The [1,5] hydrogen transfer and cis–trans-isomerizations of cis-2,3-dimethylpenta-1,3-diene: the cis–trans-isomerization of penta-1,3-diene. Kinetics and equilibrium measurements

Abstract

The kinetics of the gas-phase thermal unimolecular isomerization of cis-2,3-dimethylpenta-1,3-diene to 3,4-di-methylpenta-1,3-diene by a sigmatropic [1,5] hydrogen shift have been studied and Arrhenius equations obtained for the rate constants in both directions, viz., log k₁/s^–1= 11·13 ± 0·10 – 141·6 ± 0·9 kJ mol^–1/2·303 RT(cis-2,3-DMP to 3,4-DMP), and log k₂/s^–1= 10·97 ± 0·10 – 139·1 ± 0·9 kJ mol^–1/2·303 RT(3,4-DMP to cis-2,3-DMP). Equilibrium constants have also been measured. These results reinforce previous kinetic data on the [1,5] hydrogen shift and indicate a structure for the transition complex not unlike that of cyclopentadiene.

The thermal unimolecular cis–trans-isomerization of cis-2,3-dimethylpenta-1,3-diene has also been studied and Arrhenius equations obtained for the rate constants, viz., log k₃/s^–1= 12·25 ± 0·13 – 183·7 ± 1·6 kJ mol^–1/2·303 RT(cis to trans), and log k₄/s^–1= 12·36 ± 0·13 – 189·3 ± 1·6 kJ mol^–1/2·303 RT(trans to cis).

The reaction is interpreted in terms of a mechanism involving the intermediate formation of 1,2,3-trimethylcyclobutene. Also reported is the thermal unimolecular cis–trans-isomerization of cis-penta-1,3-diene. This reaction probably also occurs via cyclobutene formation, but a competing biradical process cannot be completely ruled out.