Kinetics of the Thermal Isomerizations of Gaseous Cycloheptene and Cyclooctene

Abstract

Single-pulse shock tube kinetic studies of the thermal isomerizations of gaseous cycloheptene (CHEP) and cis-cyclooctene (COCT), and static reactor isomerizations of COCT at lower temperatures, have revealed a mechanistic dissimilarity in the two superficially analogous cycloalkene to, a,w-alkadiene reactions observed.At 1035-1256 K, CHEP produced mostly vinylcyclopentane, loglo(k,s-') = 15.1 (k 0.7) -69.7 (k 3.3) x 103/4.576T, and some 1,6-heptadiene. From COCT, heated over the range 610-1091 K, the dominant product was 1,7-octadiene, log& s-l) = 13.8 (k 0.2) -54.6 (k 0.5) x 103/4.576T, with small amounts of vinylcyclohexane formed at the higher temperatures, loglo(k, s-I) = 15.2 (& 0.3) -64.4 (k 1.2) x lo3/ 4.576T. The activation energy for the COCT isomerization to 1,7-octadiene is too low to associate with formation of a diradical, but is consistent with a concerted retro-ene mechanism. The higher activation energy isomerization to vinylcyclohexane, however, passes through a diradical transition structure. In contrast, the structure of CHEP is not adaptable to a concerted retro-ene process, and both 1,6-heptadiene and vinylcyclopentane are formed through diradical-mediated reactions.

Kinetics of the Thermal Isomerizations of Gaseous Vinylcyclopropane and Vinylcyclobutane

“…The same inference was drawn earlier for transition structures for the interconversion of vinylcyclopentane with cycloheptene and of vinylcyclohexane with cis -cyclooctene, based on ground state Δ H f o values and E a parameters. The estimated transition state Δ H f o values agreed within probable error limits with Δ H f o values calculated for the corresponding extended ( Z )-2-hepten-1,7-diyl and ( Z )-2-octen-1,8-diyl diradicals …”

“…The estimated transition state ∆H f o values agreed within probable error limits with ∆H f o values calculated for the corresponding extended (Z)-2-hepten-1,7-diyl and (Z)-2-octen-1,8-diyl diradicals. 18 The activation energy for vinylcyclopropane f cyclopentene obtained by connecting the present high-temperature shock-tube and lower temperature static-reactor data lies about 2 kcal/mol above the values obtained in earlier studies, 2-4 a modest but not insignificant upward revision. Extrapolated to 1200 K, the rate constant for vinylcyclopropane f cyclopentene is k ) 7.7 × 10 4 s -1 .…”

“…One may infer that the transition structure for the vinylcyclopropane to cyclopentene rearrangement still retains some of the strain energy associated with the cyclic reactant and product; the allylic unit of the transition structure may not be fully planar, and other torsional strain factors may still be at play. The residual strain energy, taken to be (82.1 ( 0.6) -(77.5 ( 0.7) ) 4.6 ( 0.9 kcal/mol, is significantly larger than zero, both formally and in contrast to the absence of any similar strain energy term in the thermochemical analyses of the related interconversions of vinylcyclobutane, vinylcyclopentane, 18 and vinylcyclohexane 18 with the corresponding isomeric cycloalkenes. That the allyl units of transition structures for vinylcyclopropane to cyclopentene rearrangements are substantially twisted out of planarity has been hypothesized to account for remarkably large secondary deuterium kinetic isotope effects at the 2′-carbon of the vinyl group; k H /k D ) 1.17 ( 0.02 at 338°C for 2′,2′-d 2 -labeled vinylcyclopropane, a fact taken as an indication of torsional distortion.…”

“…A consideration of thermochemical relationships pertinent to the interconversion of vinylcyclobutane and cyclohexene based on a diradical model for transition structures, a model exercised earlier in analyses of the thermal interconversions of vinylcyclopentane with cycloheptene and of vinylcyclohexane with cis- cyclooctene, is depicted in Figure . Gas-phase Δ H f o values for ( Z )-2-hexene, cyclohexene, and vinylcyclobutane establish the lower reference energies.…”

Thermal Isomerization of (3-Butenyl)cyclopropane to Norbornane

“…Diels−Alder additions of the initially formed 1,3-butadiene and propylene, followed by inter- and intramolecular hydrogen transfers and [1,5] shifts, can account for most of these secondary products. Cycloheptene probably forms from 2 through an intramolecular ene reaction …”