FISSION OF THE EPOXIDE RING IN 6,8-DIOXABICYCLO[5.1.0]OCTA-2,4-DIENE (EPOXYOXEPIN) AND IN 8-OXABICYCLO[5.1.0]OCTA-2,4-DIENE - AN AB-INITIO MOLECULAR-ORBITAL STUDY

As part of our investigation of the role of 6,8-dioxabicyclo[5.1.0]oct a-2,4-diene (6,8-dioxaBCOD; epoxyoxepin) as the precursor of muconalde hyde in the metabolic oxidation of benzene, we have carried out ab ini tio molecular orbital calculations on the fission of the three-membere d (epoxide) ring in 6,8-dioxaBCOD and 8-oxaBCOD in which cyclic enol s tructures are formed. Electronic energies have been calculated at the MP2/6-31G (frozen core, valence orbitals active) level with full geom etry optimization at the RHF/6-31G level. With the inclusion of therm al energies derived from vibrational frequencies obtained at the RHF/6 -31G//RHF/6-31G* level, reaction energies and activation energies tha t would correspond to gas phase data at 298 K have been evaluated. Syn and anti conformers of the enol structures have been identified. The syn conformer from 8-oxaBCOD is more stable than the anti by 2.0 kcal mol(-1), in common with other enols, whereas the anti conformer from 6 ,8-dioxaBCOD is more stable than the syn by 1.9 kcal mol(-1) a differe nce attributable to a hydrogen bonding interaction with O-6 in the sev en-membered ring. The ketone formed by the tautomeric shift is 8.4 kca l mol(-1) lower in energy than the syn-enol from 8-oxaBCOD, whereas th e lactone formed by the corresponding shift in the anti-enol from 6,8- dioxaBCOD is 27.4 kcal mol(-1) lower in energy; an enhanced energy dif ference mainly attributable to the well-known stabilization in an acyl grouping. Transition states for the ring fission have been characteri zed and found to lead to the syn conformers of the enol structures. Th e activation energy for the fission in 6,8-dioxaBCOD is far larger tha n that for the cooperative fission of both rings; 32.7 compared with 1 6.5 kcal mol(-1). Hence, competition with the formation of eZzZz-mucon aldehyde in the latter reaction is negligible. The energy barriers for the two fission processes in 8-oxaBCOD, however, favor the former; 33 .7 compared with 41.5 kcal mol(-1). These results have established tha t the fission of the epoxide ring giving enol structures is an extreme ly unfavorable process, due apparently to an inherent difficulty in ef fecting the requisite 1,2 H-shift and the lack of any driving force th at might originate in changes in the bonding in the seven-membered rin g. In contradistinction, the fission of both rings, which in the case of 6,8-dioxaBCOD results in the formation of muconaldehyde, is charact erized by a profound alteration in the bonding between the heavy atoms : all the bonds but one change from double to single, or vice versa, n otably the bonding to the oxygen atoms.