Silyl anions transmetallation

The reactivity of the newly synthesized anions was studied. As expected, they can easily be protonated and alkylated, in close analogy to other silyl anions we have investigated earlier [3, 4]. Furthermore, oligosilyl anions were successfully used for the formation of interesting silicon-heteroatom bonds [9]. In a similar manner, vinylsilyl potassium compound 4b can be transmetalated into the Mg analog [10], and the respective anion can then be used as a nucleophile in the reaction... 

Summary The synthesis of a,(0-bis[tris(trimethylsilyl)silyl] alkanes was achieved by the reaction of ct,(0-ditosylalkanes with tris(trimethylsilyl)silyl potassium. Reaction of these with one equivalent of potassium /er/-butoxide resulted in clean formation of monopotassium silyl anions. Addition of another equivalent of the transmetallating agent led to the formation of the dipotassium compounds in cases where the alkyl spacer contained at least three methylene units. Partial hydrolysis of the dipotassium compounds induced an intramolecular reaction yielding a cyclic silyl potassium compound. 

This increase in stability caused by the change of the metal is in contrast to the proposed mechanism of racemization for metalated silanes. Since the rate-determining step of the racemization process is discussed in the literature [3a, 8-11] as the inversion of the free silyl anion, no drastic effect of transmetalation should be expected. 

The reason for this seems to be the enhanced reduction properties of the silyl anion, which are caused by the removal of a charge-stabilizing group. This called for a reactivity moderation, which can easily be accomplished via the transmetallation of potassium against magnesium [6], The reagent obtained then reacts smoothly with the metallocene dichloride (Scheme 2). 

Corriu and coworkers found that the reaction of (Z)-y-Af,lV-bis(trhnethylsilyl)amino-1-propenylstannane 298 with butyllithium, followed by quenching with methyl iodide and then hydrolysis, provided 3-methylamino-l-propenylsilane 299 in high yield (equation 185)446, suggesting an anionic 1,4-silyl migration from N to C in the (Z)-vinyllithium 300 to give 301 (equation 186). The transmetallation of the ( )-isomer of 298 gave no silyl migration products (equation 187)447. 

Lithiation of compound 560 with s-BuLi-TMEDA in THF at —78 °C following an inverse addition protocol provided the anion 561. It reacts with primary alkyl iodides and triflates, silyl chlorides, diphenyl disulfide, epoxides, aldehydes, ketones, imines, acyl chlorides, isocyanates and sulfonyl fluorides to yield the expected compounds 562 (Scheme 152). The transmetallation of compound 561 with ZnBr2 allowed the palladium-catalyzed cross-coupling reaction with aryl and vinyl bromides837. When the reaction was quenched with 1,2-dibromotetrafluoroethane, the corresponding bromide 562 (X = Br) is obtained838. 

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. 

Corey has utilized the Brook-generated allyl anion alkylation in several syntheses. A recent example was in the preparation of ZF-cation cyclization precmsor 28. Chemoselective addition of 2-propenyllithium to the acyl silane carbonyl of 27, followed by transmetalation and alkylation provided the (Z)-silyl enol ether 28. ... 

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