Tetraalkoxysilane Si

Effect of the Structure of Silicon Sources. Hoebbel et al. used silicic acid sols, silicic acid gels, or Aerosil as a silica source of tetramethylammonium silicate aqueous solutions (9). In the solutions at the conditions that a N/Si ratio is 1.0 and Si02 concentration is ca. 1.4 mol dm-, the distributions of silicate anions are almost the same, and the cubic octamer is a dominant species, although the degradation rates of the silica sources are different. This suggests that the cubic octamer is formed in the tetramethylammonium silicate aqueous solution, regardless of the type of silica source with tetra-functionality. Tetraalkoxysilanes (Si(0R), R denotes an alkyl group) can be used as a silica source as well (4,12,14). 

Silica is an attractive material for inorganic gel applications because it is non-toxic, non-flammable and easy to dispose of. Dry silica gels have a translucent, bluish appearance. For gel synthesis at lab-scale, a tetraalkoxysilane Si(OR)4, commonly tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS), dissolved in the respective alcohol ROH, acts as the silica source. When water is added, two chemical reactions occur the first, hydrolysis. 

CsF in the presence ol tetraalkoxysilanes also effects Michael addition of ketones lo a,/ -unsaturaied ketones, esters, and nitriles. Presumably the enolate is generated and is converted by Si(OR)a into the silyl enol ether, which reacts in situ.2 Examples ... 

Since the early studies with tetrahydrofurans, new substrates have been explored that are capable of highly diastereoselective reactions. trans-Allyl si-lyl ethers are exceptional substrates as illustrated by the conversion of 23 to 24 [Eq. (11)] [24], The reactions are highly diastereoselective (96-98% de) and in many respects outperform the standard aldol reaction. Even better substrates are tetraalkoxysilanes as shown for the reaction of tetraethoxysilane (25) [25]. In this case, both the enantioselectivity and the diastereoselectivity for the formation of C-H activation product 26 are >90% de and ee. 

An alternative method to synthesize organically functionalized mesoporous silica phases is the co-condensation method (also called one-pot synthesis or direct synthesis). Historically, the classical co-condensation reaction involved tetraalkoxysilane [(RO Si (predominantly TEOS or TMOS)] and a terminal organotrialkoxysilane of the type (R 0)3Si-R in the presence of structure-directing agents (Fig. 3.9, top). Meanwhile, not only bis- and multi-silylated precursors are used, that is, precursors of the type [(R 0)3Si]mR, m > 2 (Fig. 3.9, bottom), together with TMOS/TEOS, but... 

The ion chemistry of tetraalkoxysilanes is relevant to plasma processes. FT-ICR studies have shown a drive towards the growth of -Si-O-Si- units71. For example, the reaction of Si(OMe)3+, the major fragment ion from the El ionization of (MeO)4Si, leads to an adduct ion and to an addition-elimination product which undergoes further addition of (MeO)4Si (equation 27)71b 72. 

Silane reacts with methanol to give the alkoxysUanes (MeO) SiH4 (n = 2, 3, 4) and with higher alcohols in the presence ofbase to give tetraalkoxysilanes (equation 8). This type of reaction is, however, rarely carried out as the alkoxysi-lanes produced are more conveniently prepared from alcohols and SiCU (which is more readily available than SiELi and much more easily handled). Similarly, silyl esters, Si(OCOR)4, are also usually prepared from polyhalogenosUanes rather than from silane and a carboxylic acid. 

Silane reacts with methanol at room temperature to produce methoxymonosilanes such as Si(OCH2)4 [78-10A]y HSi(OCH2)3, and H2Si(OCH3)2 [5314-52-3] but not H SiOCH [2171 -96-2] (23). The reaction is catalyzed by copper metal. In the presence of alkoxide ions, SiH reacts with various alcohols, except CH3OH, to produce tetraalkoxysilanes and hydrogen (24). 

The usefulness of retention index versus temperature plots for the evaluation of the optimum column temperature has been confirmed422 when analysing mixtures of compounds with similar boiling points. For example, plots for the tetraalkoxysilanes (MeO) Si(OEt)4 give straight lines that converge towards higher temperatures. Hence, the best separation will be obtained at the lowest possible column temperature. 

A linear relationship exists between the logarithm of the retention volume and the carbon number for tetraalkylsilanes belonging to the series R4Si, R3SiR3 and SiR4362. For the corresponding series of tetraalkoxysilanes, an approximately linear relationship between retention index and carbon number will exist only for the series (PrO)MSi(OBu)4 M, but not for the series (MeO) Si(OBu)4 . 

However, Si-OR bonds can also be involved in a Wurtz-Fittig reaction for instance, treating tetraalkoxysilanes with tert-butyl chloride and sodium affords alkoxy-te/7-butylsilane.316... 

The preparations and performance of molecular catalysts successfully heterogenized by physical entrapment for a wide range of reactions are reviewed in detail in [86,93]. Two synthetic protocols were used, depending on the chemical properties of entrapped molecules addition of dopant solution to prehydrolyzed Si(OR)4 at pH close to 7, or together with ammonium hydroxide to tetraalkoxysilane precondensed in acidic solution. Both, followed by gelation. 

Compounds of the type Si(OR)4 are termed tetraalkoxysilanes. Consequently information in the specialist literature about orthosilicate esters is to be found under this heading [90]. 

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