Absolute Rate Constants for Some Intermolecular and Intramolecular Reactions of α-, β-, and γ-Silicon-Substituted Radicals

Article abstract of DOI:10.1021/ja00209a045

Rate constants for hydrogen atom abstraction from n-Bu3GeH (k^GeH) by Me3SiCH2^. (1^.), Me3SiCH2CH2^. (2^.), and Me3SiCH2CH2CH2^. (3^.) and from n-Bu3SnH (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-Bu3Sn^. and n-Bu3Ge^., the order of decreasing reactivity is 1-Br > 2-Br > 3-Br ca. n-alkyl bromide.The Arrhenius equations for reaction of 1^. and 3^. with n-Bu3SnH 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 rate constant ratios, k_c⁵⁺⁶/k^SnH and k_exo⁵/k_endo⁶, where k_c⁵⁺⁶ corresponds to the cyclization of α-, β-, and γ-dimethylsilyl-substituted 5-hexenyl radicals to form 5-membered (k_exo⁵) and 6-membered (k_endo⁶) silacycloalkylmethyl radicals.