Tris silyl group

Monosubstitution of acetylene itself is not easy. Therefore, trimethylsilyl-acetylene (297)[ 202-206] is used as a protected acetylene. The coupling reaction of trimethylsilylacetylene (297) proceeds most efficiently in piperidine as a solvent[207]. After the coupling, the silyl group is removed by treatment with fluoride anion. Hexabromobenzene undergoes complete hexasubstitution with trimethylsilylacetylene to form hexaethynylbenzene (298) after desilylation in total yield of 28% for the six reactions[208,209]. The product was converted into tris(benzocyclobutadieno)benzene (299). Similarly, hexabutadiynylben-zene was prepared[210j. 

There are, however, two disadvantages associated with use of the phenyldimethylsilyl group. Based on the reaction stoichiometry, for each equivalent of substrate, one silyl group is unused, and after work-up this appears as a relatively involatile by-product. Secondly, after synthetic use of such vinylsilanes involving desilylation, a similar problem of by-product formation arises. One solution to these problems lies in the use of the tri-methylsilyl group (Chapter 8), since the by-product, hexamethyldisiloxane, is volatile and normally disappears on work-up. 

It has been reported that (TMS)3SiCl can be used for the protection of primary and secondary alcohols [55]. Tris(trimethylsilyl)silyl ethers are stable to the usual conditions employed in organic synthesis for the deprotection of other silyl groups and can be deprotected using photolysis at 254 nm, in yields ranging from 62 to 95%. Combining this fact with the hydrosilylation of ketones and aldehydes, a radical pathway can be drawn, which is formally equivalent to the ionic reduction of carbonyl moieties to the corresponding alcohols. 

Recently, density functional calculations (B3LYP) on the thermal rearrangement of tris(silyl)hydroxylamines to silylamino disiloxane for model compounds concluded that the insertion of a silyl group into the N—O bond is energetically favoured if it occurs from the nitrogen atom. ... 

The reaction of (alkyl)chlorosilanes with a silica surface has been discussed and reviewed in great detail in literature [10], Although 5 different reactions are possible with di-, tri- or tetrachlorosilanes, basically two important surface species are created. The first is a monodentate silyl group, created by the monomolecular reaction of 1 silanol with 1 chlorosilane, according to reaction (A) (cfr. Figure 2). The second surface specie is a bidentate silyl group, created either by a bimolecular reaction (B) or by a consecutive reaction (C). We have reported previously [11] that the surface of MCM-48, prepared by the gemini 16-12-16 surfactant, possesses 0.9 OH/nm2. 

The anodic oxidation of silyl-substituted tetrahedranes was studied by cyclic voltammetry5. Tri-f-butyl(trimethylsilyl)tetrahedrane is oxidized more easily compared with tetra-t-butyltetrahedrane owing to the cr-donating silyl group (equation 2). 

If a carbocation or a dication at the same time is also a Hiickeloid An + 2)jt aromatic system, resonance can result in substantial stabilization. The simplest 2jt aromatic system is the Breslow s cyclopropenium ion 206.434 439 Recently, electronic and infrared spectra of the parent ion cyclo-C3H3+ (206, R = H) in neon matrices440 and the X-ray characterization of the tris(trimethylsilyl) derivative were reported.441 The destabilizing effect of the silyl groups was found to be significantly smaller than in vinyl cations. The ion was computed to be more stable than the parent cyclopropenium ion by 31.4 kcal mol1 [MP3(fc)/6-31 lG //6-31G +ZPVE level]. The alkynylcy-clopropenylium ions 207 have been reported recently.442... 

Treatment of silyl ethers of sucrose with the Vilsmeier reagent gave various sucrose formates. When only one silyl protecting group was present, the reaction afforded the corresponding monoesters in high yields. However, when di- or tri-silylated sucroses were used as substrates, a mixture of mono- and di-formates were obtained (Scheme 26).301... 

The high formal oxidation states of metals in some of these adducts is noteworthy, e.g., Fe(IV) (entries 17 and 18), Ru(IV) (entries 21 and 22), and Pt(IV) (entries 55 and 56). Such adducts are important because they provide definite examples of species often postulated as intermediates in oxidative addition-reductive elimination processes (compare Section II,G,1) and in homogeneous catalysis (134,220a, 410a). In the case of germanium, a tris(germyl) adduct of Pt(IV) has been described (57), but no more than two silyl groups per metal atom are known to result from oxidative addition. 

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