Reactions Mediated by Tin and Silicon Hydrides

In a similar way, heteroaiyl radicals can be generated by photolysis to synthesize a great number of different biaryls [31]. Alkyl radicals can be also generated by photolysis of the C—X bond (X = aryl selenide [32] or diazene [33]). 

Owing to space limitations, only a review of the most important concepts of the radical reactions mediated by tin and silicon hydrides will be discussed. However, these reactions have been excellently reviewed elsewhere [34]. 

TABLE 9.1 Rate Constants for H-Atom Abstraction of Different Types of Carbon-Centered Radicals from (TMS) SiH and Bu SnH [38] 

Another factor to be considered is that the hydrogen transfer from Z MH to R is generally faster than R addition onto arene (Table 9.1) [38]. For example, as mentioned earlier (see Section 9.1), the rate of addition of the phenyl radical to benzene is 4.5 x 10 [5]. Therefore, in order to avoid hydrogen transfer competition, the hydride is often added to the reaction mixture slowly (thereby lowering its concentration) by a syringe pump, together with the radical initiator. 

As stannyl or silyl radicals have high rates of halogen abstraction, alkyl and aryl halides are the most common sources of radicals [39]. The reactivity order is R-I R-Br R-Cl, with R being either alkyl or aryl. Radical generation by chlorine abstraction occurs only in activated alkyl chlorides, such as a-chloroketones (or esters), and polyhalogenated carbons, whereas vinyl and aryl chlorides tend to be nonreactive to organostannyl radicals. 

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