. The a-halosilane can be deprotonated but, unlike the readily available chloromethyltrimethylsilane, there are few general methods to this approach. Al-kyllithium reagents add to vinylsilanes ( ) to produce the carbanion (287). Silyl derivatives with heteroatoms, such as sulfur, selenium, silicon or tin, in the a-position (288) may be transmetallated (Scheme 41). Besides the difficulty in synthesizing the anion, alkene formation lacks specificity for simple di- and tri-alkyl-substituted alkenes. As a result, the Peterson reaction of an a-silyl carbanion with a carbonyl has found the greatest utility in the synthesis of methylene derivatives, (as discussed in Section 3.1.3), heterosubstituted alkenes and a,p-unsaturated esters, aldehydes and nitriles. 

Several procedures are available for the preparation of the requisite p-hydroxysi-lanes such as addition of a-silyl carbanions to aldehydes and ketones, reaction of organocuprates with a,P-epoxysilanes, reduction of P-ketosilanes, and addition of organometallic regents to P-ketosilanes. The selection of a particular procedure is dictated by the structure and stereochemistry of the desired alkene. 

In the P-ketosilane approach, the condensation of a-silyl carbanions with carbonyl compounds produces mixtures of diastereomeric P-hydroxysilanes. Therefore, the preparation of stereodefined alkenes via the Peterson reaction hinges on the availability of just one diastereomer. To overcome this shortcoming, procedures have been developed to prepare stereoselectively P-hydroxyalkylsilanes via the P-ketosilane or the epoxysilane routes, as exemplilRed below. 

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