Silyl anions formation

For an excellent summary of the methods of silyl anion formation see ref. 4c. 

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]. 

Several other synthetic techniques have also been described. Redistribution polymerization was outlined in COMC II (1995) (chapter Organopolysilanes, p 99) and proceeds by phosphonium salt-catalyzed redistribution of chlorodisilanes.133 Disproportionation polymerization, which is a similar process, has been described for the formation of polymers by ethoxide-catalyzed disproportionation of alkoxydisilanes via silyl anion intermediates.134 These procedures give rise to network polymeric products of rather low molecular weight (see below, Section 3.11.7.1). 

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 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... 

Stereopure epoxide 1 was prepared and treated with 3.6 equivalents of t-butyllithium in THF/HMPA at -78°C. The intention was that formation of the anion at the benzylic carbon would lead to a 4-exo-epoxide ring opening reaction a subsequent [l,2]-silyl shift (Brook rearrangement) would generate the oxetane 2 with stereocontrol at all three stereocentres. Anion formation proceeded smoothly at -78°C, then 1 ml of 1 M hydrochloric acid was added and the product isolated. Obtained pure in 40% yield, this was shown to be the aldehyde 3. No oxetane 2 was obtained. 

It has been demonstrated that quatemarization of nitrogen may be realized with alkyl halides or tosylates and iodide is found to be the best anion. Formation of N-unsubstituted pyrrolidines when using an alkyl chloride was tentatively explained by the formation of trimethylsilyl chloride in the reaction medium. This silyl halide participates in the quatemarization of nitrogen to give A-silyl pyrrolidine and finally 1V-H pyrrolidine under the hydrolytic conditions of the work-up. The fact that changing iodide for chloride allows formation of the N-unsubstituted pyrrolidine is a synthetically interesting feature.393... 

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. 

Silyl complexes also result from the reaction between a silyl anion and a metal halide, for example, in the formation of (26), which has an Si-H stretching frequency of 2103 cm (equation 41). ... 

The reactions leading to stereoselective formation of the C — Si bond fall into two main categories hydrosilylation of alkenes or dienes (cf. Houben-Weyl, Vol. 13/5, p 51) and the related addition of silyl anions to activated C-C double bonds (cf. Houben-Weyl, Vol. 13/5, p 53). Also of interest is the stereoselective silylation ofcarbanions (cf. Houben-Weyl, Vol. 13/5, p 39). 

Silyl Anions (RsSE"). For years aryl substitution was thought to be necessary for the formation of silyl alkali compounds. This is not the case, and trialkylsilyl alkali metal compounds are now readily available. The most general and convenient method of generation is disilane cleavage. For example ... 

In the reductive coupling of phenylhexenyldichlorosilane, 75% of the alkenyl groups did not react with radicals, although cyclization was highly favored in the strainless ring. This result means that the lifetime of radicals must be very short and that they can be further reduced to silyl anions prior to intramolecular cyclization. The presence of anionic intermediates is additionally supported by faster reactions in ethereal solvents, first-order kinetics of the monomer, and some model reactions. The formation of silyl... 

The macromolecular silyl chloride reacts with sodium in a two-electron-transfer reaction to form macromolecular silyl anion. The two-electron-trans-fer process consists of two (or three) discrete steps formation of radical anion, precipitation of sodium chloride and generation of the macromolecular silyl radical (whose presence was proved by trapping experiments), and the very rapid second electron transfer, that is, reduction to the macromolecular silyl anion. Some preliminary kinetic results indicate that the monomer is consumed with an internal first-order-reaction rate. This result supports the theory that a monomer participates in the rate-limiting step. Thus, the slowest step should be a nucleophilic displacement at a monomer by macromolecular silyl anion. This anion will react faster with the more electrophilic dichlorosilane than with a macromolecular silyl chloride. Therefore, polymerization would resemble a chain growth process with a slow initiation step and a rapid multistep propagation (the first and rate-limiting step is the reaction of an anion with degree of polymerization n[DP ] to form macromolecular silyl chloride [DP +J, and the chloride is reduced subsequently to the anion). 

Reaction of an enantiomeric hydrosilane with KH yields a racemic silyl anion (77). Thus, treatment of (-l-)-(l-Np)PhMeSiH with KH at 50°C in DME for 24 hours and subsequent addition of n-BuBr give racemic ( )-(l-Np)PhMeSi-n-Bu, via the formation of the corresponding silyl anion with loss of optical activity. Hydrolysis or deuterolysis also gives a racemic product. These observations clearly rule out a mechanism in which the silyl anion is formed by proton abstraction from the hydrosilane, because retention of configuration would be expected. A possible mechanism involves a pentacoordinate dihydrosilyl anion formed via coordination of H in an initial fast, reversible process, and its decomposition to the racemic silyl anion with loss of molecular hydrogen [Eq. (7)]. A gas-... 

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... 

As mentioned in the previous section, the Peterson reaction proceeds by an irreversible addition of the silyl-substituted carbanion to a carbonyl. It has generally been assumed that an intermediate p-oxidosi-lane is formed and then eliminated. In support of this mechanistic hypothesis, if an anion-stabilizing group is not present in the silyl anion, the p-hydroxysilanes can be isolated fixrm the reaction, and elimination to the alkene carried out in a separate step. Recent studies by Hudrlik indicate that, in analogy to the Wittig reaction, an oxasiletane (304) may be formed directly by simultaneous C—C and Si—O bond formation (Scheme 43). The p-hyd xysilanes were synthesized by addition to the silyl epoxide. When the base-induced elimination was carried out, dramatically different ratios of cis- to rranr-alkenes were obtained than from the direct Peterson alkenation. While conclusions of the mechanism in general await further study, the Peterson alkenation may prove to be more closely allied with the Wittig reaction than with -elimination reactions. 

For an excellent review and experimental procedures for a variety of silyl anions as well as alkene formation see ref. 4f. 

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