Silanes, structure

Jacobsen, Panek and co-workers (86) investigated the intermolecular Si-H bond insertion of diazoesters. Bis(oxazolines) and diimines were found to be effective in this reaction, with diimine enf-88a providing optimal selectivities. As expected, enantioselectivity is a function of silane structure, with bulkier silanes providing higher selectivities but lower reactivity. Both CuOTf and Cu(OTf)2 catalyze this reaction but the Cu(II) precursors leads to much lower enantioselectivity (44% vs 83% at -40°C). 

Spontaneous reaction rates—hydrolysis. Although the spontaneous hydrolysis of alkoxysilanes has been reported, there are insufficient data to develop an understanding of how structure impacts on rate or to give insights into the reaction mechanisms [35-37, 40]. The spontaneous hydrolysis usually contributes only a small amount to the observed rate of hydrolysis under most conditions. In many instances, the contribution of the spontaneous hydrolysis is so small that it is not detectable [25, 39, 42, 43]. The impact of the silane structure on the spontaneous hydrolysis of alkoxysilanes will therefore not be discussed. 

Acid catalysis—hydrolysis. Several series of alkylsilane esters were studied to determine the effect of silane structure on the hydronium ion catalyzed hydrolysis reaction. The hydronium ion catalyzed hydrolysis rate constants for a series of alkyl tris-(2-methoxyethoxy)silanes in aqueous solution were used to define the modified Taft equation log(A /Ah ) = 0.39a + 1.06ES, where Ho is the rate of hydrolysis for methyl tris-2-(methoxyethoxy)silane [42], The hydronium ion catalyzed hydrolysis rate constants and the reaction half-lives are reported in Table 2. In a similar manner, the hydronium ion catalyzed hydrolysis rate constants for a series of trialkylalkoxysilanes in 55% aqueous acetone were used to obtain the modified Taft equation log(/cH//cHo) = -0.37 a + 2.48 E where kHo is the rate of hydrolysis for trimethylalkoxy-silane. 

On industrial scale, water or water/ethanol mixtures are mostly used as a solvent for aminosilane modification of silica or glass fibres. Water has a profound effect on the silane structure in solution. Alkoxy groups are hydrolyzed to silanols. Those can combine to form a siloxane linkage between two silane molecules, with production of a new water molecule (reaction (A)). 

A suggestion of the poly silane structure based on NMR and mass balance data is depicted in Fig. 4. 

At neutral pH, high molecular weight silane structures are obtained which provide more protection against chemical and water attack at the interface, but they also reduce interpenetration of resin into the silane layers (a similar effect was discussed above for calcium carbonate coating density). 

Conversely, highly dynamic hydrido( /2-silane) structures are found for [IrH2(i/2-HSiEt3)2(PPh3)2]+55 and TpRuHO/2-HSiR3XPPh3)20 which will be further dis-... 

Keywords Silanes / Structures / Silanides / Silylenes / Disilenes... 

Even though the final performances of the cured rubber compounds are very important, the process stability is also of great importance in manufacturing and cannot be neglected when choosing the silane structure. Figure 13 shows strain/modulus curves of three silica-filled rubber compounds using TESPT, TESPD, and mercaptopropyitriethoxysilane, respectively. 

By using t i, which is determined from the crystal growth kinetics, one can eliminate the use of I, an aforementioned difficult quantity to measure. At the present time, no quantitative study of the effect of silane structures on the transcrystal formation has been reported. 

Although the rheology of silane-treated filled systems has been reported (36-38X few reports deal with the silane structure/rheology correlation. 

Silanes as metal pre-treatments W J VAN OOIJ Silanes, structure and effectiveness... 

Silane hydrolysis and condensation take place on the surface of the mineral filler, thus, forming oligomeric silane structures. Oligomeric silanes (Figure 4.3) are commercially available under the Dynasylan trade name. They are low-viscosity... 

In order to explain the marked effect of silane structure on the stereochemical outcome, it seems reasonable, in the absence of contrary evidence, to assume that diastereomeric a-siloxyalkyl-rhodium intermediates are formed in the rate determining step, where a predominant configuration and the extent of enantiomeric excess of the product would have already been determined. It is, therefore, evident that the steric demands of not only the chiral phosphine ligand but also the substituents on the silicon bound to the rhodium catalyst exhibit a remarkable effect on the selection of enantiotopic faces of a prochiral ketone. An extreme example is given by the reversal of preferred configuration in the reactions of pivalophenone with dimethylphenylsilane (VIII) on the one hand and with trimethylsilane (DC) on the other. 

Chatgilialoglu, C., Guerrini, A. and Lucarini, M. (1992) The Me3Si Substituent Effect on the Reactivity of Silanes. Structural Correlations between Silyl Radicals and Their Parent Silanes, /. Org. Chem. 57, 3405-3409. 

Finally, we examined the effect of varying the silane structure on reaction rate and selectivity. The use of tetramethyldisiloxane (TMDSO) (entry 5) showed a small but significant increase in the azide/alkane ratio (84 16 vs. 77 23 with ethanesulfonyl azide (7)). The reaction with poly(methylhydrosiloxane) (PMHS) was too slow (entry 6). However, addition of a sub-stoichiometric amount of phenylsilane was enough to give useful conversion (entry 7). Triethylsilane and triethoxysilane (entry 8 and 9) could not be used. Finally, combining tosyl azide (17) and TMDSO gave full conversion of 4-phenylbut-l-ene (3) in 3h with an improved azide/alkane ratio of 96 4 (entry 10) and 86% isolated yield. 

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