Hypervalent silanes

While the last elass of eomplexes considered in this section, the compounds 145, closely resemble the usual silane a-complexes, other multicenter H Si interactions discussed above have spectroscopic and structural features common to both the IHI and a-complexes. This enigmatic situation can be explained well by the structure 132 in terms of a a-coordination of the Si-H bonds of the hypervalent ligand (H +iSiX3)" 1) to metal, which thus includes both the hypervalent interaction of the silicon with the hydride atoms and the a-complexation of the Si-H bonds to metals. The key features of complexes with multicenter H Si interactions are summarized in Table VIII, where a comparison with the IHI and the residual H-Si interactions in silane a-complexes is given. 

It is obvious that studying interligand Si-H interactions has reached a great extent of sophistication. At least three classes of nonclassical Si-H bonding can be identified. These are the electron-deficient residual Si H interactions in silane a-complexes and agostic complexes, electron-rich IHI MH SiX, and the more recent multicenter H Si interactions, which are the subject of current debate and have features common to both IHI and a-complexes. This surprising diversity stems from the special role the substituent at silicon can play in tuning the extent of Si H interaction, and from the propensity of silicon to be hypervalent. 

Reaction of allylic silanes with aldehydes and ketones can also be induced by fluoride ion, which is usually supplied by the THF-soluble salt tetrabutylammonium fluoride (TBAF). Fluoride adds at silicon to form a hypervalent anion with much enhanced nucleophilicity.73 An alternative reagent to TBAF is tetrabutylammonium triphenyldi-fluorosilicate.74... 

The kinetics of nucleophilic substitution at the silicon atom assisted by uncharged nucleophiles have been studied by Corriu et at. (248-251). Hydrolysis of triorganochlorosilanes induced with HMPA, DMSO, and DMF was used as the model. The reaction proceeded according to the third-order kinetic law, first order with respect to the nucleophile, the silane, and the silylation substrate. Very low values of activation enthalpy and high negative entropy of activation were observed (Table VI). These results were taken as evidence for the intermediacy of silicon hypervalent species (249,251) however, they are also perfectly consistent with... 

Summary BusSnH is an effective reagent for partial conversion of Si-Cl into Si-H groups. The hydrogenation mechanism postulates the coordination of the catalyst or the solvent to silicon giving a hypervalent intermediate in the first step, followed by the attack of tributyltin hydride by a single electron transfer or a synchronous hydride transfer. This mechanism implies that the intermediate containing a hypervalent silicon atom reacts faster than the starting tetracoordinate silane. 

The single NMR resonance moved to low frequency with each addition of HMPA, and finally remained as a singlet at 8 = -78 ppm at all ratios of HMPA to silane of 3 1 or greater. The only reasonable structure for this new species is 5. These complex changes for a relatively simple system illustrate the subtle relationship between coordination and reactivity for silicon. It was observations such as these and others that stimulated us to try to make quantitative measures on hypervalent silicon compounds. 

Hypervalent silicon componnds have found wide utility in organic synthesis. In general, pentacoordinated anionic silicates are more reactive toward nucleophiles than are tetracoordinated silanes. For example, Mes2SiF2 is nmeactive toward water, while (the 18-crown-6 potassium salt of) Mes2SiF3 is completely hydrolyzed within minutes. Similarly, the pentacoordinate anion HSi(OEt)4 is an effective reducing agent for aldehydes, ketones, and esters at or below room temperature (Scheme 2) no snch reaction occurs with HSi(OEt)3. The difference in relative reactivities of hypervalent and nonhypervalent species is relevant to the intermediates proposed in Section 7.6. 

Coupling of organosilane derivatives to RX species is known as the Hiyama reaction. This process is seen as more difficult to catalyze than the Stille reaction since a C-Si bond is less reactive than a C-Sn bond. However, addition of additives, such as B114NF or CsF, that form hypervalent silicon intermediates, or the use of more reactive silane precursors, such as Me3SiSiMc3, ArSiMc20H, or Cl2MeSi(vmyl), can overcome this limitation (equation 29). 

A completely different approach has been taken in the use of hypervalent silicon hydrides. It has been shown that pentacoordinated silanes possess enhanced reducing power. Thus, in the presence of a nucleophile, which may be external (reaction (a) in Scheme 2) or internal (reaction (b) in Scheme 2), hy-... 

In an extension of the high temperature reaction of hypervalent silanes with carboxylic acids to produce aldehydes (see Section 1.11.2), it was found that acyl chlorides reacted at room temperature with... 

The first step of the overall reaction is the attack of the Lewis base catalyst at the silicon atom forming a hypervalent silicon atom and stretching the Si-Cl bond. In a second step this species reacts with the stannane to give the hydrogenated silane and trialkylchlorostannane ... 

Keywords Hypervalent Silanes / Heterobutadienes / Oligomers / Polymers... 

Summary The synthesis of the pentacoordinated silane (2-Me2NCH2C6Hi)-(CH=CH2)Si(H)2 (1) is described. A comparison of the chemical behavior of hypervalent 1 with tetravalent silicon compounds is carried out. 

Silicon compounds with coordination number larger than four are the object of many studies first with respect to their application as catalysts in organic and inorganic syntheses and second as starting materials for the preparation of a broad variety of organosilicon compounds [1]. Additionally, hypervalent silicon hydride compounds can successfully be used as model compounds to study, for instance, the mechanism of nucleophilic substitution reactions, which is of great interest since the silicon atom is able to easily extend its coordination number [1]. Moreover, hypervalent silanes are suitable as starting materials for the synthesis and stabilization of low-valent silanediyl transition metal complexes [2-5]. 

The hypervalent silane 1 can also be used to prepare donor-stabilized low-coordinated silicon compounds, such as l-thia-2-sila-l,3-dienes or l-metalla-2-sila-l,3-dienes [3, 6b, 9]... 

On treatment of the vinyl-fiinctionalized hypervalent silane (2-Me2NCH2C6H4)(CH=CH2)Si(H)2 (1) with sulfur in chloroform at 25 °C the formation of the l-thia-2-sila-1,3-diene 9 is observed. It appears that 9 is extremely sensitive to moisture By elimination of H2S oligomeric 3 is formed [6]. 

In addition to the head-to-tail hydrosilylation described above we could show that monomeric vinyl-functionalized hypervalent silanes can be used for the preparation of polycarbosilanes containing silicon atoms in a trigonal-bipyramidal environment [7b, 8] Treatment of... 

The mechanism of the reactions of the other types of silanes and stannanes to be discussed in this chapter has hardly been studied, but the general outline given in the introduction is likely to be an adequate framework for thinking about these reactions. Fluoride-catalyzed reactions of vinylsilanes only work when there are anion-stabilizing groups present, which implies that there is substantial anionic character on carbon in the intermediate nucleophile, but this does not identify it as a carbanion it could still be a hypervalent silyl species. 

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