Thermal isomerization of cyclobutenes. Part 8.—Cis- and trans-1,2,3,4-tetramethylcyclobutene and bicyclo[4.2.0]oct-7-ene

Abstract

The thermal isomerizations of cis and trans-1,2,3,4-tetramethylcyclobutene and bicyclo[4.2.0]oct-7-ene have been studied in the gas phase. In the temperature range 157-201°C, cis-tetramethylcyclobutene isomerizes by a homogeneous first-order process to yield cis, trans-3,4-dimethylhexa-2,4-diene and the rate constants fit the Arrhenius equation k₁= 10^14.10 exp (–37,360/RT) sec^–1. In the temperature range 125-167°C the trans tetramethylcyclobutene isomerizes to trans, trans-3,4- dimethylhexa-2,4-diene, but under the conditions employed this product undergoes surface-catalyzed polymerization. The rate of disappearance of the reactant obeyed first-order kinetics and the rate constants obtained fitted the Arrhenius equation, k₂= 10^13.85 exp (–33,590/RT) sec^–1. The isomerization of bicyclo[4.2.0]oct-7-ene yields cycloocta-1,3-diene by a homogeneous first-order process, and in the temperature range 236-285° the rate constants fit the Arrhenius equation, k₃= 10^14.13 exp (–43,180/RT) sec^–1. Above 1 mm the rate constants of the three isomerizations are independent of pressure. The results obtained are compared with those for other cyclobutenes with special reference to the mechanism of the transformation.