Silyl anions aryl-substituted

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. 

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 ... 

A stereoselective approach to tetrasubstituted (ii)-/ -hydroxy silyl enol ethers from aryl-substituted oxiranyl anions and acylsilanes is reported to occur via a sequential addition - [l,2]-Brook rearrangement - epoxide-opening (Scheme 87). ... 

The resulting dark green solution was converted into the expected 1-methyl-1-(trimethylsilyl)-l-silafluorene by reaction with an excess of MesSiCl. The Si NMR chemical shift for 128 (S = —22.09 ppm) is in the range of aryl-substituted silyllithium compounds, e.g. Ph2MeSi Li+, which have no delocalization of the negative charge on silicon to the phenyl substituents. Downfield shifts of the ring carbons are also consistent with a localized silyl anion. 

The field of R3C lithium organic structures is wide and we wiU concentrate on those with alkyl-, aryl- and silyl-substituted anions. Heteroatom-snbstitnted lithinm organics will just be mentioned briefly. 

Use as an Electrophile. The chlorosilane function is highly electrophilic and can react with a variety of nucleophiles, for instance with an aryllithium carbanion, to provide silyl derivatives. The (bromomethyl)chlorodimethylsilane can also be utilized as a bis-electrophilic reagent, thanks to the bromomethylene function. Initial substitution of chlorine by an aryllithium or an aryl Grignard followed by the displacement of bromine by a phenoxide anion provided valuable linkages for solid phase s)m-thesis. Based on the same idea, Martin proposed a new synthesis of unsymmetrical C-aryl glycosides. Orthometallation of a furyl moiety followed by silylation sets a diene on a silicon tether. The masked dienophile (a benzyne) is then introduced by 0-alkylation. The cycloadduct was then converted to various naphthol derivatives (eq 25). 

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