Group silicon hydrides

Unfortunately, because self-condensation of silanols on the same silicone can occur almost spontaneously, the reaction of disdanol or trisilanol compounds with telechelic sdanol polymers to form a three-dimensional network is not feasible. Instead, the telechelic polymers react with cross-linkers containing reactive groups such as alkoxysdanes, acyloxysdanes, silicon hydrides, or methylethyloximesilanes, as in the reactions in equations 18—21 (155). 

Phenyl or mixed alkyl/phenyl substituted silicon hydrides show similar reactivities to trialkylsilanes. Indeed, by replacing one alkyl by a phenyl group the effect on the hydrogen donating ability of SiH moiety increases only slightly. ... 

Figure 2.6 Reagents used for the deactivation of silanol groups on glass surfaces. A - disilazanes, B > cyclic siloxanes, and C -silicon hydride polysiloxanes in which R is usually methyl, phenyl, 3,3,3-trifluoropropyl, 3-cyanopropyl, or some combination of these groups. The lover portion of the figure provides a view of the surface of fused silica with adsorbed water (D), fused silica surface after deactivation with a trimethylsilylating reagent (E), and fused silica surface after treatment with a silicon hydride polysiloxane (F).

Table I includes the relative bond dissociation enthalpies obtained for some group 14 hydrides by photoacoustic calorimetry,7 10 The data demonstrate that, for the trialkyl-substituted series, the bond strengths decrease by 6.5 and 16.5 kcal/mol on going from silane to germane and to stannane, respectively. The silicon-hydrogen bonds can be dramatically weakened by successive substitution of the Me3Si group at the Si-H functionality. A substantial decrease in the bond strength is also observed by replacing alkyl with methylthio groups.

Silicones can be prepared in such a way that they contain only one percent or less of vinyl groups and silicon hydrides, which undergo a catalytic hydrosilylation reaction to give the desired cross-linking (Figure 18.3). Vinyl silanes are made by Si-H addition to acetylene, and thus two hydrosilylations are involved. 

PhSeSiRs reacts with BusSnH under free radical conditions and affords the corresponding silicon hydride (Reaction 1.8) [19,20]. This method of generating RsSi radicals has been successfully applied to hydrosilylation of carbonyl groups, which is generally a sluggish reaction (see Chapter 5). 

The use of free-radical reactions in organic synthesis started with the reduction of functional groups. The purpose of this chapter is to give an overview of the relevance of silanes as efficient and effective sources for facile hydrogen atom transfer by radical chain processes. A number of reviews [1-7] have described some specific areas in detail. Reaction (4.1) represents the reduction of a functional group by silicon hydride which, in order to be a radical chain process, has to be associated with initiation, propagation and termination steps of the radical species. Scheme 4.1 illustrates the insertion of Reaction (4.1) in a radical chain process. 

Secondary alkyl selenides are reduced by (TMS)3SiH, as expected in view of the affinity of silyl radicals for selenium-containing substrates (Table 4.3) [40]. Reaction (4.23) shows the phenylseleno group removal from the 2 position of nucleoside [50]. Similarly to 1,3-dithiolanes and 1,3-dithianes, five- and six-membered cyclic selenoacetals can be monoreduced to the corresponding selenides in the presence of (TMS)3SiH [51]. The silicon hydride preferentially approached from the less hindered equatorial position to give transicis ratios of 30/70 and 25/75 for the five-membered (Reaction 4.24) and six-membered cyclic selenoacetals, respectively. 

The addition reaction of alkyl and/or phenyl substituted silicon hydrides to acetylenes has limitations mainly due to the hydrogen donation step (cf. Scheme 5.1). Reaction (5.17) shows that the replacement of Ph by MesSi group in silanes made the reaction easier, the effect being cumulative. Indeed, the reaction time decreased from 88 h for PhsSiH to 3h for (TMS)3SiH [39], together with the amelioration of yields, and a slightly better cis stereoselectivity. 

The reduction of ketones with silicon hydrides has been occasionally performed by radical chemistry for a synthetic purpose. The radical adduct is stabilized by the a-silyloxyl substituent and for RsSi (R = alkyl and/or phenyl) the hydrogen abstraction from the parent silane is much slower than a primary alkyl radical (cf. Chapter 3). On the other hand, (TMS)3SiH undergoes synthetically useful addition to the carbonyl group and the reactions with dialkyl ketones afford yields > 70% under standard experimental conditions, i.e., AIBN, 80-85 °C [45,51]. Reaction (5.25) shows as an example the reduction of 4-tcrt-butyl-... 

H MAS NMR clearly shows five signals at 0.8, 1.9, 4.4, 10.1 and 12.1 ppm. The signals at 0.8, 1.9 and 4.4 are assigned to alkyl fragments bonded to silicon atoms, residual silanol groups on the silica surface and to silicon hydride species, respectively. The two downfield signals at 10.1 and 12.1 ppm are an indication of the presence of two types of zirconium hydride surface species [111]. DQ rotor-synchronized 2D H MAS NMR was used to discriminate these zirconium hydride... 

In this paper detailed methods for the determination of placement and assay of silicon hydride (Si-H), silicon hydroxide (Si-OH) and silicon phenyl (Si-0) functional groups in molecular weight components of silicones of the Sylgard (Dow-Corning Co.) type will be described. The methods are illustrated with the analysis of Sylgard addition prepolymers and of model polydimethylsiloxanes (PDMS). 

TX, MHSMP-85-06 (January 1985). Kohn, E. "High Performance Size Exclusion Chromatography (VII) Determination of Silicon Hydride Functional Groups in Components of Silicones" ibid, MHSMP-83-28 (August 1983). Both available through National Technical Information Service (NTIS),... 

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