Cope rearrangements of cis and trans hepta-1,5-diene and 3-methylhexa-1,5-diene

Abstract

The isomerization of trans hepta-1,5-diene (I) to cis hepta-1,5-diene (III) occurs via 3-methylhexa-1,5-diene (II). Both reactions which are reversible and unimolecular are examples of the Cope rearrangement. No direct cis-trans isomerization occurs. The Arrhenius equations for all four rate constants have been determined in the temperature range 220–300°C. Equilibrium values have been measured up to 390°C and hence enthalpy and entropy differences evaluated. The A factors for the isomerizations are in reasonable agreement with those expected for a cyclic transition complex and are also similar to values obtained for other Cope rearrangements. The reasons for the discrepancies between the present work and a recent study of this system are considered.