Trimethylsilyl anion

Thus removal of water from classical rather inactive fluoride reagents such as tetrabutylammonium fluoride di- or trihydrate by silylation, e.g. in THF, is a prerequisite to the generation of such reactive benzyl, allyl, or trimethylsilyl anions. The complete or partial dehydration of tetrabutylammonium fluoride di- or trihydrate is especially simple in silylation-amination, silylation-cyanation, or analogous reactions in the presence of HMDS 2 or trimethylsilyl cyanide 18, which effect the simultaneous dehydration and activation of the employed hydrated fluoride reagent (cf, also, discussion of the dehydration of such fluoride salts in Section 13.1). For discussion and preparative applications of these and other anhydrous fluoride reagents, for example tetrabutylammonium triphenyldifluorosilicate or Zn(Bp4)2, see Section 12.4. Finally, the volatile trimethylsilyl fluoride 71 (b.p. 17 °C) will react with nucleophiles such as aqueous alkali to give trimethylsilanol 4, HMDSO 7, and alkali fluoride or with alkaline methanol to afford methoxytri-methylsilane 13 a and alkali fluoride. 

Thus, if trimethylsilyl anion were used as the nucleophile, then requisite diastereomer 30 could become available. Of the various counterions, Li+, Na+, and K+ have been examined by others. None of these species were suitable for direct 1,2-addition to carbonyl compounds. For example, trimethylsilyl lithium (Me3SiLi) adds nicely 1,4- to enones by a one-electron-transfer process, but does not provide a-silyl alcohols from ketones or aldehydes. In contrast tris(trimethylsilyl)alumi-... 

SUyl anions. A symmetrical disilane such as hexamethyidisilane is cleaved by TBAF (or CsF, but not by KF) to trimethylsilyl fluoride and the trimethylsilyl anion 1. An unsymmetrical silane is cleaved selectively at the less hindered Si atom. 

Preparation of phenyl-substituted silyl anion is a simple process either from chlorosilanes or disilanes (equation 15). Trimethylsilyl anions can he prepared from hexamethyldisi-lane by reaction with RM (RM=MeONa, MeLi ) in a strongly basic solvent such as HMPA (hexamethyl phosphoramide). 

Trimethylsilyl anions also give the substitution products by the reaction with alkyl chloride in almost quantitative yield (equation 14), but considerable amounts of electron-transfer products are produced as well in HMPA. Indeed, Mc3SiNa/HMPA is an excellent reagent to produce anion radicals (Table 9) from aromatic hydrocarbons for ESR (electron spin resonance) studies (equation 65). ... 

A metal-free trimethylsilyl anion is formed from hexamethyldisilane by cleavage with TBAF in HMPA in equilibrium concentration, as revealed by H and F NMR analysis of the reaction mixture. Treatment of trisilane with TBAF provides Me3SiMe2Si /NBu4 and Me3SiF the fluoride anion attacks at the terminal silicon of trisilane [Eq. (21)] (59). Synthetic application of this species is described in Section VII. Other metal-free silyl anions are described in Section V. 

Nucleophilic substitition of aromatic halides with trimethylsilyl anions competes with reduction. The ratio of substitution/reduction increases in the direction of I < Br < Cl, and appears relatively insensitive to the metal cations (30a,b). When an electrophile is a poorer electron acceptor, such as Ph3GeBr, nucleophilic substitution proceeds smoothly to form the coupling product in good yield ( ). 

Thus, under the reaction conditions the intermediate a-trimethylsilyl anion is configurationally stable. One should also mention that the Haller-Bauer cleavage proceeds with complete retention of configuration when Me, and possibly F and... 

It was also demonstrated that organosilylstannanes can be used as the trimethylsilyl anion source. In this case, the acid chlorides gave poor results and it was found that aryl iodides were suitable substrates. Reaction of buta-1,3-diene with Phi and BujSnSiMej gave the 1,4-carbosilylation product in 50% yield as an E/Z 84 16 mixture. The use of phenyl triflate as the aryl source did not give the desired 1,4-addition product but afforded the 2 1 telomerization product from two molecules of diene and one trimethylsilyl group, in good yield. 

Trimethylsilyl anion, free of metal ions, is generated when [n-Bu4N]F is added to (Me3Si)2 in HMPA. 

In the reaction of the trimethylsilyl anion 10 with N20 the anion does not suffer from the transfer of a CH2 group, but the reaction occurs exclusively by oxygen transfer without loss of any substituent from the anionic part (reaction 129). 

Trimethylsilyl Anion. Treatment of hexamethyldisilane (1) with an alkyllithium, metal alcoholate, or fluoride ion generates the trimethylsilyl anion (2) (eq 1). Reaction of (2) with aryl halides gives trimethylsilylarenes in 63-80% yields along with reduced products (ArH, 4-27%). tetra-w-butylammonium Fluoride (TBAF) catalyzed reaction of MegSi2 with butadienes gives l,4-disilyl-2-butenes (15-81% yield) with high ( ) selectivity. ... 

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