作者:A. P. Dickinson、H. E. O'Neal、M. A. Ring
DOI:10.1021/om00056a021
日期:1991.10
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 x 10(9) M-1 s-1 and 3.1 x 10(9) M-1 s-1, respectively, give limiting high-pressure Arrhenius parameters for the butyl- and pentylsilylene decompositions of log A infinity = 15.5 +/- 0.1, E infinity = 22.5 +/- 0.3 kcal. The activation energies are consistent with the decomposition reaction thermochemistries (DELTA-H(dec) = 26.6 +/- 3.4 kcal, DELTA-E(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 + DELTA-E; log A(d) = 16.9, E(d) = 26.1 + DELTA-E, where DELTA-E = (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.