Conversion Of A 1,2,3-Dithiazole Into A 3h-Pyrrole-3-Thione And A 3h-Pyrrol-3-Ylidenephosphorane

Treatment of the readily available dicyanomethylenedithiazole 1 with excess of morpholine or triphenylphosphine gives the 3-azacyclopentadienethione (3H-pyrrole-3-thione) 5 and the 3-azacyclopentadienylphosphorane (3H-pyrrol-3-ylidenephosphorane) 16 respectively. Both products are deeply coloured and highly stabilised by extensive electron delocalisation: H-1 and C-13 NMR spectra show that rotation of the morpholine groups in 5 is hindered. Structure 16, the first azacyclopentadienylphosphorane reported, is proved by X-ray crystallography. Mechanisms are reported for these transformations in which the initial step is considered to be opening of the dithiazole ring of 1 by nucleophilic attack by the amine or the phosphine at the central heteroatom.