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
- 1 January 1971
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc. A
- p. 2642-2646
- https://doi.org/10.1039/j19710002642
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.Keywords
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