Halogenated silyl radical

Kim and coworkers introduced silyl radical mediated addition of alkyl radical to silyloxy enamine 76. The silyloxy enamine moiety is readily accessible from a variety of functionalities. The mechanistic concept is illustrated in the Scheme 12 and involves the addition of R radical to 76 to give the radical adduct 77 and the subsequent homolytic cleavage of N-O bond to yield the desired product 78 and a silyloxy radical 79. The latter undergoes 1,2-phenyl migration to give the silyl radical 80 that abstracts halogen from the alkyl halide to regenerate the R radical. 

One of us, as well as Bottoni, recently investigated homolytic substitution reactions of silyl radicals at the halogen atom of halomethanes (CH3X, X = Cl, Br, I), and the chalcogen atom in methanechalcogenols (CH3EH, E = S, Se, Te) and alkyltellurols, with expulsion of alkyl radical (equations 12 and 13)40-43. 

TABLE 3. Some calculated energy barriers (A/A in kJ mol-1) for homolytic substitution by some silyl radicals at the halogen atom in some alkyl halides (RX), and the chalcogen atom in some alkylchalcogenols (REH) (equations 12 and 13)... 

If organosilicon compounds have an acetoxy group on the carbon adjacent to the silicon atom, a facile 1,2-shift of the acetoxy group from carbon to silicon has been observed1 s, in addition to capture of the initially formed silyl radical by abstraction of halogen from the CC14 solvent (equation 3). Similar behavior was shown by a related benzyldimethylsilyl... 

Since Si—halogen bonds are considerably stronger than Si-H bonds, the converse to the carbon analogues, the halogen transfer reactions of silyl radicals are fast. This is illustrated by the reactions between alkyl halides and trichlorosilane initiated by irradiation from a medium-pressure mercury arc [376—378]... 

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. 

Bottoni also reported the results of computational studies into the abstraction of hydrogen atom by silyl and trichlorosilyl radicals from chloromethane, dichloromethane and chloroform (equation 15)42, and concluded that hydrogen abstraction does not effectively compete with halogen abstraction in the systems investigated. B3LYP/6-31G calculated energy barriers for these reactions fall in the approximate range of 46-72 kJmol-1. 

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