Silyl anions reactivity

Unlike silyl radicals and silylium ions that have been elusive for a long time, silyl anions (silanides) are well known since the pioneering work of Gilman etal. in the 1960s. Two literature reviews on silyl anions by Lickiss and Smith as well as Tamao appeared in 1995. Many structural elucidations and reactivity studies of silyl anions have been... 

The mechanism of the polycondensation reaction remains unclear. A variety of possible reactive intermediates have been suggested, including silyl radicals and silyl anions. An anionic propagation mechanism (100,101,103) has been strongly suggested, although the case is by no means settled (104). Other Synthetic Methods. 

An (alkoxysilyl)lithium, ( -BuO)Ph2SiLi, can be prepared from (t-BuO)Ph2SiSnMe3 by treatment with n-BuLi in THF (Scheme 10). This species is stable at -78°C for a few hours and behaves as an ordinary nucleophilic silyl anion 54d). (f-BuO)Ph2SiLi also behaves as an electrophile under certain conditions It undergoes self-condensation smoothly or butylation in the presence of an excess amount of n-BuLi and TMEDA (Scheme 10). (t-BuO)Ph2SiLi thus exhibits an ambiphilic nature, that is, silylenoid character. The ambiphilic reactivity of (t-BuO)Ph2SiLi, how-... 

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... 

Some years ago Hengge et al. published a feasible synthesis of KSi6Men (1) and did basic research on the reactivity of this silyl anion including the preparation of transition metal complexes [1-3]. Our group contributed two crystal structures of such compounds [4, 5]. In an effort to get further insight into the reactivity of 1 and the structures of its transition metal conq>lexes, we reacted it with various transition metal conq>ounds. 

Because silyl anions are much more reactive in substitution reactions than the isosteric phosphanes [10], we examined the reaction of 1 with transition metal carbonyls. 

For lithiated allylic phosphonates, the nature of the carbon substituent affects the regioselectivity of the initial silylation as well as the reactivity of the resulting silylated anion.Whereas diethyl... 

In this section we will discuss various reactive intermediates that contain silicon. In particular, the silicon analogues of the classic organic reactive intermediates such as carbenes (silylenes), carbenium (silicenium) ions and carbanions (silyl anions) have attracted the interest of experimentalists and theoreticians. In light of the elusive nature of these species it is not surprising that much of what we know about their properties comes from theory, in particular ab initio molecular orbital calculations. 

The distinct reactivity of silyl anions versus carbanions towards N20 was used149 to probe the structure(s) of [M —1]" generated via deprotonation of trimethylsilane with NH2. The fact that the [M — H] ion m/z 73 upon reaction with N20 generates an ion at m/z 75 and not at m/z 89 clearly demonstrates that deprotonation has generated ion 9 and not 10 (reaction 131). 

Useful alternative preparations of stannanes include palladium-catalysed decarboxylation of stannyl esters or coupling of halo compounds with hexa-alkyldistannanes coupling with hexaalkyldisilanes requires rather more vigorous conditions. Trialkylstannyl and -silyl anions are highly reactive and will displace halogen without the use of a catalyst. ... 

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). 

Polysilanes prepared by anionic polymerization have reactive silyl anion end-groups. Such polymers can be utilized for preparing novel block copol5miers. They also can be used for preparing new polymers that contain novel end-groups. 

Organosilicon chemistry has expanded its scope considerably in the last two decades. One of the most remarkable achievements is the progress which was made in the elucidation of the mechanisms in silicon chemistry, which now become comparable to those in carbon chemistry. The behavior of reactive intermediates such as silylenes, silyl radicals and silyl anions are well explored, although the chemistry of silyl cations is still controversial. Doubly-bonded silicon species are now well understood " but triply-bonded silicon is still elusive. 

The electronic and structural features of disilene radical anions make them highly sought after species. The above-mentioned disilene radical anion possesses radical character at one of the central Si atoms whereas the other Si atom has silyl anion character. This structural feature can be exploited toward novel reactivity investigations. 

For reviews on silyl anions with emphasis on synthesis, structure and reactivity perspectives, see ... 

The combination of silyl enol ethers and fluoride ion provides more reactive anions to give alkylated nitre compounds in good yields after oxidation v/ith DDQ, as shovm in Eq. 9.22. This process provides a new method for synthesis of indoles and oxyindoles fsee Chapter 10, Symhesis of Hatarocydic Compoioids). 

The reactivity pattern (1) of silyl enol ethers and ketene acetals is based largely on their synthetic equivalence to enolate anions. Recently, a different spectrum of behaviour has been revealed, particularly in those reactions that involve direct reaction without prior generation of the enolate anion. Indeed, the historic development of silyl enol ethers can be seen in three separate phases, involving... 

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. 

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