Homolytic organometallic reactions. Part 13. The homolytic reactivity of stannacycloalkanes

Abstract

t-Butoxyl, trimethylsiloxyl, benzoyloxyl, and phenylthiyl radicals, X·, were generated by photolysis of the appropriate precursor, X₂, and caused to react with the acyclic butyltin compounds Bu_nSnCl_4–n(n= 1–4) and the cyclic 1,1-dialkylstannacycloalkanes R₂[graphic omitted]H₂]_n(R = Me, Et, Buⁿ, or Bu^t, n= 4, 5, or 6). The resulting radicals were observed by e.s.r. spectroscopy.

t-Butoxyl and, particularly, trimethylsiloxyl radicals react with tetra-n-butyltin at hydrogen in the α-methylene group to give the radical Bu₃SnĊHPr. The stannacycloalkanes, on the other hand, react also at the tin centre to yield the ring-opened radicals XSnR₂[CH₂]_n–1CH₂·, and this tendency increases as the size of the ring is reduced. The stannacyclopentanes R₂[graphic omitted]H₂]₄ react with the radicals Me₃CO·, PhCO₂·, and PhS· exclusively by ring opening, and the rate constants of the reaction of t-butoxyl radicals with 1,1-dimethyl- and 1,1 -dibutyl-stannacyclopentane have been measured to be 1.0 × 10⁶ and 5.5 × 10⁵ I mol^–1 s^–1, respectively, at 213 K. This reactivity is ascribed mainly to the relief of angle strain in the formation of a five-co-ordinate transition state or intermediate.

Abstraction of hydrogen from an α-methylene group in organotin compounds is accompanied by abstraction from the β-methylene group to give the radical R₃SnCH₂ĊHR′, but this radical apparently rap'idly undergoes fragmentation to give the trialkylstannyl radical R₃Sn· and the alkene CH₂CHR′.