Metal silyl anions

Basically the same methods known from the synthesis of classical metal-silyl complexes can also be applied to the preparation of low valent Si compounds. The procedures given here are summarized with the focus on silylene complexes These are a) reactions of appropriate metal anions with halosilanes, which are the most important methods for the formation of M-Si bonds. Alternatively, silyl... 

Reactions of Silyl Anions with Metal Halides... 

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]. 

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... 

Anionic Transition Metal Silyl Complexes New Building Blocks in Coordination Chemistry... 

The currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

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 chemistry of metalated organosilicon compounds has been the subject of several reviews1, the most recent ones by Lickiss and Smith13 and Tamao and Kawachilb, which cover the literature up to the year 1994. This chapter will now take into account the developments in the chemistry of metalated silanes up to the middle of 1996 however, for completeness there will be some overlap with former reviews. The emphasis of this review is on the synthesis and structure of these metalated silanes. However, some examples of their utilization for synthetic purposes will also be given where appropriate. For more information about synthetic applications of silyl anions the reader is referred to some leading references in this fieldla,b h k. 

Another point which needs to be clarified from the start is the nomenclature of metalated silanes We will frequently use the term silyl anion in this chapter when we talk about metalated silanes. Although the term anion defines, literally taken, an ionic compound, this expression, when used by us, does not necessarily imply that the compound in question is of ionic nature, but covers, as well, in analogy to the use of the term carbanion , silicon compounds with a polarized covalent silicon-metal bond. 

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. 

IR, UV and, most importantly, NMR spectroscopy have proven to be excellent tools for elucidation of the molecular and electronic structures of silyl anions. Whereas 13C NMR data of metalated alkylsilanes are quite rare, the wealth of 13C NMR data which are available for metalated arylsilanes allows one to draw important conclusions concerning the question of charge delocalization in these compounds (Table 2)36,47. 

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... 

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). ... 

While silyl metal carbonyl derivatives are unreactive towards neutral metal carbonyls (7), metal carbonyl anions give rise to various displacement processes (entries 1-3). The order of displacing ability seems generally similar to that noted earlier for nucleophilicities (Section II, A,1) ... 

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). 

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