Alkyl radical, 127 silicon hydride

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. ... 

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

Table 3.1 Rate constants (at 80 °C) and Arrhenius parameters for the reactions of primary alkyl radicals with silicon hydrides ...

In Table 3.2 are collected the rate constants of some silicon hydrides with a variety of radicals like the ir-type secondary or tertiary alkyl radicals, the o-type phenyl or acyl radicals, and the halogenated carbon-centred radicals. A few Arrhenius parameters are also available and reported here below. 

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 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. 

Figure 5.7. Proposed mechanism for surface hydrosilylation. The initial loss of silicon hydride generates a silicon dangling bond. Reaction between the silicon and an alkene molecule leads to an attached alkyl radical, which may abstract a hydrogen atom from a neighboring silicon.

The silicon hydrides do not spontaneously add to alkenes either. However, the hydrosilation, or hydrosilylation reaction, of olefins is of significant utility in the preparation of alkyl-subtituted silanes with the use of either radical or transition metal catalysis. The preferred metal catalysts for hydrosilation are platinum complexes. Chloro-platinic acid will catalyze hydrosilations with halosilanes, alkylarylhalosilanes, alkoxy-silanes, and siloxanes that in many cases are quantitative under ambient conditions. Yields and conversions are generally poorer for alkyl,- and arylsilanes. Many other coordination complexes have been found to catalyze the hydrosilation reaction, and these can provide certain advantages, particularly in regiochemistry. Some typical hydrosilation reactions are shown in Table... 

The rate constants Sh and SiH were determined in cyclohexane at 60 °C relative to 2kt for the self-termination of the thiyl radicals, using the kinetic analysis of the thiol-catalysed reduction of 1-bromooctane and 1-chlorooctane by silane, respectively.24 The advantage of using a silicon hydride-thiol mixture lies in the low reactivity/solubility of alkyl- and/or phenyl-substituted orga-nosilanes in reduction processes that can be ameliorated in the presence of a catalytic amount of alkanethiol. 

Some examples of more elaborate radical cyclizations accompanied by I-transfer are illustrated in Scheme 3. The cyclization of alkyl radicals onto propargyl esters has been demonstrated in synthesis of a-methylene butyrolactones [17]. This procedure uses thermolysis in the presence of benzoyl peroxide in order to induce initiation, and appears to progress in the absence of a distannane reagent. Attempts to carry out the cyclization under tin hydride conditions led to uncyclized, reduced substrate. A series of more complex radical cyclizations involving both I-transfer and unimolecular H-transfer have recently been reported. In these reactions, the radical initially formed by I-abstraction underwent 5>-exo cyclization to generate a vinyl radical. This radical, in turn, abstracted H from silicon in an intramolecular... 

If one of the reactions in a radical chain sequence is too slow to compete effectively with radical-radical reactions, the chain will collapse. Slow reactions of simple silanes such as Et3SiH with alkyl radicals precludes their use in the tin hydride method. Although quite reactive with alkyl radicals, thiols and selenols fail in the tin hydride method because the thiyl and selenyl radicals do not react rapidly with organic halide precursors. Nonetheless, it is possible to use thiols and selenols in tin hydride sequences when a Group 14 hydride is used as a sacrificial reducing agent. The thiyl or selenyl radical reacts with the silane or stannane rapidly, and the silicon- or tin-centered radical thus formed reacts rapidly with the organic halide [8], In practice, benzeneselenol in catalytic amounts has been used in radical clock studies where BusSnH served as the sacrificial reductant [9]. 

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