Carbanions silicon-stabilized

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

This Li-phenyl interaction is similar to the r/ -Ph contact observed in the silicon-stabilized carbanion complex [Li C(SiMe2Ph)3 (THF)] (140). Addition of tmeda to 49 yields the monomeric compound [Li C(SiMe2Ph)(PMe2)2 (tmeda)l in which the lithium is bound by the two P atoms of the phosphinomethanide ligand and the two N atoms of the tmeda in a distorted tetrahedral geometry (139). There... 

The carbon-carbon bond formation is accomplished by the reaction of the silicon-stabilized carbanions with electrophiles. Magnus and Roy have reported that methoxy(trimethylsilyl)methane is deprotonated with sec-butyllithium in... 

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. 

Since silicon stabilizes an a-carbanion by d jr-p jr interaction, silicon should promote electron transfer to a carbon-halogen bond which generates the a-carbanion. In fact,... 

The use of silicon is illustrated by Scheme 3.50 involving a Grignard reagent containing a trimethylsilyl group in which the silicon stabilizes the carbanion. Elimination of the silyl group from the P-hydroxysilane leads to the formation of the alkene. 

The preparation and use of silicon-stabilized carbanions is well documented the vast majority of reports are concerned with the reaction of these reagents with carbonyl compounds to form alkenes. 

Generation of the silicon stabilized benzyl carbanion 153 from 149 required the presence of hexamethylphosphotriamide. In its absence lithiation occurred at least in part in the aromatic ring. Alkylation of 153 with 152 produced 154 in 94% yield as a 2 1 isomer mixture at the benzylic site. Methiodide formation, addition of caesium fluoride, and refluxing in acetonitrile for 1.5 hours then afforded ( )-estrone methyl ether 28 (containing 7-8% of the C-9 p-H isomer) in 86% yield. Pure 28 was obtained on recrystallization. 

Generally, the two-electron reduction of organic halides produces carbanion species. In fact, cathodic reduction of organic halides under certain conditions gives the product derived from the corresponding carbanion intermediates. Silicon is known to stabilize the carbanion at the a position by dn-pn interaction. Therefore, we can expect that silicon promotes the electron transfer from carbon-halogen bonds and the formation of the carbanion at the a position. 

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... 

However, there is evidence against d-orbital overlap and the stabilizing effects have been attributed to other causes.88 An a silicon atom also stabilizes carbanions.89... 

While there are few examples of conjugate additions of either a-oxygen- or a-silyl-stabilized carbanions, Tamao and Posner have reported two hydroxymethyl synthons ["ClfcOH] (246 and 247) which show synthetic promise. Additions with the silicon-based synthon (246) is restricted to 2-cyclohexen-l-ones and work-up requires a successive acid and base procedure that is incompatible with sensitive molecules,188a-b while the tin-based synthon (247) is more versatile and the hydroxyl group is obtained under neutral conditions (Scheme 83).,88c... 

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. 

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