Reactions of group III metal alkyls in the gas phase. Part VII. The thermal unimolecular decomposition of tri-isobutylboron

Abstract

The kinetics and mechanism of the gas-phase thermal decomposition of tri-isobutylboron in the presence of excess of ethylene have been studied in the temperature range 407–469 K. Elimination of 2-methylpropene and rapid addition of ethylene to the resulting hydride leads to the formation of ethyldi-isobutylboron, diethylisobutylboron, BBuⁱ₃ [graphic omitted] EtBBuⁱ₂ [graphic omitted] Et₂BBuⁱ [graphic omitted] Et₃B (A) and triethylboron by a consecutive mechanism (A). The rate constants are those for the elimination of 2-methylpropene from the trialkylboron species and fit the following Arrhenius relationships: log k₁/s^–1= 12·3 ± 0·5 –(30·4 ± 1·0)/θ log k₂/s^–1= 12·5 ± 0·6 –(30·7 ± 1·1)/θ log k₃/s^–1= 11·4 ± 0·8 –(28·9 ± 1·6)/θ where θ= 4·58 × 10^–3T in units of kcal mol^–1.

The rate constant for the overall loss of isobutyl groups (k₀) fits the equation log k₀/s^–1= 12·3 ± 0·5 –(31·1 ± 1·0)/θ. These results are compared with those from studies of other Group III metal alkyls in the gas phase and are shown to be consistent with a polar 4-centre transition state.