Application of 13C NMR to the study of the tautomerism of some disymmetric triazenes: The 3‐aryl‐1‐(3,4‐dimentyl‐5‐isoxazolyl)triazenes

The 13C NMR spectra of some 1,3‐diaryltriazenes, 3(1)‐aryl‐1(3)‐hetaryltriazenes and their methylated derovatives have been recorded. From the comparison of the 13C chemical shifts between 3‐aryl‐1‐(3,4‐dimethyl‐5‐isoxazolyl)triazenes 1 and compounds with fixed structure, such as 1‐aryl‐3,3‐dimentyltriazenes and methylated derivatives of 5‐amino‐3,4‐dimethylisxazole, we found that the triazenes 1 have the sturcture (A) in which the azo group is conjugated with the heterocycllic moiety. para‐Substituent on the aromatic ring have a week influence on the displacement of this tautomeric equilibrium. 13C NMR gives only qualitatives results. Theoretical calculation (HMO, CNDO‐2) are also consistent with the greater stability of tautomeric form A.