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

• Published in: ChemistrySelect (Weinheim) 2020-03, Vol.5 (12), p.3586-3592
• Main Authors: Kolodina, Alexandra A., Tsaturyan, Arshak A., Galkina, Maria S., Borodkina, Inna G., Vetrova, Elena V., Demidov, Oleg P., Berezhnaya, Alexandra G., Metelitsa, Anatoly V.
• Format: Article
• Language: English
• Subjects: aryl benzyl ketones; cleavage reactions; solvatochromism; tautomerism; triazolothiadiazine
• ISSN: 2365-6549; 2365-6549
• DOI: 10.1002/slct.202000732

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. The cleavage mechanism of thiadiazine cycle, which is formed as intermediate in the reactions of coupling of aromatic aldehydes with benzyl halides via 4‐Amino‐4H‐1,2,4‐triazole‐3‐thiol, is described. The dependence of the reaction pathway on the nature of aryl substituents is established. The correlation of tautomerism and solvatochromism of products is determined.