Unexpected formation of benzo[b]pyran-2,4-dione-3-ylides by thermal S_NAr cyclisation of fluorinated α-benzoyl(alkoxycarbonyl)methylenetriphenylphosphoranes

Thermal elimination of Ph₃PO from suitably functionalised 2-oxophosphonium ylides is a well-established and useful method for alkyne synthesis. Using both conventional thermolysis and flash vacuum pyrolysis, it has been used to prepare hundreds of substituted alkynes including many highly fluorinated examples. Whilst using this method to access ethyl pentafluorophenylpropiolate using two alternative regioisomeric ylide precursors, we have found that, while one reacts as expected to give the alkyne product, the other may undergo an unexpected intramolecular S_NAr reaction with loss of ethyl fluoride to give a heterocyclic ylide. The cyclisation is most easily achieved by simply heating the ylides at 250 °C and has been extended to a total of four ylides giving three heterocyclic ylides. The structures of the precursor and product for the key reaction are confirmed by X-ray diffraction.