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George P , Bock CW , Glusker JP , Greenberg A , Gallagher JD
Thermal Rearrangements of Bicyclo 5.1.0 Octa-2,4-Diene and Its 8-Oxa, 6-Oxa, and 6,8-Dioxa Derivatives - an Ab-Initio Molecular-Orbital Study
Journal of Organic Chemistry. 1995 Jul 14;60(14) :4385-4394
PMID: ISI:A1995RK02700015   
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Abstract
We have carried out calculations at the MP2/6-31G*//RKF/6-31G* level on bicyclo[5.1.0]octa-2,4-diene (BCOD), 8- oxabicyclo[5.1.0]octa-2,4-diene (8-oxaBCOD), 6- oxabicyclo[5.1.0]octa-2,4-diene (6-oxaBCOD), and 6,8- dioxabicylo[5.1.0]octa-2,4-diene (6,8-dioxaBCOD), otherwise 2,3-epoxyoxepin, to determine whether the remarkable instability of 6,8-dioxaBCOD with respect to the fission of both the three- and the seven-membered ring giving eZzZz- muconaldehyde-a key step in the metabolic oxidation of benzene- is already apparent in either or both monooxygen derivatives. The effect of oxygen substitution is traced from reactions in which the overall structure is conserved, i.e. the cisoid/transoid interconversion, the degenerate Cope rearrangement, and the 1,5-hydrogen shift in the bicyclic molecules, to the fission of both rings giving acyclic isomers. Oxygen substitution has little effect on the interconversion and the 1,5-hydrogen shift, but the Cope rearrangement of 6,8- dioxaBCOD is much slower than that of BCOD. On the other hand, oxygen substitution has an incremental destabilizing influence on the ring fission reaction with respect to both thermodynamic and kinetic parameters. Kinetically, the double substitution in 6,8-dioxaBCOD exerts a destabilizing influence over and above the combined effects of the single substitutions in 8-oxaBCOD and 6-oxaBCOD, decreasing the activation energy further by some 10 kcal mol(-1). The activation energies for the fission reactions of the three-membered ring in BCOD, in which cyclooctatriene and methylcycloheptatriene are formed, are far in excess of the activation energy for the fission of both rings. These results suggest that the fission of both rings of BCOD is a cooperative process.
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Times Cited: 2 Article RK027 J ORG CHEM