Alkyl trialkoxysilanes

Thus, to summarize the hydrolysis data, the alkyl substitutents on silicon were found to have a significant effect on the rates of aqueous hydrolysis of alkyl-trialkoxysilanes under basic and acidic conditions. The base- and acid-catalyzed rate constants were found to correlate well with modified Taft equations. The Taft coefficients, p and s, suggest a change in mechanism as the aqueous solution is varied from basic to acidic pH. Large positive. v and p under basic conditions suggest a two-step mechanism with a pentacoordinate intermediate. The smaller, positive p and s under acidic condition are consistent with an SN2-type mechanism. 

Stationary phase technology has also seen significant improvements over the past years. The silica base material is nowadays often a hybrid material, synthesized from tetraalkoxysilanes and functionalized trialkoxysilanes, for example, methyl-trimethoxysilane (MTMS). The introduction of alkyl-trialkoxysilanes into the silica backbone makes the material more resistant to hydrolytic attack and also improves their separation behavior for basic analytes.30 C18 (= octadecylsilane) stationary phases are still the materials typically used in environmental analysis, and the enormous choice of materials with gradually different properties allows columns to be selected that are particularly well suited to a given separation task.31 Reversed phase separations with materials of shorter alkylsilane chain length (C8, C4, and Cl) are less frequently used. 

Silylation of MTS surface Firstly, the wall of preformed MTS is functionalized by the covalent linkage of organic moieties carried out by silylation of the surface with alkyl trialkoxysilane in anhydrous conditions in apolar solvent (biphasic conditions). 

Fig. 16.5 The chemical reactions associated with the sol-gel process, (a) For a tetraalkoxysilane (b) For the cohydrolysis and cocondensation of tetraalkoxysilane and an alkyl-trialkoxysilane to form a hybrid class II ORMOSIL...

Diphosphine-rhodium catalysts bound to inorganic supports via isocyana-to-alkyl-trialkoxysilane linkers meet all requirements to be of practical use i. the preparation method is probably the most simple, general and efficient known so far, ii. immobilized cationic rhodium complexes with "molecular weights" as low as 5 kD exhibit a comparable performance in enantioselective catalytic... 

Tospearl is a spherical silicone resin particle made by the controlled hydrolysis and condensation of alkyl trialkoxysilanes (equation 1) and produced by Toshiba Silicones, a joint venture of GE Silicones. The first report of these materials appeared in 1985 when the aqueous amine catalyzed condensation of methyl trimethoxysilane was described (1). Since the initial disclosure, a number of other patents have issued in this area (2-8). The product is a tluee-dimensional network that is intermediate between inorganic and organic particles. This paper describes the physical and chemical properties of this network as well as its utility in a number of applications. 

This is a very diverse group of polymeric materials. Particles have mostly spherical form. Spherical silicone resin (commercial name - Tospearl) is produced in several grades, most of which are spherical (some are elongated and irregular). Controlled hydrolysis and condensation of alkyl trialkoxysilane permits tight control of properties." Particles of some grades are monodisperse, containing only a small admixture of particles of other sizes than attempted in s mthesis. 

Alkyl alkoxysilanes undergo quick hydrolysis and polycondensation in acidic medium. This has been exploited in combination with self-assembly of long-chain alkyl trialkoxysilanes driven by hydrophobic interactions that opened prospects for surfectant- and template-free synthesis of ordered mesoporous silicas. This approach has been developed strongly by Shimojima and Kuroda [24]. 

Brook has effectively modified a procedure (introduced by Hosomi) which employs a trialkoxysilane as the stoichiometric reducing agent which, in the presence of amino acid anions reduces aryl alkyl ketones or diaryl ketones to the corresponding (A)-secondary alcohols, albeit in modest ee (generally 25 40%). ... 

It is possible to modify the matrix hydrophilicity-lipophilicity balance (HLB) by using different alkyl-modified trialkoxysilanes as co-reagents with tetraethylorthosilicate (TEOS), to greatly alter (reduce) the release rate of heparin, as then water access to the entrapped drug is considerably reduced (Figure 2.10).4... 

