Silyl anion

Silyl anions are not commonly mentioned in introductory inorganic courses, but they should be Many silyl anions are considerably more stable than their carbanion analogs and many can be generated with surprising ease. Thus a tertiary amine suffices to deprotonate trichlorosilane  

The trichlorosilyl anion so produced can be put to a variety of uses. With a simple alkyl halide, it carries out an Sn2 displacement, producing an alkyltrichlorosilane  

With hexachloroethane, the Cl3Si anion does an E2 elimination to produce tetra-chloroethylene, as shown below  

Tris(trimethylsilyl)silane is much more acidic than tris(trimethylsilyl)methane (p a = 36.8) and triphenylsilane (p Ta = 35.1). Can you think of an explanation for these relative acidities Also, can you suggest a rationale for the high acidity of trichlorosilane (recall that it can be deprotonated by triethylamine). 

In 1968, Gilman and Smith showed that tetrakis(trimethylsilyl)silane can be cleaved by methyllithium to generate tris(trimethylsilyl)silyllithium (A in the reaction below), sometimes also called supersilyllithium (J. Organomet. Chem. 1968,14, 91)  

A variety of other silyl anions has been studied. Hopkinson and Lien, using a dz basis set, found that the potentially conjugated NCSiH2, 0=CHSiHj and HC=CSiH2 anions314,352 are pyramidal at silicon and have substantial barriers to inversion (35.2, 19.9 and 34.3 kcalmol-1, respectively)314. This indicates that contributions 

The most stable C2SiH3 anion is 167, but 168 lies only 5.7 kcal mol 1 higher in energy. 169 and 170 are 16.0 and 17.2 kcal mol 1 higher in energy than 167352. 

The potential aromatic silacyclopentadienyl anion 101246a 259 was discussed in Section VI.C. Despite the potential aromaticity of the planar C2v structure, 101 adopts a pyramidal Cs structure259. Silacyclopentadiene is also by only 2.2 kcal mol 1 more acidic than SiH4259. Significant stabilization is however present in the planar structure, as reflected in the lower inversion barrier of 16.2 kcal mol 1 in Cs 101 compared with an inversion barrier of about 26 kcal mol-1 in SiH3 at a comparable level of theory352. 

A silyl substituent also stabilizes adjacent N and P anions as shown by the increased calculated acidities of H3SiNH2, (H3Si)2NH2, H3SiPH2 and (H3Si)2PH relative to the corresponding parent and methyl substituted amines and phosphines356. 

3-21G). Thus, SiH3 stabilizes the model transition state for the SN2 reaction (171) by 14.2 kcal mol-1 relatively to CH3. The energy difference between the transition state models 171d and 171e is even higher 20kcalmol 1 in favor of the silylated system. 

Metal-substituted silicon analogs of alkenes, the disilenides, possess a prefabricated silicon—silicon double bond (1). Such compounds are important from the perspective of being sp -type silyl anions which are a powerful synthetic tool in organosi-licon chemistry (2). In particular, the disilenyl anions have been a focus of intensive investigation since these can act as excellent disilene (Si=Si) transfer agents (1). 

Schlenk line, 25-mL Schlenk flask, dry ice/acetone cooling bath, syringe, solvent cold trap, filter funnel, magnetic stirrer, safety glasses, laboratory coat, and protective gloves. 

Tetrakis(di- butylmethylsilyl)disilene, LiNp, THF, benzene. Experimental procedure 

A THF solution of lithium naphthalenide (0.79 mmol) was added to disilene (252 mg, 0.37 mmol) in THF (1.5 mL) at 78°C, and then the reaction mixture was allowed to warm to room temperature over 2 h. There was a rapid color change from dark blue to red due to the formation of the dianion. After removal of THF in vacuo, benzene was added to the reaction mixture and the solution was filtered. The NMR spectra of the benzene solution showed the formation of disilenyllithium and tBu2MeSiLi, and disappearance of the dianion. The solution was cooled to give red crystals of disilenyllithium (104 mg, 42%). Mp 159—161°C. 

The synthesis of the disilenyllithium 2 is carried out by the treatment of tBuLi with a tetrasila-1,3-butadiene derivative namely [(tBu2MeSi)2Si=Si(Mes)(Mes)Si=Si(SiMetBu2)2] (Mes = 2,4,6-trimethylphenyl) (5). 

---

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. 

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]. 

A variety of routes are available for the preparation of allylsilanes (/) with the simplest and most direct being the silylation of allyl-metal species. Other routes exemplified in this chapter include Wittig methodology, the use of silyl anions/anionoids in allylic substitution, and hydrometallation of... 

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

Silyl Anions with Functional Groups (Hydrido)silyllithiums ... 

Germyl, Stannyl, and Plumbyl Anions The preparative methods for the synthesis of the germyl, stannyl, and plumbyl anions are essentially the same as those mentioned above for the silyl anions. The most widely used methods are (1) reduction of halides R3EX (R = alkyl, aryl E = Ge, Sn, Pb X = Cl, Br) with alkali metals and (2) reductive cleavage of the E-E bond of R3E-ER3 (R = alkyl, aryl E = Ge, Sn, Pb) with alkali metals or organolithium reagents. Due to the favorable polarization of the (E = Ge, Sn, Pb) bond, the direct metalation... 

K2 was absolutely planar, the silyl anionic centers at the 1,3 positions were expectedly pyramidal. 

Tamao, Kohei, Kawachi, Atsushi, Silyl Anions. ... 

In order to avoid the SET process, we chose diphenylmethylsilyl anions (PI MeSiM 8a, M = K 8b, M = Na 8c, M = Li) as initiators for 7 instead of alkyllithium and benzene as a solvent. The polymerization did not take place in benzene with silyl anions alone. However, in the presence of an equimolar amount of suitable cryptands, the silyl anions initiated the polymerization. The results are summarized in Table 2. The molecular weights of polysilylenes thus obtained were in good agreement with the calculated values within experimental error. 

Table 2. Anionic Polymerization of Masked Disilene 7 initiated by Silyl Anions in the Presence of Cryptand in Benzene at Room Temperature...

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

Vinylsilanes TASF is superior to Bu4NF for cleavage of (CH3)3Si-—Si(CH3)3 to a silyl anion species that reacts with vinyl halides in the presence of a Pd(0)... 

Silyl Radicals, Silyl Cations and Silyl Anions 418... 

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 currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

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. 

---