Silsesquioxanes reactions

Fig. 9.11 In situ ATR FTIR spectra of the silsesquioxane reaction mixture as a function of the reaction time for a window of 670-1000 cmf1. Analytical parameters were IR analysis (670-4000 crrT1) was performed for 10 h with the acquisition of a spectrum every 2 min, 16 scans per spectrum and a resolution of 4 cmf1.

The reactions of POSS molecules fall into two categories, those involving the silsesquioxane core of the molecule and those involving reactions of the peripheral substituents. The silsesquioxane core in POSS compounds is fairly unreactive to many reaction conditions and this inertness has led to the successful application of POSS species in many materials (see Section V for further... 

Evaluation of the peak pattern gives evidence of silsesquioxanes and their homo-derivatives as well as high molecular reaction products with ladder-like structures. The presence of cage-like polysiloxanes is remarkable since their formation generally requires completely different preparative conditions [10],... 

A bulkyl silsesquioxane has been used to prepare metathetically active Mo complexes [91]. A silsesquioxane mimics the manner in which a metal might be attached to silica in a heterogeneous metathesis system. The conjugate acid of the silsesquioxane appears to have a relatively high pKa, a hint that the silsesquioxane itself may be relatively electron withdrawing. Silsesquioxane complexes were found to be highly active catalysts in metathesis reactions. 

Alkene cross-metathesis has also been recently used for the modification of silsesquioxanes and spherosilicates, by Feher and co-workers [46]. Reaction of vinylsilsesquioxane 28 with a variety of simple functionalised alkenes, in the presence of Schrock s molybdenum catalyst 3, gave complete conversion of the starting material and very good isolated yields of the desired products (75— 100%) (for example Eq. 28). 

A similar reaction sequence starting from the completely condensed silsesquioxane octamer cage (c-C,jI 1i iSi) Oi 2 513 affords the isolated intermediates 514 and 515, eventually providing the octasiloxanediol (r-C4I h iSiHOjoiOH 516 (Scheme 69).471... 

The chemistry of silicone halides was recently reviewed by Collins.13 The primary use for SiCU is in the manufacturing of fumed silica, but it is also used in the manufacture of polycrystalline silicon for the semiconductor industry. It is also commonly used in the synthesis of silicate esters. T richlorosilane (another important product of the reaction of silicon or silicon alloys with chlorine) is primarily used in the manufacture of semiconductor-grade silicon, and in the synthesis of organotrichlorosilane by the hydrosilylation reactions. The silicon halohydrides are particularly useful intermediate chemicals because of their ability to add to alkenes, allowing the production of a broad range of alkyl- and functional alkyltrihalosilanes. These alkylsilanes have important commercial value as monomers, and are also used in the production of silicon fluids and resins. On the other hand, trichlorosilane is a basic precursor to the synthesis of functional silsesquioxanes and other highly branched siloxane structures. 

The fact that silsesquioxane molecules like 2-7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties. This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest. As mentioned above, incompletely condensed silsesquioxanes of the type R7Si70g(0H)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces. Metal... 

The hydrolysis/condensation process takes a somewhat different course when cyclopentyl and cycloheptyl substituents are employed instead of cyclohexyl. In 1991 Feher et al reported the preparation of 2 and 4 via hydrolytic condensation reactions of c-CsHgSiCls (9) and c-C7Hi3SiCl3 (10), respectively. It was reported that these straightforward syntheses produce multigram quantities of incompletely condensed silsesquioxanes within a few days. In these two cases no completely condensed silsesquioxane derivatives were formed. While 2 was obtained as the sole... 

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