P. George et al., 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, 60(14), 1995, pp. 4385-4394
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.