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

We have carried out calculations at the MP2/6-31G//RKF/6-31G* level o n bicyclo[5.1.0]octa-2,4-diene (BCOD), 8-oxabicyclo[5.1.0]octa-2,4-die ne (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-epoxy oxepin, to determine whether the remarkable instability of 6,8-dioxaBC OD with respect to the fission of both the three- and the seven-member ed ring giving eZzZz-muconaldehyde-a key step in the metabolic oxidati on of benzene-is already apparent in either or both monooxygen derivat ives. The effect of oxygen substitution is traced from reactions in wh ich the overall structure is conserved, i.e. the cisoid/transoid inter conversion, the degenerate Cope rearrangement, and the 1,5-hydrogen sh ift in the bicyclic molecules, to the fission of both rings giving acy clic isomers. Oxygen substitution has little effect on the interconver sion and the 1,5-hydrogen shift, but the Cope rearrangement of 6,8-dio xaBCOD is much slower than that of BCOD. On the other hand, oxygen sub stitution has an incremental destabilizing influence on the ring fissi on reaction with respect to both thermodynamic and kinetic parameters. Kinetically, the double substitution in 6,8-dioxaBCOD exerts a destab ilizing influence over and above the combined effects of the single su bstitutions in 8-oxaBCOD and 6-oxaBCOD, decreasing the activation ener gy further by some 10 kcal mol(-1). The activation energies for the fi ssion reactions of the three-membered ring in BCOD, in which cycloocta triene and methylcycloheptatriene are formed, are far in excess of the activation energy for the fission of both rings. These results sugges t that the fission of both rings of BCOD is a cooperative process.