A-Silyl anion

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)... 

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. 

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. 

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 electron affinities of a number of a-silyl substituted silyl and carbon radicals were determined in photodetachment experiments and confirmed by data obtained from ab initio calculations. The authors conclude in this study that the stabilization a carbanion experiences through a-silyl substitution is approximately 14-20 kcalmol-1 per silyl group that of a silyl anion is approximately 6-14 kcal mol-1. The larger stabilization in the carbanionic systems is readily explained by stronger hyperconjugation of the anionic carbon center with the silyl groups as compared to that of the silyl anion with a silyl group. 

Cathodically induced silylation of unsaturated compounds such as phenylacetylene, styrene and cyclohexene has also been reported (equations 94 and 95)118119. Since chlorotrimethylsilane is reduced at a less negative potential (E1/2 = —1.95 V vs Hg pool) compared with phenylacetylene ( 1/2 = —2.05 V vs Hg pool), cathodic reduction of chlorotrimethylsilane to generate a silyl anion is possible in the presence of phenylacetylene. 

Despite the paucity of data for 9 itself, there now exists a wide range of derivatives in the cyclopropabenzene and -[Z ]naphthalene series that have been expanded upon since a 1987 accountThe preparation of these derivatives can be effected by one of three distinct routes depending upon the particular nature of the compound sought. Each has its limitations and none has provided a parent methylenecycloproparene. The first method depends upon the availability of the cycloproparenyl anion (Section IV.B) which can be intercepted by trimethylsilyl chloride to give silane 93 (R=H). In turn, deprotonation of 93 at the benzylic position affords the stabilized a-silyl anion that gives alkylidene derivatives 94 (R=H) from interaction with an appropriate carbonyl compound in a Peterson olefination (Scheme 12). The reaction sequence can be effected as a one-pot operation... 

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). ... 

To a solution of 3.8 g (11.9 mmol) of [(-)-menthyl] l-(trimethylsilyl)ethanesulfonate in 500 mL of THF is added at —78 °C 1.1 equiv of BuLi in hexane. After stirring for 10 min, the solution of the a-silyl anion is siphoned into an excess of sulfur dioxide in THF at — 78 °C. Stirring is continued for 10 min and then an excess of 2,3-dimethyl-l,3-butadiene is added at —78 °C. After warming up to r.t., the mixture is poured into a sat. aq NH4C1. The organic layer is separated, dried and concentrated m vacuo. Chromatography [silica gel, petroleum ether (60-80°C)/EtOAc] gives the pure cycloadduct as an oil yield 1.52 g (34%) de 38% (HPLC). 

As noted in Equation (46), thiasiliranide 86 was prepared through deprotonation of a silyl thiol. The reagent was prepared by sulfuration of an a-silyl anion and quenching with aqueous NH4CI <2002JOM504>. Thiastannirane 87 was prepared by a reaction analogous to Equation (52) <19930M4>. 

Another important system for which (p-d)i bonding has been assumed, but calculations deny, is the a-silyl anion, R3Si-CH2, which is unusually stable and easy to form. Current thinking is that a-carbanion stabilization is due to the high polarizability of Si and the presence of low-lying o- orbitals. 

Reactions of silylcuprates provide additional examples of a silyl anion based mechanism (Scheme 3). The reagent (19), prepared from silyllithium and copper(I) cyanide, reacts with a C=C bond to give, after aqueous work-up, c/j-hydrosilylated products (20). Conjugate addition of (21) to a,P-unsaturated... 

Peterson olefination using the a-silyl anion 449 with ketones 450 or 451 provided Tt-extended 1,3-dithioles 452 and 453 in 13% and 6% yields, respectively (Scheme 64) <2004EJ0138>. 

Silicon stabilizes an adjacent C- metal bond despite being more electropositive than hydrogen or carbon. The origins of this stabilization were discussed earlier in Section II.F. Whilst stabilization of the free anion is usually discussed, it should be remembered that the free anion is rarely involved. The stabilization of the a-silyl anionic species is illustrated by the ease and variety of methods available for generating a-silylcarbanions, namely ... 

Early transition-metal silyl compounds have received much less attention, largely due to the fact that general, straightforward synthetic routes have not been available. Syntheses based on oxidative additions are less applicable, given the more electropositive character of the early metals. Particularly for d° silyl complexes, most syntheses are based on nucleophilic displacement of halide by a silyl anion reagent, usually an alkali metal derivative. 

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