Alkoxysilanes determination

The exact mass determination of emitted secondary ions yields informations about the molecular structure of the thick film. In the low mass area the fragmentation of the quasimolecular cation m/e = 163, i.e. (EtO)3Si+ derived from the alkoxysilane monomer gives rise to the positive ions m/e = 135, 119, 107, 91, 79, 63. The resulting fragmentation pattern can be described as follows ... 

It is important to note that catalysts for alkoxysilane hydrolysis are usually catalysts for condensation. In typical silane surface treatment applications, alkoxysilane reaction products are removed from equilibrium by phase separation and deposition of condensation products. The overall complexity of hydrolysis and condensation has not allowed simultaneous determination of the kinetics of silanol formation and reaction. Equilibrium data for silanol formation and condensation, until now, have not been reported. 

Equilibrium constant determination for alkoxysilane hydrolysis. Triethyl-silanol was selected as a model compound for determination of the equilibrium constant for equation (1), since under neutral conditions the condensation to disiloxane was observed to take place only over an extended period of time (i.e. years), eliminating equilibria (2) and (3) as interfering factors. 

New data have been presented in the context of a review of the aqueous behavior of silanes which elucidate their behavior, including mixed alkoxysilane hydrolysis kinetics, silane solubility, and the determination of the equilibrium constant for the alkoxy hydrolysis reaction. 

The hydrolysis of p-trifluoromethylphenyldimethylethoxysilane was carried out at four different acid concentrations and the rate constants were calculated (Table 8). If the reaction is first order in acid, the rate constant should not change as shown in Table 8. Each of the alkoxysilanes was hydrolyzed with a ten-fold excess of water. If the rate equation is correct, water does not react until after the rate determining step and should not affect the rate constants (Table 9).- Again, this is the case. If the mechanism proceeded through a pentavalent state such as Smith suggested, we would expect to see water involved in the rate equation. At this point, Jada s mechanism seems more likely. 

Silicon tethers based on bis-alkoxysilanes (silyl ketals) are commonly prepared from the dichlorosilanes by reaction with an alcohol in the presence of base. These conditions are not compatible with some base labile compounds. To make unsymmetrical bis-alkoxysilanes requires a method for breaking the symmetry of the dichlorosilane. Without such a method, one must accept a statistically determined mixture of mono-alkoxy and bis-alkoxy products. This may be acceptable for inexpensive readily available alcohols, but it precludes the use of bis-alkoxysilane tethers for high-value synthetic intermediates. To overcome these limitations to... 

Oxidative addition of the silane to the metal is fast and reversible 30 therefore unless the pentacoordinated silane drastically slows down the oxidative addition process, pentacoordination will not alter the rate of the reaction at this stage of the cycle. The increased reactivity of le may be explained by the attack of the alcohol on the pentacoordinated silane that would form after oxidative addition (Figure 9A). The rate of the alcohol addition is increased by the higher reactivity of the pentacoordinated silicon center. This may explain the slower reactivity for those alkoxysilanes that cannot form this intramolecular coordination complex due to the absence of a nearby Lewis basic atom. We had observed during the comparison of aliphatic alcohol to benzyl alcohol that the nucleophilicity of the alcohols has an effect on the rate of the reaction. This is evidence that the alcohol and the silane are involved in the rate-determining step with 10 % Pd/C catalytic system. 

From these results we concluded that the rate determining steps for both catalytic systems involves the alcohol and the mono-alkoxysilane. This rate is influenced by both the nucleophilicity of the alcohol and the reactivity of the silane. 

The equilibrium constants K corresponding to Eq. 3 have been determined in dioxane solution starting with definite concentrations of silanol or alkoxysilane, alcohol, and water and measuring the equilibrium concentrations of silanol and alkoxysilane by GC. The results given in Table 1 show that K is within the order of 0.1. This means that the silanol alcoholysis is, especially at an excess of alcohol, a reaction not to be neglected in the system mentioned above. 

As expected, octasilacubane possesses high reactivity toward electrophiles. As shown in Scheme 3, mCPBA oxidation of silyl-substituted 1 resulted in a hi -yield formation of octasilsesquioxane (3) (Tg). Silsesquioxanes are well studied, but were previously only prepared by the hydrolysis-dehydration of halo- or alkoxysilanes. When the substituents are bulky, the reaction proceeds no further than the silanol stage, and silsesquioxanes are not obtained. For example, hydrolytic condensation of t-butyldimethylsilyltrichlorosilane gave only partly hydrolyzed silanols. Thus, 3 is the octasilsesquioxane with the bulkiest substituents, and the only silyl-substituted one ever reported. The structure was determined by X-ray ciystallography [7], and the bond lengths and angles are similar to those of known octasilsesquioxanes. 

In this study we describe a meftiod used to measure the reactions that silylated latexes can undergo in coatings formulations. Si NMR is shown to reveal crosslinking by silanol condensation occurring in silylated latex synthesis and cured films. In addition, a complementary method for quantitative determination of degree of alkoxysilane hydrolysis is described. Low-temperature separation of the latex solids from the volatile components followed by gas chromatographic analysis of the distillate can provide accurate and reproducible measurement of the alcohol generated by the hydrolysis of the alkoxysilanes used in the formulation. 

Different n-alkoxysilanes with increasing hydrophobicity were employed as silica precursors. The porosity parameters as determined by nitrogen sorption measurements are summarised in table 2. 

General SOG Characteristics. Both the SOG products are a solution of hydrolyzed alkoxysilanes in common organic solvents such as alcohols and ketones. The solutions are filtered through 0.2 micron membrane filters and do not form particles or crystals on standing for long periods of time or during the spln-on process. Trace metal contamination levels of the SOGs were determined by atomic absorption spectrophotometry to be well below 0.5 ppm for K, Fe, Cr, Cu, Ni, and Mn and less than 0.1 ppm for Na. 

Ultimately, it was determined that protected allyl alcohols, tetralkoxy silanes, and silyl ethers were the best substrates for these transformations. Alkyl or allyl silyl ethers (120) provided products such as 121 in >85% de and good 74-92% ee with Rh2(S-DOSP)4 as the catalyst (Scheme 27) [98], Extension of the reaction to alkoxysilanes, generated 122 in 88 to >94% de and 81-96% ee. Interestingly, this catalyst did not seem to be compatible with benzyl silyl ethers, and the products were formed with low selectivity (<40% ee). It was found that the use of either a chiral auxiliary or phthalimide Rh2(PTTL)4 resulted in improved results [31]. 

Asymmetric induction at the silicon atom of transient prochiral silaethylenes has been observed recently (67). As shown in Scheme 10, prochiral silaethylenes were generated by photolysis of substituted silacyclobutanes. Trapping the shortlived silaethylenes with a chiral alcohol resulted in the formation of unequal amounts of diastereomeric alkoxysilanes. The diastereomer ratio was determined by NMR. 

Chirality can be introduced by use of an asymmetric alcohol and/or an asymmetric catalyst. The optical yield and predominant configuration around silicon were determined by conversion of the diastereomeric alkoxysilanes to a trisub-stituted silane of known configuration and maximum specific rotation. Reaction of Grignard reagents to this end is both quantitative and stereospecific. 

Enantiomeric excess, determined after conversion of the alkoxysilanes to a trisubstituted silane. 

S = solvent. Ligands DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane47 NMDPP = neomenthyldiphenylphosphine MDPP = menthyldiphenylphosphine48. b Enantiomeric excess, determined after conversion of the alkoxysilane to a trisubstituted silane. 

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