Reduction of aryl alkyl ketones with moderate to good (ee 30-80%) entantio-selectivity has been achieved using trialkoxysilanes in the presence of chiral quininium fluorides (or hydroxides) [20]. Greater selectivities were noted (ee >65%) when tris(trimethylsiloxy)silane was used. 

Type III (no homodimerization) Acrylonitrile," protected 3° allylamines" Vinyl trialkoxysilanes, vinyl siloxanes 1,1-Disubstituted olefins, " non-bulky trisubstituted olefms, vinyl phosphonates, " vinyl phosphine oxides,phenyl vinyl sulfone, acrylonitrile, 4° allylic carbons (all alkyl substituents), protected 3° allylic alcohols, 7,Aolefm of 2-subst. 1.3- butadienes, 7,Aolefin of electronically deactivated 1.3- butadienes ... 

Base catalysis—hydrolysis. Pohl studied the hydrolysis in aqueous solutions of a series of trialkoxysilanes of R Si(OCH,CH,OCH,), structures in which R was an alkyl or a substituted alkyl group [42]. The reactions were followed using an extraction/quenching technique. Silanes were studied at concentrations ranging from 0.001 to 0.03 M and pHs adjusted from 7 to 9. The hydrolysis was found to be first order in silane. The order in water was not determined because the reactions were carried out in a large excess of water (water was the solvent). The rate constants for the hydroxide anion catalyzed hydrolysis reactions and reaction half-lives are reported in Table 1. 

Asymmetric reduction of alkyl aryl ketones with trialkoxysilanes is promoted by a catalytic amount of chiral nucleophiles [39]. The reactive species is a transiently prepared hypervalent silicon hydride. 2, 4, 6 -Trimethylacetophenone was reduced with equimolecular amounts of trimethoxysilane in the presence of the monolithio salt of (R)-BINAPHTHOL (substrate Li=20 l) in a 30 1 ether-TMEDA mixed solvent at 0 °C to afford the R product in 90% ee (Scheme 21) [40]. The presence of TMEDA was crucial to achieve high yield and enantiose-lectivity. Reduction of less hindered ketonic substrates preferentially gave the... 

Since the position of the aldehyde function on the alkyl chain will probably affect the quality of a fixation, we have studied the regioselectivity of the synthesis of aldehyde-functionalized mono- and trialkoxysilanes, using rhodium-hydrido-carbonyl-tris(triphenylphosphine) as catalyst. In general, the hydroformylation of the terminal double bond in organoalkenylsilanes results in the formation of isomeric aldehydes (Eq. 1). 

Silanes are widely recognized as efficient reagents for reduction of carbonyl and heterocarbonyl functionality. In the case of alkyl and arylsilanes, the reaction requires catalysis by Lewis acids or transition metal complexes 1, 3] however, with more Lewis acidic trichloro or trialkoxysilanes, an altemative metal free activation can be accomplished. Thus, it has been demonstrated that extracoordinate silicon hydrides, formed by the coordination of silanes to Lewis bases, such as tertiary amines 7a], DMF [7b] or MeCN, and so on [7], can serve as mild reagents for the reduction of imines to amines [8]. In the case of trichlorosilane, an inexpensive and relatively easy to handle reducing reagent, and DMF as a Lewis basic promoter, the intermediacy of hexacoordinate species has been confirmed by Si NMR spectro scopy [7b]. 

Schmidt (35) demonstrated that alkyl substituents enhance ligand electron donation. Hydrolysis is enhanced under acidic conditions, but is retarded under basic conditions, by methyl substitution of ethoxysilane derivatives. This result is important for the generation of organic-inorganic hybrid materials. When TEOS is reacted with silicon monomers containing alkyl substituents (or organic polymers with trialkoxysilane groups), the relative reaction rates just described must be controlled to produce true hybrids and avoid blocky materials. 

---