L2. Stereochemistry of the Thermal Decompositions and Isomerizations of Deutrated Cyclohexenes and Vinylcyclobutane; Evidence of Nonconcerted Thermal Decomposition of Cyclohexene

David K. Lewis

Department of Chemistry

Connecticut College, New London, CT 06320

An effort has been underway in our laboratory to examine the mechanism of the retro-Diels Alder thermal decomposition of gaseous cyclohexene. Samples of 3,3,6,6-d₄-cyclohexene and the cis-4,5 isomer of 1,2,3,4,5,6-d₆-cyclohexene were heated to T>1000 K in a single-pulse shock tube. Tunable diode laser and FTIR spectroscopic analyses of the products indicated the presence of substantial quantities of deuterated ethylenes apparently produced by stereochemically nonconserative pathways: 1,1-d₂-ethylene from the first and trans-1,2-d₂-ethylene from the second [Lewis, et al, J. Am. Chem. Soc. 1993, 115.11728]. Their rates of formation nicely fit a kinetic model involving vinylcyclobutane intermediate, but only if it is assumed that reactant stereochemistry is totally lost in the ethylenes formed via vinylcyclobutane. That assumption was recently verified experimentally: samples of 1-vinyl-2,3-cis-dideuterio-cyclobutane shock-heated to 945-963 K produced equal amounts of cis- and trans-1,2-d₂-ethylene [Lewis, et al, Israel J. Chem. 1996, 36, 233]. Other studies supporting this effort have been the measurement of the high temperature rates of vinylcyclobutane is isomerization to cyclohexene and fragmentation to ethylene plus buta-1,3-diene (see paper by Kalra, Lewis and Baldwin), and the rates of cis-trans isomerization of 1,2-d₂-ethylene (in progress).

This paper will summarize the experimental studies and kinetic modeling which together provide strong evidence of a substantial non-concerted pathway in the thermal isomerizations and decomposition of cyclohexene.