Silicon silyl anions

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

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]. 

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

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. 

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. 

Another point which needs to be clarified from the start is the nomenclature of metalated silanes We will frequently use the term silyl anion in this chapter when we talk about metalated silanes. Although the term anion defines, literally taken, an ionic compound, this expression, when used by us, does not necessarily imply that the compound in question is of ionic nature, but covers, as well, in analogy to the use of the term carbanion , silicon compounds with a polarized covalent silicon-metal bond. 

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. 

Spectroscopic studies have also been devoted to the question of ion-pairing phenomena in silyl anions. It was found that in arylated silyl anions the 13C NMR as well as the 7Li NMR chemical shifts are only slightly influenced by the polarity of the solvent36,51. This is in clear contrast to the NMR-spectroscopic behavior of aryl-substituted carbanions, which show a marked solvent dependence. This was interpreted in terms of a significant covalent interaction between silicon and lithium in metalated silanes36. Further evidence for a significant covalent nature of the Si—Li bond arises from the observation of a scalar... 

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. 

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... 

RR SiCr, Na+ (equation (7)). The propagation step (equation (8)), which is rapid but nevertheless rate determining, is the reaction of anion-terminated chains with dichlorosi-lane to add one silicon unit and produce a chlorine-ended chain. The latter is thought to be reduced rapidly to the anionic form by reaction with sodium. The polysilane chains will alternately be terminated with silyl anions or Si-Cl. This is consistent with evidence from chemical derivatization that both may be present in the products of the reaction. ... 

From these observations, Woerpel and Cleary proposed a mechanism to account for allylic silane formation (Scheme 7.23).85 Silacyclopropane 94 is formed from cyclohexene silacyclopropane 58 through silylene transfer. Coordination of the Lewis basic benzyl ether to the electrophilic silicon atom86-88 generates pentacoordinate siliconate 95 and increases the nucleophilicity of the apical Si-C bond.89 Electrophilic attack by silylsilver triflate 96 forms silyl anion 97. Intramolecular deprotonation and elimination then affords the silylmethyl allylic silane. 

Optimization of geometry for some silylenium cations was made (22,24). Such calculations predict that silylenium ions will adopt a planar structure in contrast to silyl anions, which are predicted to be pyramidal. It should be noted that theoretical studies also indicate a high ability of silicon to accommodate negative charge. The parent silyl anion H3Si was calculated to be more stable than its carbon analog by about 50 kcal/mol (24). This implies a remarkable affinity of silylenium ions toward electron-rich species. 

Since silicon is a positive element, covalent bonds of most silicon compounds polarize as Si+-X . However, silicon can form bonds with alkaline and alkaline earth metals to give silylmetals in which silicon compomids polarize as SD-M+. These silylmetals are the source of silyl anions and react with haloaUcanes to give the corresponding organosilanes (equation 14). 

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