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

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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 BunSnCl4–n(n= 1–4) and the cyclic 1,1-dialkylstannacycloalkanes R2[graphic omitted]H2]n(R = Me, Et, Bun, or But, 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 CH2 CHR′.

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