Silanol intermediates, production

M2D-C3-0-(E0)re-H (2)3 type products. A combination of hydrolysis at (a) and (b) would also yield HO-(EO) -H type products. If hydrolysis at (6) did occur then shorter chain analogues of M2D-C3-0-(E0) -H, HO-(EO) -H and HO-(EO)n-CH3 would also be expected due to the comparable nature of the bonds being cleaved. Hydrolysis of the Si—O bond (c) will lead to mono- and di-silanol intermediates, (5 and 6), with the concomitant generation of (CH3)3Si-OH (TMS-OH). 

Subsequent condensation of the organofunctionalised silanol intermediates (3, 5, 6) will yield a broad range of linear (7) and cyclic (8) products as shown in Fig. 5.5.4, whilst the condensation of two molecules of TMS-OH to yield (TMS)2-0 is expected to be rapid. 

The halides are liquids that are readily hydrolyzed by water, usually in an inert solvent. In certain cases, the silanol intermediates R3SiOH, R2Si(OH)2, and RSi(OH)3 can be isolated, but the diols and triols usually condense under the hydrolysis conditions to siloxanes which have Si—O—Si bonds. The exact nature of the products depends on the hydrolysis conditions and linear, cyclic, and complex cross-linked polymers of varying molecular weights can be obtained. They are often referred to as silicones the commercial polymers usually have R = CH3, but other groups may be incorporated for special purposes.78... 

Figure 36 Production of silyl triflates and silanol intermediates.

In addition, the infrared examination of the mechanism of propane and oxygen interaction with the sample (Fig. 6) indicates the different mechanism of interaction of the intermediate propylene as compared to other supported vanadium catalysts such as V-Ti02 (10). In particular, the formation of a 7t-bonded complex stabilized by a nearlying silanol with weak basic character due to the inductive effect of vicinal vanadium is shown. This indicates the relative inertness of the V sites in the silicalite towards 0-insertion or allylic H-abstraction on the adsorbed propylene. It is evident that the reduced reactivity of V sites in these reactions limits the consecutive reactions of intermediate propylene, thus enhancing the selectivity in the formation of this product. 

Oxidation of an a-silyl selenide with hydrogen peroxide leads to an aldehyde, silanol and selenol as the primary products [253]. Evidently, the fragmentation proceeds only after contrapolarization at Se, the intermediate now having an a-d-a array. 

The condensation of silanols in solution or with surfaces has not been as extensively studied and therefore is less well understood. The limitation until recently has been the lack of suitable analytical methods necessary to monitor in real time the many condensation products that form when di- or trifunctional silanols are used as substrates. With the advent of high-field wSi-NMR techniques, this limitation has been overcome and recent studies have provided insights into the effects of silanol structure, catalysts, solvent, pH, and temperature on the reaction rates and mechanisms. Analysis of the available data has indicated that the base catalyzed condensation of silanols proceeds by a rapid deprotonation of the silanol, followed by slow attack of the resulting silanolate on another silanol molecule. By analogy with the base catalyzed hydrolysis mechanism, this probably occurs by an SN2 -Si or SN2 -Si type mechanism with a pentavalent intermediate. The acid catalyzed condensation of silanols most likely proceeds by rapid protonation of the silanol followed by slow attack on a neutral molecule by an SN2-Si type mechanism. 

The trihalogeno-substituted silanes are very readily hydrolyzed, and the initially formed silanols condense with themselves and with halo-genosilanes with extreme ease. Consequently, few simple intermediate hydrolysis products of this class have been isolated or characterized. Linear halogen-substituted silanols and siloxanes are certainly first... 

Alternatively, the reaction may proceed via 1,6-ring closure giving the isolated 2,3-disilanaphthalene 372109. It should be mentioned, however, that under the reaction conditions it is possible that silyl lithium compound 375 formed from the precursor of the silene 376 (obtained from metalation of 377) by a 1,3-trimethylsilyl shift adds to the intermediate silene 371 and that subsequent ring closure with elimination of lithium silanolate gives rise to the observed products 372 and 373 (equation 105). 

The generation of methane in the reaction was evidenced by the 111 NMR spectrum of the reaction mixture. It was also shown that the newly obtained complex 29 reacts catalytically with silanol 28 to give the trimer 30 (presumably from trimerization of 31) with the evolution of hydrogen gas. In the presence of MesSiOMe the same reaction resulted in the formation of an insertion product of the intermediate silanone 31 as shown in the lower part of Scheme 12. The proposed catalytic cycle for the dehydrogenation of 28 with 29 is shown in Scheme 13. It should be noted, however, that spectroscopic evidence for the proposed silanones was not presented. 

One of the reasons for that is the high stability of the O-H bond in the newly formed silanol group (125-130 kcal/mol), and H-r can be not only a hydrocarbon molecule, but also H-OH, H-NH2, etc. The diamagnetic dioxasily-rane groups are also the generators of alkyl radicals (see subsection 4.2). Many intermediates can be obtained as the products of thermal or thermo oxidative transformations or photo transformations of other initial structures. For example, vinoxyl radicals were obtained by the photolysis of peroxide radicals of the vinyl type [119] ... 

Table 2. Structure of intermediate surface compounds and the products of elimination in reactions of some chlorides and oxochlorides with isolated silanol groups.

There appears to be no reports of any silyl chloroformate derivatives, RjSiOCOCl. Although these materials may be postulated as reaction intermediates, the corresponding silyl chlorides are the thermodynamically and kinetically favoured products of the reactions between phosgene and silanols. Equation (10.36) [1407,1407a,1409] ... 

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