Silanol isolated

Figure 5.27 In vacuo. total hydroxyls + geminal silanols isolated silanols n vicinal silanols.

Fig. 1. Silanol groups of amorphous silica surface, where 0= Si Q — O and = H (a) isolated, (b) vicinal, and (c) geminal.

As mentioned above, hydrolysis of bulky RSiCls precursors tends to give a variety of silanols, these may be isolated and used for the preparation of Tg derivatives. Thus, simple silanetriols RSifOHls or disiloxanetetraols [RSi(OH)2l20 (R = fBu, fHexyl, or 2,4,6-iPr3C6H2) may be dehydrated and... 

As has been seen in the preceding section concerned with the industrial uses of silanols as intermediates, the most likely reaction that a silanol may undergo is that of self-condensation to form a siloxane. This can be a very serious problem, because silanols are sensitive to base, acid, and heat. The following points should therefore be considered when planning the synthesis and isolation of a silanol. 

Although organosilanes appear to react slowly (if at all) with water alone, in the presence of acids or bases (e.g., alkali metal hydroxides), reactions to give a silanol and H2 are rapid, with bases being particularly powerful catalysts. The evolution of H2 in this type of reaction may be used as both a qualitative and a quantitative test for Si-H bonds, and the mechanism of the acid and the base hydrolysis has been discussed in detail (30,31). This hydrolytic method is not very common for the preparation of silanols that are to be isolated, because both acids and bases catalyze the condensation of silanols to siloxanes, and therefore, only compounds containing large substituents are conveniently made in this way. If an anhydrous alkali metal salt is used, a metal siloxide may be isolated and subsequently hydrolyzed to give the silanol [Eq. (10)] (32). 

Unusual monosodium salts of ethyl- or phenylsilanetriol RSi-(OH)2ONa. 1.37H20 (R = Et or Ph) are reported to be obtained by treating the polymers obtained from the simple hydrolysis of RSiCl3 with 20% aqueous NaOH/EtOH (103). Relatively few silanols have been isolated from the cleavage of siloxanes, as the conditions required to carry out such reactions are likely to cause further reactions of any silanols formed. 

The increasing tendency of silanols to condense as the number of OH groups on a particular silicon increases means that relatively few silanetriols have been isolated and even fewer structurally characterized. A wide range of substituted arylsilanetriols have been prepared (102), but none have been studied by x-ray crystallography. The first silanetriol to be structurally characterized was c-C6HuSi(OH)3, which has a sheet structure comprising double layers of molecules joined in a... 

Figure 6 Schematic of hydrogen-bonding between the biocide OIT and isolated silanol groups...

On this basis the porosity and surface composition of a number of silicas and zeolites were varied systematically to maximize retention of the isothizolinone structures. For the sake of clarity, data is represented here for only four silicas (Table 1) and three zeolites (Table 2). Silicas 1 and 3 differ in their pore dimensions, these being ca. 20 A and 180 A respectively. Silicas 2 and 4, their counterparts, have been calcined to optimise the number and distribution of isolated silanol sites. Zeolites 1 and 2 are the Na- and H- forms of zeolite-Y respectively. Zeolite 3 is the H-Y zeolite after subjecting to steam calcination, thereby substantially increasing the proportion of Si Al in the structure. The minimum pore dimensions of these materials were around 15 A, selected on the basis that energy-minimized structures obtained by molecular modelling predict the widest dimension of the bulkiest biocide (OIT) to be ca. 13 A, thereby allowing entry to the pore network. 

An asymmetric variant of this reaction was developed using chiral Pd complex 111 with either silanes or disiloxanes [66-68]. Both relative and absolute stereochemistries were controlled in this system and good yields (60-85%) were obtained after oxidation (Eq. 18). Formation of the silane-containing product was inhibited by the presence of water due to competitive formation of the palladium hydrides and silanols [68]. The use of disiloxanes as reductants, however, provided expedient oxidation to the alcohol products without decreasing the isolated yields enantioselectivity was 5-15% lower in this more robust system [66]. Benzhydryldimethylsilane proved to be a good compromise between high yield and facile oxidation [66]. Palladium com-... 

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