Decomposition kinetics and thermochemistry of butyl- and pentylsilylenes

Article abstract of DOI:10.1021/om00056a021

Reaction kinetics of alkylsilylenes (R = butyl, pentyl, and methylpentyl) and silacyclopropane intermediates produced by silylene additions to 1-butene, 1- and 2-pentene, 2-methyl-1-pentene, and 4-methyl-1-pentene are reported. Observations are consistent with a homogeneous, Barton²-type mechanism, which describes alkylsilylene isomerization and decomposition in terms of silacyclopropane (SCP) intermediates. Modeling studies on this basis conclude that SCP-forming and -opening reactions are at least an order of magnitude faster than SCP decompositions to olefins and SiH₂. All reactions are pressure dependent at 400 Torr. Decomposition versus trapping comparative rate data at 410 Torr, based on butylsilylene- and pentylsilylene-trapping reactions with silane of 4.8 × 10⁹ M^-1 s^-1 and 3.1 × 10⁹ M^-1 s^-1, respectively, give limiting high-pressure Arrhenius parameters for the butyl- and pentylsilylene decompositions of log A_∞ = 15.5 ± 0.1, E_∞ = 22.5 ± 0.3 kcal. The activation energies are consistent with the decomposition reaction thermochemistries (Δ_dec = 26.6 ± 3.4 kcal, Δ_dec = 25.3 ± 3.4 kcal), and A factors indicate surprisingly loose transition states for both alkysilylene decompositions as well as their reverse silylene/olefin additions. A loose silylene/olefin addition complex is suggested for the transition state as the thermochemistry of decomposition precludes the intermediacy of biradicals. Generic high-pressure Arrhenius parameters (A, s^-1: E, kcal) are deduced for silacyclopropane ring-opening (o), -closing (c), and -decomposition (d) reactions: log A_c = 12.3, E_c = 10.4; log A_o = 14.0, E_o = 14.7 + ΔE; log A_d = 16.9, E_d = 26.1 + ΔE, where ΔE = (49.6 - SCP strain energy). The low SCP ring-closing activation energy indicates zero ring strain development in the transition state and is consistent with a reanalysis of prior estimates of the activation energy of the SiH₂ + CH₄ strain free, C-H bond insertion reaction.