Photooxidation of Fulvenes in a Continuous Flow Photoreactor using Carbon Dioxide as a Solvent

A range of 3-substituted oxepinones have been synthesised by the photooxidation of fulvenes with photochemically generated singlet oxygen (¹O₂) using carbon dioxide (CO₂) as a solvent and a CO₂-soluble porphyrin photosensitizer. The reactive intermediates generated in this reaction decompose to yield a range of useful and underutilized building blocks that are not easily accessible using more traditional reaction setups. Fulvenes containing an exocyclic allylic hydrogen are shown to form three products through potentially competing intramolecular pathways, which can be tuned to some extent by adjusting the conditions in the flow reactor. A range of substrates containing a variety of functional groups was examined; this led to lower yields of the desired lactones being observed in the case of compounds containing polar electron-donating groups. We also demonstrate that one of the lactones can be further hydrogenated over a Pd/C catalyst to the unsaturated 7-membered ring in high yield.