Thermolyses of cis- and trans-2,2,3,4-tetramethyloxetane

Abstract

The thermolyses of cis- and trans-2,2,3,4-tetramethyloxetane were studied at temperatures between 408.0 and 477.0 °C and initial reactant pressures from 5.0 to 10.0 Torr.^† Both compounds undergo parallel unimolecular reactions producing either 2-methylbut-2-ene and acetaldehyde [path (a)] or but-2-ene and acetone [path (b)].

For the trans isomer the following rate expressions were found: log₁₀(k_a/s^–1)=(14.50±0.35)–(237.0±4.18 kJ mol^–1/)2.303 RT log₁₀(k′_b/s^–1)=(15.00±0.40)–(261.5±5.02 kJ mol^–1/)2.303 RT. The ratio of cis-but-2-ene to trans-but-2-ene was 0.25±0.02.

For the cis isomer the corresponding rate expressions are: log₁₀(k′_a/s^–1)=(13.90 ± 0.76)–(229.0 ± 10.0 kJ mol^–1/)2.303 RT log₁₀(k′_b/s^–1)=(14.10±0.83)–(246.6±10.9 kJ mol^–1/)2.303 RT. The ratio of cis-but-2-ene to trans-but-2-ene in this case was 3.45 ± 0.30. All error limits are the 95% confidence limits.

Comparison of the Arrhenius parameters with those for other oxetane decompositions and with thermochemical estimates make it unlikely that both paths proceed via biradical intermediates. It is suggested that for both isomers the data are consistent with a biradical path for the production of but-2-ene but that the major path to 2-methylbut-2-ene is a concerted reaction involving a twisted activated complex.