Small rings. Part 32. The gas phase kinetics, mechanism, and energy hypersurface for the thermolyses of syn- and anti-tricyclo[4.2.0.0]-octane

Abstract

The title reactions have been studied at low pressure (1–10 Torr) and in the temperature ranges 390–419 {syn-tricyclo[4.2.0.0]octane (syn-TCO)} and 412–445 K (anti-TCO). The major products from both compounds were cis,cis- and cis,trans-cyclo-octa-1,5-diene (ccCOD and ctCOD) and, in addition, anti-TCO was formed from syn-TCO. Minor products from both compounds were cis- and trans-1,2-divinylcyclobutane (cDVC and tDVC) and 4-vinylcyclohexene (VCH). The disappearance of the reactant cyclo-octanes followed clear first-order kinetics (independent of pressure) with the rate constants given by equation (i) and (ii) where θ=syn-TCO log k/s^–1=(13.37 ± 0.40)–(31.39 ± 0.74 kcal mol^–1)/θ(i)anti-TCO log k/s^–1=(14.69 ± 0.02)–(35.58 ± 0.04 kcal mol^–1)/θ(ii)RTIn 10.§ Both reaction systems were subjected to kinetic analyses which allow for known interconversion pathways amongst the products. These analyses support a mechanism in which the bicyclo[4.2.0]octane-2,5-diyl diradical is a key intermediate. This diradical decomposes to form ctCOD, cDVC, and ccCOD in the approximate proportions of 60, 15, and 25%. The full mechanism is discussed in terms of a likely potential energy hypersurface for C₈H₁₂ hydrocarbons. Combustion calorimetry has been used to arrive at ΔH°_f(I) values for both syn- and anti-TCO.