Conversion of a 1,2,3-dithiazole into a 3H-pyrrole-3-thione and a 3H-pyrrol-3-ylidenephosphorane

Abstract

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: ¹H and ¹³C 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.