Reactions of 1,2,3-dithiazoles with halogenated malononitriles

Abstract

Dibromomalononitrile 3a reacts with 4-chloro-1,2,3-dithiazole-5-thione 9 and monobromomalononitrile 3c reacts with 4,5-dichloro-1,2,3-dithiazolium chloride 4 (Appel salt) to give 4-chloro-5H-1,2,3-dithiazole-5-ylidenemalononitrile 1 in 76 and 73% yields, respectively, thus providing the best available synthesis of this key intermediate. The reactions were accompanied by the formation of 1-(4-chloro-5H-1,2,3-dithiazol-5-ylideneamino)-1,2-dihaloethene-2-carbonitriles 5, the yields of which increased with reaction temperature. In refluxing toluene, dibromomalononitrile 3a and dithiazolethione 9 give directly 3-bromoisothiazole-4,5-dicarbonitrile 12 (59%); the probable intermediate (dicyanomethylene)dithiazole 1 is readily converted into bromoisothiazole 12 on treatment with anhydrous gaseous HBr at ca. 20 °C (83%). The addition of bromine to dithiazolethione 9 gives 5-bromosulfanyl-4-chloro-1,2,3-dithiazolium bromide 11 almost quantitatively. Mechanisms are proposed for all these reactions.