Chemistry of 4-dicyanomethylene-1,2,6-thiadiazines

Abstract

The chlorine atoms in 3,5-dichloro-4-dicyanomethylene-1,2,6-thiadiazine 1 are readily displaced by thiophenols in the presence of Hünig’s base, the first at −78 °C and the second at 20 °C to give the orange mono- and bis-arylthio derivatives in high yield (Table 1). Similarly secondary amines give the red mono- and blue bis-amino compounds in high yield (Table 2); piperidine, for example, gives the former at −78 °C and the latter at −30 °C. A low limit of reactivity is reached with diisopropylamine which gives the mono derivative only, in low yield (30%). Reactions with ammonia and with primary amines are complex since the amines formed can cyclise onto the adjacent cyano group, aniline giving only a low yield of the pyrrolo[2,3-c][1,2,6]thiadiazine 8. The 4-dicyanomethylene compound 1 is more reactive than the known 4-oxo analogue 2, but can suffer the added complications of cyano group cyclisation and hydrolysis of dicyanomethylene to the keto group. 3,5-Dimorpholino-4-dicyanomethylene-1,2,6-thiadiazine 7c is oxidized to the sulfoxide 9 with MCPBA or N₂O₄, and 9 reverts to 7c with triphenylphosphine–tetrachloromethane, all in high yield. Thiadiazine 1 undergoes a complex reaction with DMSO at 20 °C to give the three furo[2,3-c][1,2,6]thiadiazines 10, 11 and (tentatively) 12 which could all arise from the initial product 13 of displacement of chlorine by DMSO, by cyclisation and sulfoxide type rearrangements. Mechanisms are proposed for all new reactions.