Silicon condensation sequence

Silicone nanospheres with different particle diameters, crosslinking density, and chemical functionalization are accessible by aqueous hydrolysis-condensation sequences of silane and siloxane precursors [1-3] and subsequent isolation. Grafting of functionalized particles with organopolymers [1] or surface modification [2] results in nanosized silicone domains which are readily dispersible in monomeric and polymeric systems. A variety of these versatile, tailor-made products will soon be launched by Wacker on a commercial scale. 

When alkoxysilanes are used as the silicon source, both hydrolysis and condensation reactions will be taking place simultaneously for most of the sol-gel reaction. The picture is considerably simplified when sodium sihcate is used as the silicon source, since only condensation reactions need to be considered. The sequence of reactions shown initially in Fig. 1 is a gross oversimplification of the sol-gel mechanism. The reaction actually involves a large number of potential reactant species and intermediates, even in the simplest case of a gel being created from a single Q-type alkoxide precursor. Each sihcon center in the reaction mixture can undergo a total of four hydrolyses and condensations, but the sequence in which each of these steps takes place, with the exception of the first hydrolysis, can vary considerably. For a single Q-type sihcon center, there... 

The structure of Ti3SiC2 is shown in Fig. 22. The titanium and silicon atoms are close packed with the stacking sequence (hhhc)2. Thus, Ti3SiC2 adopts a structure typical for a metal-rich carbide. The titanium octahedra are filled by carbon atoms, and these Ti C octahedra are condensed via common edges, forming double layers which are separated by the silicon atoms. No silicon-carbon bonds are observed. In compounds of similar composition Si—C bonds (0.193nm) have been found only in U3Si2C2 [102]. 

A copper enolate is also regarded as an intermediate in a hetero-Diels-Alder-like sequence between a ketone and an analog of Danishefsky s diene, modified at silicon ii.e., (0Et)3 in place of Mes Scheme 1-12)J A fluoride-initiated (tetrabutylammonium difluorotriphenylsilicate [TBAT]) cleavage of the starting silyl enol ether leads to a nonracemically ligated (with Walphos) copper enolate, which condenses and then closes to the observed dihydropyranones in modest ee s. Noteworthy in this scheme is generation of the quaternary center. 

As described earlier, the use of organosilanes grew from work sponsored by the US Air Force to find products that performed better than the original chrome complexes in glass fiber reinforced unsaturated polyester resin composites. The first success was achieved with vinyl trichlorosilane and its mixtures with allyl alcohol. The unsaturation provided the polymer reactive functionality while the chloro-groups provided the fiber reactive ones. The fiber reactivity comes about by hydrolysis of the chloro-groups to form silicon hydroxyls, which are then able to condense with surface hydroxyls, as shown in the following simplified reaction sequence. 

The aldol reaction has long been recognized as one of the most useful synthetic tools. Under classical aldol reaction conditions, in vhich basic media are usually employed, dimers, polymers, self-condensation products, or a,j5-unsaturated carbonyl compounds are invariably formed as byproducts. The lithium enolate-mediated aldol reaction is regarded as one useful synthetic means of solving these problems. Besides the vell-studied aldol reaction based on lithium enolates, very versatile regio- and stereoselective carbon-carbon bond forming aldol-type reactions have been established in our laboratory by use of boron enolates (1971), silicon enolates-Le vis acids (1973), and tin(II) enolates (1982). Here we describe the first t vo topics, boron and silicon enolate-mediated crossed aldol reactions, in sequence. 

Another interesting feature of these reactions is the appearance sequence of resonances. Fully hydrolyzed silicons form condensation bonds first. End groups with one methoxy appear after hydrolyzed end groups. This must be due... 

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