Solution and solid state structure of highly delocalised 5-triphenylphosphoranylidene-cyclopentene-3,4-diones

The fully assigned C-13 NMR spectra of the cyclic ylides 4 and 5 show that the C-1-C-2 bond is very strongly polarised consistent with substantial delocalisation of the ylide negative charge on to C-2. The X-ray structure of 5 confirms the occurrence of extensive delocalisation involving a significant contribution from up to six separate resonance forms.</p