Structure and synthesis of the metribuzin blue dye. Synthesis and reactions of 2,5-dihydrazino-1,3,4-thiadiazole and its derivatives

A blue pigment which sometimes accompanies the manufacture of the triazine herbicide metribuzin ( 5 ) was identified as the novel thiadiazole formazan 10 which arises from the reaction of two impurities 3 and 7 . The latter compound is a keto acid derivative of 2,4-dihydrazinothiadiazole which in turn is an impurity in carbonothioic dihydrazide. Compound 7 has the interesting property of undergoing a facile oxidative decarboxylation in the presence of alcohols to form hydrazonate esters and in the presence of aliphatic amines to form red amidrazone dyes 9 . The N-substituted dyes can be oxidized to ditriazolothiadiazoles 12 .