The thermal isomerization of bicyclo[2,1,1]hex-2-ene and tricyclo[3,3,0,02,6]oct-3-ene

Abstract

The thermal isomerization of bicyclo[2,1,1]hex-2-ene to bicyclo[3,1,0]hex-2-ene has been investigated in the gas phase in the temperature range 149–190 °C. In an aged reaction vessel the reaction is homogeneous, of the first order, and the rate constant is independent of pressure down to 0·5 Torr. The rate constants fit the Arrhenius equation (i)(where R= 1·987 cal. deg.^–1 mole^–1). A consideration of these Arrhenius parameters leads to the conclusion that the isomerization is a concerted process. It is however likely that some substituted bicyclo-[2,1,1]hex-2-enes will isomerize by a two-step process involving a biradical intermediate. k= 10^{13·95 ± 0·07} exp (–35,170 ± 140/RT) sec.^–1(i)

The thermal isomerization of tricyclo[3,3,0,0^2,6]oct-3-ene to tricyclo[3,3,0,0^2,8]octa-3-ene has been studied in the temperature range 140–178 °C. The reaction is homogeneous and of the first order and the rate constants yield the Arrhenius equation (ii). It is believed that the isomerization proceeds by a two-step mechanism involving k= 10^{14·35 ± 0·06} exp (–35,320 ± 130/RT) sec.^–1(ii) the formation of an allylically stabilized biradical. From this it has been possible to estimate the rate equation for the isomerization of tricyclo[3,3,0,0^2,6]octa-3,7-diene to semibulvalene.