Silyl anion reactions

Reactions of Silyl Anions with Metal Halides... 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

Finally, the reaction of silyl anions with dichlorides of Zr and Hf, which provides the silyl complexes 28 and 29, should be mentioned [116]. 

An extensive study (6) of Sn displacement reactions of allyl halides using silyl anions/anionoids has provided the following regioselective alternatives ... 

Ultrasound also promotes the reaction of potassium hydride with some silicon hydrides to give silyl anions in excellent yields and... 

Numbers of reactions have been developed by utilizing the strong affinity of the fluoride anion to the silicon atom. In this context, the use of chiral non-racemic ammonium fluorides137 41 43 751 for asymmetric silyl mediated reactions will be further investigated in future. 

In this method, attack by an anionic initiator ( -BuLi, potassium alkoxides/cryptand[2.2.2],62 or silyl anions in benzene)63 occurs regioselectively on the less hindered silicon of 9, resulting in an anionically terminated disilanyl-lithium which then attacks another monomer at the less hindered silicon atom. The process continues rapidly (the reaction is usually complete within a few minutes) in a living polymerization fashion to yield 10 on alcohol workup. 

The reaction is considered to proceed via a silyl anion mechanism, although the possibility of a radical-based mechanism has also been discussed.115,125 In order to clarify the mechanism, coupling experiments on a 1 1 mixture of chlorotrimethylsilane, 27 (reduction potential <—3.0 V),126 and chlorotriphenylsilane, 28 (reduction potential vs. standard calomel electrode (SCE) < —3.0 V),120 were performed, in which the mixed coupling product 1,1,1-trimethyl-2,2,2-triphenyldisilane, 29, and the homocoupling product hexaphenyldisilane, 30, only, were found,125 as indicated in Scheme 15. 

Some important reactions of silyl anions are as follows and they have emerged on the basis of the carbanion chemistry. 

Silyl radicals have been produced by one-electron oxidation of silyl metals [11]. This is found to be the method of choice for the generation of persistent silyl radicals and allowed the preparation of the first isolable silyl radical (see later in this chapter). Reactions (1.5) and (1.6) show two sterically hindered silyl anions with Na+ as the counter-cation, and their oxidation by the nitrosyl cation [12] and the complex GeCh/dioxane [13], respectively. 

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. 

The first step of this conversion is assumed to be the formation of the silyl anion, which undergoes a subsequent nucleophilic attack on the starting material283. The resulting disilane may be isolated, when stoichiometric amounts of metal are used. Flowever, in contrast to peralkylated disilanes, disilanes which bear at least one aryl substituent at each silicon are susceptible to further reduction. Accordingly, the Si—Si bond of the fully or partially arylated disilane is easily cleaved under the reaction conditions by slow electron transfer from excess metal, eventually transforming both silyl units of the disilane into the desired metalated silane. 

The first evidence for the existence of short-lived (alkoxy)silyl anions (e.g. 32) was obtained by Watanabe and coworkers in the reaction of NaOMe with the corresponding disilanes (31), which yielded in the presence of alkyl halides the expected trapping product of the intermediate (alkoxy)silyl sodium compounds 32 (equation 43)93. 

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

The final product of the deprotonation depends strongly on the deprotonation reagent and/or the reaction conditions. Thus, in THF and using MeLi (or NaH) as base, an anionotropic l,3-Si,0-trimethylsilyl migration occurs in the alkoxymethylsilane 191 with formation of the silyl anion 192 instead of elimination of silanolate. Therefore, after hydrolytic work-up only trimethylsiloxy(bis(trimethylsilyl)silyl)alkanes 193 were obtained (equation 48).108,117... 

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). 

Treatment of hydrosilanes with potassium hydride is a known route to silyl anions and has been used to generate silacyclopentadienide anions. Reinvestigation of the reaction of KH with C4Ph4SiMeH has shown that the dominant pathway is simple addition of the hydride anion to the silicon centre to give the pentavalent silicate [Ph4C4SiMeH2] K+ (129)189 (equation 75). The Si—H coupling constants in 129 (Jsi-H = 192.5 and... 

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