Silyl anions preparation

A disadvantage is that solutions of silyl anions prepared from the halosilanes by one of these methods inevitably contain metal halides as by-product. Salt-free perarylated silyl anions can be prepared by the reaction of a disilane with lithium, sodium or potassium in polar solvents such as THF or DME (Table i)28b,29 Alternative procedures for the preparation of halide-free Ph3SiK utilize the reductive cleavage of the Si—C bond of Ph3Si—CMe2Ph by a sodium-potassium alloy in Et20 (equation 17), as well as via the reaction of PhsSiH with KH in DME . 

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... 

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... 

Silyl radicals have been produced by one-electron oxidation of silyl metals [11]. This is found to be the method of choice for the generation of persistent silyl radicals and allowed the preparation of the first isolable silyl radical (see later in this chapter). Reactions (1.5) and (1.6) show two sterically hindered silyl anions with Na+ as the counter-cation, and their oxidation by the nitrosyl cation [12] and the complex GeCh/dioxane [13], respectively. 

The efficiency of the amination methodology of enolates with 0-phoshinylhydroxyl-amine-type reagents was further demonstrated by Boche and Schrodt in the high-yield amination of 0-(trimethylsilyl)cyanohydrin anions with 4c (Scheme 44)". 0-(Trimethyl-silyl)cyanohydrins prepared by treatment of aryl, hetaryl and conjugated aldehydes with trimethylsilyl cyanide were lithiated to the eniminates. This is a practical method that enables the easy conversion of aldehydes to iV,Ai-dimethylamides under mild conditions. 

IV. ALKYL-SUBSTITUTED SILYL ANIONS A. Methods of Preparation... 

The procedure is successful with ketene silyl acetals prepared from the enolate anions of methyl 2-methylpropanoate and methylpropanoate, and provides good yields of the propionic esters 7 in a single step (Scheme 10.20). The latter are known to be intermediates in the preparation of analgesic compounds. No significant difference in the yields is observed when 4-chloroaniline is used as the starting material in place of its dimethylated analogue [31]. 

Few structural data are available on type (B) silyl anions. The SiH3 anion may be prepared from SiH4 and the alkali metals in the solvents CH3OCH2CH2OCH3 or OP[N(CH3)2]3. While the vibrational spectrum of SiH3 has not yet been obtained, the nmr in CH3OCH2CH2OCH3 solutions has been reported 108). 

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... 

The standard method for synthesis of polysilanes follows the original preparation of (Me2Si) by Burkhard. Diorganodichlorosilanes are treated with finely dispersed sodium metal in an inert diluent, usually above the melting point of sodium. Homopolymers are obtained from single dichlorosilanes, while cocondensation of mixtures of dichlorosilanes yields copolymers (equation 33). Toluene is the most commonly used solvent, but other aliphatic or aromatic solvents are also effective. After completion of the reaction, the mixture is quenched with alcohol and/or water to destroy any excess sodium and silyl anion, then filtered and... 

Preparation of phenyl-substituted silyl anion is a simple process either from chlorosilanes or disilanes (equation 15). Trimethylsilyl anions can he prepared from hexamethyldisi-lane by reaction with RM (RM=MeONa, MeLi ) in a strongly basic solvent such as HMPA (hexamethyl phosphoramide). 

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>. 

In this section, silyl anions are classified in terms of counter-cations, and their preparative methods are summarized. 

Silyllithiums are the most popular and most extensively investigated species among the silyl anions, and can be prepared by the following methods ... 

An (alkoxysilyl)lithium, ( -BuO)Ph2SiLi, can be prepared from (t-BuO)Ph2SiSnMe3 by treatment with n-BuLi in THF (Scheme 10). This species is stable at -78°C for a few hours and behaves as an ordinary nucleophilic silyl anion 54d). (f-BuO)Ph2SiLi also behaves as an electrophile under certain conditions It undergoes self-condensation smoothly or butylation in the presence of an excess amount of n-BuLi and TMEDA (Scheme 10). (t-BuO)Ph2SiLi thus exhibits an ambiphilic nature, that is, silylenoid character. The ambiphilic reactivity of (t-BuO)Ph2SiLi, how-... 

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... 

Some years ago Hengge et al. published a feasible synthesis of KSi6Men (1) and did basic research on the reactivity of this silyl anion including the preparation of transition metal complexes [1-3]. Our group contributed two crystal structures of such compounds [4, 5]. In an effort to get further insight into the reactivity of 1 and the structures of its transition metal conq>lexes, we reacted it with various transition metal conq>ounds. 

Since the preparation of anionic species or organosilicon compounds is largely restricted by its reaction conditions, the synthetic utility of silyl anions for the synthesis of organosilicon compounds is limited. However silyllithiums possessing one or more phenyl groups on a silicon atom are readily prepared by the reductive metallation of the corresponding halosilanes with lithium. Allylsilanes are synthesized by the reaction of allyl acetates with the cuprate prepared from such a reagent (eq (47)) [43]. 

Reactions of methyllithium or methyl Grignard reagents resulted in the formation of optically active silyl anions which were characterized after hydrolysis and reaction with allyl bromide (243). The observed overall predominant retention of configuration provides evidence for the optical stability of silyl anions. Their first preparation was reported by Sommer and Mason (245) and they appeared configurationally less stable than the chiral germyl anions (31). 

Use of main-group metal silyls to prepare transition-metal silyls appears to be a generally applicable method that is primarily limited by the availability of suitable starting materials, since these silyl anion reagents are sometimes rather difficult to obtain. Typically an alkali-metal silyl is generated in solution and then treated with the appropriate transition-metal halide. Displacement of halide by the silyl anion, with salt elimination, then leads to product (equations 17-19)46 49. The lithium silyl in equation... 

Two reports describe the use of alkali metal silyls to prepare anionic silyl complexes via displacement of neutral ligands (equations 23 and 24, COD= 1,5-cyclooctadiene)57,58. 

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