Thermal unimolecular reactions of bicyclo[3,2,0]hept-2-ene

Dynamic isotope dilution kinetic study of the thermal conversions of bicyclo[3.2.0]hept‐2‐ene to bicyclo[2.2.1]hept‐2‐ene and to cyclopentadiene and ethene

“…Let kj be the corresponding rate constants for reactions involving &-labeled substrates 1-dz and 2-d2. If 1 and 2-dz are heated simultaneously, Equations (1) and (2) apply.…”

“…The unimolecular gas-phase reactions of bicyclo [ 3.2.01hept-2-ene ( I ) were investigated over the temperature range 307-353 OC by Cocks and Frey in 1971. ' The overall decomposition of 1 was kinetically first order at initial reactant pressures above 2 Torr; product mixtures were found t o contain norbornene, cyclopentadiene plus ethene, and five other products (Scheme 1). Three of the five were identified as methylcyclohexa-l,3-dienes by comparison of GC retention times with those of authentic samples on four different columns.…”

“…The other two products had similar retention times to those of truns,truns-hepta-l,3,5-triene and 'cis-hepta-l,3,5-triene. ' Kinetic data were analyzed in terms of three primary reactions (Scheme 1): olefin 1 reacts through [ 1,3] carbon migration of C-7 from C-1 t o C-3 to form bicyclo 2), cleavage of the C-1 -C-7 and C-5-C-6 bonds to give cyclopentadiene (3) and ethene, and fragmentation of the cyclobutane unit in the alternative sense, with cleavage of C-1-C-5 and C-6-C-7 to yield hepta-l,3 Z,6-triene (4). Both norbornene and the heptatriene reacted further under the reaction conditions t o give cyclopentadiene and ethene, respectively, through a retro-Diels-Alder process, and a mixture of the five isomers found in the product mixture.…”

“…[2(t)1 = kl (k2 -kO)-' { exp( -kot) -exp( -k2t) 1 [2(0)1 (1) Here ko is the rate constant for disappearance of 1; ko = kl + k3 + k4.…”

“…When [2(t)] is very small, small absolute errors in measuring it are large relative errors, leading to large errors in estimates Several facets of the thermal chemistry of bicycle[ 3.2.01hept-2-ene as delineated by Cocks and Frey attracted our attention. It is one of the simplest bicyclic olefins giving a [ 1,3] carbon sigmatropic migration product through a supraficial process, and thus with only two stereochemical options; it offers a direct competition between a possibly allowed and concerted [ 1,3 1 shift with inversion and cyclobutane cycloreversion pathways, which are generally considered to be stepwise, diradical-mediated reactions; and it presents some daunting challenges to experimentalists concerned with detailed definitions of stereochemistry and reaction rates for specific individual paths.…”