Fragmentation patterns in the gas-phase pyrolysis of some bi- and tri-cyclic sulfolanes related to the 8-thiabicyclo[4.3.0]non-3-ene 8,8-dioxide ring system

Depending upon the degree of ring strain, the thermal breakdown of cis-8-thiabicyclo[4.3.0]non-3-ene 8,8-dioxide 5 and related ring systems in the gas phase follows widely differing pathways. Decomposition of 5 occurs only under forcing conditions, resulting in complete fragmentation of the sulfolane ring to give benzene and toluene, while pyrolysis of the 2,5-bridged analogues 6-8 proceeds by a retro-Diels-Alder reaction at much lower temperatures to give 1,3-dienes and the decomposition products of 3-sulfolene, buta-1,3-diene and SO2. Epoxidation of the double bond in these compounds results in a marked change in their thermal fragmentation behaviour, only SO2 is lost to produce novel divinyl epoxides. The corresponding N-ethoxycarbonylaziridines, formed by photolysis of the unsaturated sulfones in ethyl azidoformate, undergo extensive decomposition on pyrolysis and do not yield any useful products. The saturated sulfone 28 gives the expected octa-1,7-diene upon flash vacuum pyrolysis (FVP), but only under relatively severe conditions. Three isomeric diene sulfones 30-32 have also been examined and show a varied pattern of reactivity under FVP conditions.