Carbanions silyl-substituted, reactions

The fact that a-silyl substitution leads to a significant stabilization of carbanionic species is well-known and has been exploited in synthetic chemistry. On the other hand, silyl anions themselves are in general much more stable than their carbon analogues. The stabilization of carbanions by silyl substituents in the a position has been measured by Brauman and coworkers37. The anions were generated via nucleophilic displacement reactions (equation 2) of a silyl group with F- (see also Section III.B). 

Very few reports deal with the substitution reaction of a silyl group by an alkyl group. In this instance, the intermediate carbanion resulting from the cleavage of a C-Si is intercepted by an electrophile that can be a very reactive bromide or a Schifif base, as in the following examples.130,240... 

Reactions involving ambient silyl-substituted carbanions 2.5332 Metallated allylaminosilanes... 

As mentioned in the previous section, the Peterson reaction proceeds by an irreversible addition of the silyl-substituted carbanion to a carbonyl. It has generally been assumed that an intermediate p-oxidosi-lane is formed and then eliminated. In support of this mechanistic hypothesis, if an anion-stabilizing group is not present in the silyl anion, the p-hydroxysilanes can be isolated fixrm the reaction, and elimination to the alkene carried out in a separate step. Recent studies by Hudrlik indicate that, in analogy to the Wittig reaction, an oxasiletane (304) may be formed directly by simultaneous C—C and Si—O bond formation (Scheme 43). The p-hyd xysilanes were synthesized by addition to the silyl epoxide. When the base-induced elimination was carried out, dramatically different ratios of cis- to rranr-alkenes were obtained than from the direct Peterson alkenation. While conclusions of the mechanism in general await further study, the Peterson alkenation may prove to be more closely allied with the Wittig reaction than with -elimination reactions. 

Silicon-substituted cyclopropanes can also be generated by the reaction of electron-deficient alkenes with silyl-substituted carbanions bearing a leaving group. This transformation proceeds via sequential Michael addition and intramolecular ring closure (MIRC reaction). 

Silyl-Substituted Carbanions (Homo-Peterson Reaction)... 

This reaction type necessitates, after intermolecular ring opening of the oxirane by a stabilized silyl-substituted carbanion, interaction between oxygen and silicon atoms situated on adjacent carbon atoms, to give substituted cyclopropanes via an intramolecular Spji process. The homo-Peterson reaction is nonstereospecific and occurs at low temperatures. This is in contrast to the cyclopropanation of oxiranes with phosphorous ylides (see Section 1.2.2.1.) or di-ethoxyphosphoryl-substituted carbanions (see Section 1,2.2.2.), which require heating for a long period to accomplish the same transformation. 

With trimethylsilyloxirane, ring opening occurs at the silyl-substituted Cl, so reaction with bis(alkylsulfanyl)-substituted silyl-substituted carbanions gave the corresponding cyclo-propanes 3. ... 

Summary Reacting 2-neopentyl substituted silacyclobutanes la,b with MeLi/HMPA (hexamethylphosphoric triamide) anionic polymerization to give polymers 3a,b plays only a minor role for product formation. Instead, the head-to-head dimers 2a,b are isolated as main products. Their formation is explained by a complex reaction mechanism, in which various carbanionic, highly reactive intermediates are discussed. Obviously, the bis-a-silyl substituted carbanions 10a,b are remarkably stable, as can be concluded from Si NMR spectroscopic investigations at low temperature and from the products formed by trapping reactions with alcohols. 

Performing the reactions of silacyclobutane la,b with MeLi/HMPA in the presence of MeOH, the doubly a-silyl-substituted carbanions 10a,b are protonated and transformed into the corresponding... 

The mechanism begins with the addition of a silyl-substituted carbanion 14 to a carbonyl compound 15 an aqueous work-up then leads to a diastereomeric mixture of yff-hydroxyalkylsilanes, often isolable and sometimes separable. The stereo-selectivity of the reaction can be controlled by the steric demands of the silyl group the use of more sterically demanding silyl groups results in the erythro isomer as the major product. 

The carbanion -type reactivity of dienamine activation can be promoted by a silyl-protected diarylprolinol catalyst in the presence of reactive electrophilic species, such as nitroolefins or diarylmethanols (Scheme 2.10) [20]. Although both direct addition (a-addition) and vinylogous addition (y-addition) are viable reaction pathways, the a-reactivity is the more typical one and usually provides better selectivities due to more effective shielding of the a-position by the aminocatalyst. Interestingly, the a-/y-selectivity of dienamine-mediated carbanion -type addition reactions seems to be strongly influenced by the substitution pattern of the enal reactant For example, it was demonstrated that y,y-disubstituted enals favor the a-addition product while y-functionaUzation is the predominant reaction pathway for y-monosubstituted enals. 

Silyl-substituted, three-membered compounds such as cyclopropanes, cyclopropenes, oxiranes, and aziridines are activated by nucleophilic catalysts such as phosphines and fluorides to generate the corresponding naked carbanionic species, which then undergo aldehyde addition reactions at relatively high temperatures (Schemes 3-137, 3-138, 3-139, and 3-140). 

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

The domino reaction is initiated by the chemoselective attack of the carbanion 2-458 on the terminal ring carbon atom of epoxyhomoallyl tosylate 2-459 to give the alkoxides 2-460 after a 1,4-carbon-oxygen shift of the silyl group. The final step to give the cyclopentane derivates 2-461 is a nucleophilic substitution. In some cases, using the TBS group and primary tosylates, oxetanes are formed as byproducts. 

Silyl homoallylic alcohols are obtained with high y-regioselection and E-stereoselection on reaction of chiral alkoxy- and aminomethyl-substituted a -silylallyl carbanions with aldehydes factors which influence the diastereomeric excess have been identified. 

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. 

Not only alky] groups, but also aryl [492, 493], vinyl [494], acyl [276, 495—497], alkoxycarbonyl [498], aminocarbonyl [499-501], silyl [502-504], or phosphoryl groups [279, 280] can migrate to a vicinal carbanion (Scheme 5.68). Because some of these groups can be used to stabilize a-heteroatom-substituted carbanions by chelate formation, migration of these groups to the carbanion is a potential side reaction in the generation and alkylation of chelate-stabilized carbanions. 

Takeda and coworkers showed that the reaction of a silylmethyllithium with a halo-methyloxirane gave the corresponding cyclobutane derivatives via a 1,4-silyl migration of the initial lithium alkoxide followed by an intramolecular nucleophilic substitution of the resulting carbanion (equation 151)370. 

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