Silyl anions, silane reactions

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. 

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... 

As a freely ionic, monomeric species, the silyl anion may undergo pyramidal inversion about the silicon center (equation 1). For the parent system H3Si , Nimlos and Ellison have obtained quantitative information about the inversion barrier from the photoelectron spectrum in the gas phase2. The photoelectron spectrum could be simulated by a model of the vibrational frequency as a linear oscillator perturbed by a Gaussian barrier. The out-of-plane angle (the deviation of one H from the plane defined by Si and the other two Hs) was found to be 32 2° and the barrier to inversion 9000 2000 cm 1 (26 6 kcal mol-1). This is the only experimental measurement to date of the barrier to inversion about trivalent, negative silicon. The anion was produced by reaction of silane (SiH4) with ammonia, and the photoelectron spectrum of the m/z 31 peak was then recorded. 

The standard method for synthesis of poly silanes 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 dichlorosUanes, whUe 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... 

Summary Alkynyl-substituted oligosilanes can be easily converted to potassium silanides. Tbe reaction proceeds very fast, exceeded only by that of the phenylethynyl -substituted silanes. Linking two or three hypersilyl moieties via an ethynylene or ethynylphenylene bridge leads to multifunctional silyl anions. 

Three groups of workers have reported the use of the addition of allyltrimethyl-silyl anion to aldehydes, giving P-hydroxy-silanes, and their subsequent elimination to give dienes. Yamamoto et al. have reported the diastereoselective synthesis of the threo-o coho (209) by the boron- or aluminium-mediated reaction of (208), and of the erythro-dXcohols (210) by using the tin-mediated reaction of (208). A similar approach, using the titanium-catalysed addition... 

Recently the unprecedented example of stereoselective C—Si bond activation in cu-silyl-substituted alkane nitriles by bare CQ+ cations has been reported by Hornung and coworkers72b. Very little is known of the gas-phase reactions of anionic metal complexes with silanes. In fact there seems to be only one such study which has been carried out by McDonald and coworkers73. In this work the reaction of the metal-carbonyl anions Fe(CO) (n = 2, 3) and Mn(CO) (n = 3, 4) with trimethylsilane and SiH have been examined. The reactions of Fe(CO)3 and Mn(CO)4 anions exclusively formed the corresponding adduct ions via an oxidative insertion into the Si—H bonds of the silanes. The 13- and 14-electron ions Fc(CO)2 and Mn(CO)3 were observed to form dehydrogenation products (CO) M(jj2 — CH2 = SiMe2) besides simple adduct formation with trimethylsilane. The reaction of these metal carbonyl anions with SiFLj afforded the dehydrogenation products (CO)2Fe(H)(SiII) and (CO)3Mn(II)(SiII). ... 

Fluoride ion-catalysed addition of trifluoromethyltrimethylsilane to acyl silanes occurs to give l,l-difluoro-2-trimethylsilyloxyalkenes (silyl enol ethers of difluoromethyl ketones), through nucleophilic addition of trifluoromethyl anion, Brook rearrangement and loss of fluoride. These compounds could be isolated when tetrabutylammonium difluorotriphenylstannate was used as a catalyst use of tetrabutylammonium fluoride gave the product corresponding to subsequent aldol reaction with the difluoromethyl ketone (Scheme 78)m. 

In contrast to substitution reactions of lithium salts of OMCTS with fluoroboranes, -phosphanes and -silanes no mono- or disubstituted OMCTS with a triorganylsilyl group is known. According to chemical experiments, the anion of the OMCTS contracts in reactions with Me3SiCl and forms a silyl-substituted six-membered-ring isomer 20,31,56 58,60 (Scheme 29). 

The (dienyl)iron cations of type (248) and (265) are susceptible to reaction with nucleophiles. For the (cyclohexadienyl)iron cations, nucleophilic attack always occurs at a terminal carbon, on the face of the ligand opposite to the metal, to afford / -cyclohexadiene products. Typical nucleophiles used are malonate anions, amines, electron-rich aromatics, silyl ketene acetals, enamines, hydrides, and aUyl silanes intramolecular nucleophilic addition is also possible. The addition of highly basic organometaUic nucleophiles (Grignard reagents, organolithiums) is often problematic this may be overcome by replacing one of the iron carbonyl... 

Functionally substituted benzylic, allylic, and vinylic compounds containing alkoxides, esters, ethers, nitriles, or amides can be reacted with halosilanes under Barbier conditions using HMPT to yield C- and O-silylated products, 1,2- or 1,4-addition products, as well as reductive dimers. Radical and anionic intermediates are postulated, based on SET reactions from the metal, and multiple silated species can be obtained. The use of the TMSCl-Mg-HMPT system has been extensively investigated by Galas group [85] at the University of Bordeaux, and their work has greatly advanced the science of the Barbier reaction with silanes. 

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