6,7-Dihydro-5H-1,2,4-triazolo[3,4-b][1,3,4]thiadiazine Ring Cleavage and Tautomerism of the Products: Experimental and Theoretical Study

Electronic effects of aryl substituents at the C(7)-atom play a crucial role in the cleavage of a six-membered ring of 6,7-dihydro-5H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines. Our experiments show that electron-withdrawing 2-nitrophenyl and 4-nitrophenyl groups promote the N-N-bond cleavage, whereas a phenyl group and a bulky 4,5-dimethoxy-2-nitrophenyl substituent are favored for the C-S-bond cleavage, affording novel triazole derivatives. UV-vis, electrochemical, and dynamic NMR studies as well as DFT calculations of product's tautomeric equilibria were performed.