Reaction of tetracyanoethylene with SCl2; new molecular rearrangements

Abstract

The chloride ion catalysed addition of SCl₂ to TCNE gives the 4-dicyanomethylene-1,2,6-thiadiazine 2 (60%) with two minor products (5%) of extensive molecular rearrangement, the pyrimidine 4 and the pyrroloimidazothiadiazine 5. Mechanisms proposed for the formation of all three products involve the interaction of SCl₂ with one cyano group and neighbouring group participation by geminal and vicinal cyano groups. The dicyanomethylene thiadiazine 2 reacts further with SCl₂ to give the pyrrolo-1,2,6-thiadiazine 19. The two chlorine atoms of the pyrroloimidazothiadiazine 5 are successively replaced by pyrrolidine to give the substitution products 20 and 21 in high yield. ¹H and ¹³C NMR spectra show that rotation of the pyrrolidine ring on the thiadiazine is considerably slower than for that on the pyrrole ring, and there is electron delocalisation across the new 14π tricyclic heteroaromatic system.