Homolytic organometallic reactions. Part 14. Homolytic reactivity of β-C–H groups in organotin compounds. An alternative source of trialkyltin radicals
Rate constants (at –84°) for the reaction of t-butoxyl radicals with tetraethyltin at the α-methylene group to give the radical Et3SnCHCH3, and at the β-methyl group to give the radical Et3SnCH2ĊH2, are 1.2 × 104 and 4.8 × 103 l mol–1 s–1, respectively ca. 103 and 5 × 102 times greater (per hydrogen atom) than the reaction of t-butoxyl radicals with ethane. The abstraction of hydrogen from the β-position in alkyltin compounds is followed by reversible elimination to give an alkene and a trialkyltin radical, and, if di-t-butyl peroxide is photolysed in the presence of trimethylisobutyltin, the e.s.r. spectrum of the trimethyltin radical can be observed [reaction (i)]. This reaction Me3SnCH2CHMe2+ Me3CO·→ Me3SnCH2ĊMe2→ Me3Sn·+ CH2 CMe2(i) provides a useful alternative source of trialkyltin radicals for e.s.r. studies, particularly for the generation of alkyl radicals by the reaction with alkyl bromides [reaction (ii)]. This formation of a β-trialkylstannylalkyl radical and Me3Sn·+ RBr → Me3SnBr + R·(ii) thence of a trialkyltin radical and alkene can constitute two of the three steps of a radical chain reaction with the overall equation Me3SnCH2CHMe2+ X–Y → Me3SnY + CH2 CMe2+ HX, and examples of these reactions have been established where X–Y = Cl–CCl3, Br–CCl3, and Cl–OCMe3.