Silsesquioxanes silylation

Silylation of the silsesquioxane [(C6H11)7Si709(0H)3] (see Section IV,E for its structure) leads to disilanols [(C6Hn)7Si709(0H)2SiMe2R] (R = Me or Ph). The structure of the phenyl compound 59 shows the... 

Siloxane compounds, in vitreous silica manufacture, 22 414 Siloxane materials, 20 240 Siloxane oligomers, in silicone polymerization, 22 555-556 Siloxanols, silylation and, 22 703 Silsesquioxane hybrids, 13 549 Silsesquioxanes, 15 188, 22 589-590 SilvaGas process, 3 696, 697 Silver (Ag), 22 636-667. See also Silver compounds. See Ag entries Argentothiosulfate complexes Batch desilverizing Lead-silver alloys Palladium-silver alloy membranes analytical methods for, 22 650-651 applications of, 22 636-637, 657-662 as bactericide, 22 656, 657, 660 barium alloys with, 3 344 in bimetallic monetary system, 22 647-648 in cast dental gold alloys, 8 307t coke formation on, 5 266 colloidal precipitation color, 7 343t colloidal suspensions, 7 275 color, 7 334, 335... 

Several titanocene derivatives containing silsesquioxane ligands have also been prepared and characterized. It soon turned out that reactions of 3 or its mono-silylated derivative 12 with titanocene dihalides are not straightforward and usually lead to the formation of product mixtures. A common feature appears to be the formation of p-oxo species despite the use of carefully dried solvents. Although at this stage the occurrence of partial hydrolysis cannot completely be ruled out, we... 

The silylation of [(CH3) NOSiOj jjg by hexamethyldisiloxane in an aqueous propanol solution in the presence of hydrochloric acid affords octa(trimethylsiloxy-silsesquioxane) in good yield (75 %)... 

Silsesquioxanes. At the same time as silanetriols were stabilized, another significant observation was made in the hydrolysis of silyl trihalides. Incompletely condensed silsesquioxanes, RySiyOgfOH (Chart 6) are formed along with other products during the hydrolysis of RSiCl3 (R =... 

We have obtained silsesquioxane-based polymer (HS-Polymer) by using the hydro-silylation reaction of vinyl-functionalized macromonomer (DD-2V) with some SiH-functionalized siloxanes (Scheme 4). 

Most of the contributions in Chapter IV deal with this area of research. Commercial interest is centered on the field of silicones with defined silicon substitution patterns, molecular weight, crosslinking degree and the study of structure-property correlations. In constrast the academic interest focusses on topics that may be recognized as "old" problems, such as "silyl modified surfaces", "synthesis and reactivity of silsesquioxanes", and "luminescent silicon". The reader will find answers to these problems in this chapter. 

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. 

Summary Two catalytic reactions, i.e. silylative coupling (mms-silylation) (SC) catalyzed by complexes containing or generating Ru-H and/or Ru-Si bonds (I, II, V, VI) and cross-metathesis (CM) catalyzed by mthenium-carbene (i.e. 1st and 2nd generation mthenium Grubbs catalyst (ID, IV)) of vinyl and allyl-substituted hetero(N,S,B)organic compounds with conunercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxane have been overviewed. They provide a universal route toward the synthesis of well-defined molecular compounds with vinylsilicon functionality. 

Previous reports on the reaction of vinyl-substituted silanes [9] and silsesquioxane [15] with vinyl allQrl ethers catalyzed by ruthenium complexes containing or generating Ru-H and/or Ru-Si bonds show that the process proceeds according to the nonmetallacarbene mechanism as in SC and yields (usually in 5-fold excess of alkene) l-silyl-2-alkoxy-ethenes with a high preference for the E-isomer (Eq. 1). 

In yet another approach, titanium(IV) silsesquioxanes were shown to be excellent homogeneous catalysts for epoxidations with TBHP [55]. A heterogeneous variant was prepared by adsorbing the Ti(IV) silsesquioxane in the pores of MCM-41 that had been silylated with Ph2SiCl2 to passivate the external surface. The resulting material was reported to be a stable, recyclable catalyst for epoxidations with TBHP [56]. 

In the silsesquioxanes, it is oxygen that couples the two radical functions, as shown schematically in the model compound 9. Quantum chemical calculations on 4 and 9 substantiate the qualitative assertions (Table 30.1). The singlet state of the C2 conformation is substantially lower in energy (117.7 kJ/mol) than the planar structure (C2V geometry), in agreement with the known strong pyramidalization forces in silyl radicals. 

Interestingly, the quantum chemical calculations on the TM-substituted silsesquioxanes, and on the various investigated substituted silyl model compounds indicate that the Si-[M] vibrations are easily identified from the other vibrations of the compounds. This is shown for the silyl-[M] structure (Figure 30.3). 

Figure 9.2 Surface modification chemistries of nanocellulose for PLA/nanocellulose biocomposites, a, Acetylation b, Esterification with various organic acids c, d, e, Grafting of PCL, PLA, P(CL-fi-LA) f, Silanization g, Silylation h, Carbojymethylation combined with hexanoation i, PEG grafting j, Modified with polyhedral oligomeric silsesquioxane (POSS).

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