Silicon hydride , hydrogen

The hydrogen abstraction from the Si-H moiety of silanes is fundamentally important for these reactions. Kinetic studies have been performed with many types of silicon hydrides and with a large variety of radicals and been reviewed periodically. The data can be interpreted in terms of the electronic properties of the silanes imparted by substituents for each attacking radical. In brevity, we compared in Figure 1 the rate constants of hydrogen abstraction from a variety of reducing systems by primary alkyl radicals at ca. 80°C. ... 

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. ... 

Two closely related reactions, (a) and (b), illustrated by Eq. (12) (Rj = HPhj, Etj, Phj, CI3, CljPh) and (13), of silicon hydrides with transition metal complexes generate compounds with Si—M bonds with elimination of hydrogen (a) cleavage of metal-metal bonds and (b) reaction with transition metal hydrides. Reactions discussed in this section are relevant to... 

The relationship between the electronegativities of silicon, carbon, and hydrogen X(Si) < X(H) < X(C) is the reason for the polarity of the Si H bond in the sense Si+H which is reversed with respect to C-H. Therefore, silicon hydrides can easily be decomposed by hydrolysis. A fundamental conclusion from electronegativity can be formulated as follows. 

Limited studies of the germanium and tin hydride analogs of the silicon hydrides show that they share this ability to function as hydride sources in ionic hydrogenations however, their relatively greater reactivity toward acids appears to restrict their practical applications in organic synthesis.24,25... 

In some cases, elements having electronegativities too low to give ionic bonding with hydrogen also tend to be unreactive, so that direct combination of the elements is not feasible. In such cases, the procedure just described can be used to prepare the hydride. For example, silicon hydride, SiH4 (known as silane), can be produced by the reactions... 

As was discussed in Chapter 13, hydrogen does not react directly with some elements, so the hydrides must be prepared in a different way. Alfred Stock prepared silicon hydrides by first making the magnesium compound, then reacting it with water. 

I thank Keith U. Ingold for having introduced me to this subject. When I arrived in Ottawa at the National Research Council of Canada in 1979 for three years postdoctoral work with him, very little was known on the reactivity of silyl radicals. At that time, several papers dealing with kinetics of silyl radicals were published, which allowed the reactivity of silyl radical to be translated into a quantitative base. Special thanks go to David Griller for his collaboration on the initial work on hydrogen donor abilities of silicon hydrides during the late 1980s. 

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. 

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-... 

As an example, the propagation steps for the reductive alkylation of alkenes are shown in Scheme 7.1. For an efficient chain process, it is important (i) that the RjSi radical reacts faster with RZ (the precursor of radical R ) than with the alkene, and (ii) that the alkyl radical reacts faster with the alkene (to form the adduct radical) than with the silicon hydride. In other words, the intermediates must be disciplined, a term introduced by D. H. R. Barton to indicate the control of radical reactivity [5]. Therefore, a synthetic plan must include the task of considering kinetic data or substituent influence on the selectivity of radicals. The reader should note that the hydrogen donation step controls the radical sequence and that the concentration of silicon hydride often serves as the variable by which the product distribution can be influenced. 

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