Homolytic organometallic reactions. Part 14. Homolytic reactivity of β-C–H groups in organotin compounds. An alternative source of trialkyltin radicals

Abstract

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·+ CH₂CMe₂(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 + CH₂CMe₂+ HX, and examples of these reactions have been established where X–Y = Cl–CCl₃, Br–CCl₃, and Cl–OCMe₃.