Thermischer Zerfall isomerer Dihydro‐1,3,4‐thiadiazol‐1,1‐dioxide 1)

Thermal Decomposition of Isomeric Dihydro‐1,3,4‐thiadiazole 1,1‐Dioxides 1) After a review of both the thermolysis and the photolysis of 2,5‐dihydro‐1,3,4‐thiadiazoles 3 , containing sulfur of various levels of oxidation, the thermolysis of the isomeric di‐ tert ‐butyl‐dihydrothiadiazole 1,1‐dioxides cis ‐ 6 , trans ‐ 6 , and 17 is described. Loss of sulfur dioxide and formation of the azine 16 from cis ‐ 6 occur smoothly already at 50°C. In contrast, only above 145°C trans ‐ 6 as well as 17 split into the alkylidenehydrazide 18 and sulfur monoxide which disproportionates into sulfur and sulfur dioxide. A concerted [4 + 1]cycloelimination of sulfur dioxide from trans ‐ 6 is avoided because of unfavourable steric interactions. Since on the linear cheletropic path, decomposition had to proceed in the disrotatory way the result would be the sterically impossible ( E , Z , s ‐ cis )‐conformation of the azine 16 .