| Résumé | Rate constants for hydrogen atom abstraction from n-Bu 3GeH (k GeH) by Me 3SiCH 2 • (1 •), Me 3SiCH 2CH 2 • (2 •), and Me 3SiCH 2CH 2CH 2 • (3 •) and from n-Bu 3SnH (k SnH) by 1 • and 3 • have been determined at ambient temperatures. The order of decreasing radical reactivity is 1 • > n-alkyl > 3 • > 2 •. However, for bromine abstraction from the parent bromides by n-Bu 3Sn • and n-Bu 3Ge •, the order of decreasing reactivity is 1-Br > 2-Br > 3-Br ∼ n-alkyl bromide. The Arrhenius equations for reaction of 1 • and 3 • with n-Bu 3SnH were also determined: log (k SnH(1 •)/(M -1 s -1)) = (10.2 ± 0.5) - (3.90 ± 0.62)/θ and log (k SnH(3 •)/(M -1 s -1)) = (8.4 ± 0.7) - (2.81 ± 0.95)/θ, where θ = 2.3RT kcal/mol. These kinetic data are discussed in relation to previously measured 8 rate constant ratios, k c 5+6/k SnH and k exo 5/k endo 6, where k c 5+6 corresponds to the cyclization of α-, β-, and γ-dimethylsilyl-substituted 5-hexenyl radicals to form 5-membered (k exo 5) and 6-membered (k endo 6) silacycloalkylmethyl radicals. © 1988 American Chemical Society. |
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