Dimroth rearrangement in the thiadiazole—triazole system

Dimroth rearrangement in a thiadiazole—triazole system was first observed by Kindt—Larsen and Pedersen [1] when 5-mercapto-l-phenyl-l,2,3-triazole was formed from 5-anilino-l,2,3-thiadiazole. A similar rearrangement has been ob­ served to take place with 4-substituted 5-amino-l,2,3-thiadiazoles [2] and 4-acyl-5-arylamino-l,2,3-thiadiazoles [3]. In the present work the kinetics of the rearrangement of substituted 5-amino-l,2,3-thiadiazoles to 1,2,3-triazoles in piperidine or its methanolic solu­ tion has been monitored under the conditions of a pseudomonomolecular reaction (excess of piperidine). The rate of the rearrangement was followed by the u.v. spectroscopy in the range 35—60°C. Below 30°C the rearrangement was a slow reaction. The course of the reaction was deduced from the decrease of the extinction of the corresponding 1,2,3-thiadiazole in the range ( — 300—340 nm) where the formed triazole shows no absorption maximum. The rearrangement was studied on two types of compounds: one of them was the conversion of 5-(R-amino)-l,2,3-thiadiazoles to l-R-5-mercapto-l,2,3-triazoles and the other was that of 4-methyl-5-(R-amino)-l,2,3-thiadiazoles to l-R-4-methyl-5-mercapto-l,2,3-triazoles. The objective of the kinetic measurements was to ascertain whether the substituents at the position C-4 of the benzene ring affect the rate of the rearrangement i.e. the rate of the cleavage of the 1,2,3-thiadiazole ring (the S—N bond) and, based on the found data, to discuss the mechanism of the process. The