Hammett correlations triazoles

The role of substituents X on the mononuclear heterocyclic rearrangement (MHR) of 20 phenylhydrazones 54 of 3-benzoyl-5-phenyl-l,2,4-oxadiazole into the triazoles 55 (Equation 2) has been investigated, allowing the influence of X on the product distribution to be evaluated and first-order rate constants and Hammett correlations to be determined <1999T12885>. 

The effects of the 1,2,3-triazole and 1-amino-1,2,3-triazole heterocycles on the chemical shift of the carbon atoms in the phenyl ring of compounds (40) and (41) (X = H) are compared with the value for benzene. With regard to the effects of substituent X para and meta) on the 1,2,3-triazole carbons, C(4) is deshielded by electron-withdrawing groups, whereas C(5) is shielded. Reasonable correlations with Hammett a values are observed for the chemical shifts of both C(4) and C(5) of (41) <86MRC53>. The chemical shifts of C(4) and C(5) in 5-amino-l-aryl-l,2,3-triazoles are shifted upheld by 2-5 ppm compared with the corresponding 5-anilino-1,2,3-triazoles. In 5-anilino-... 

Pevzner et al. widely used NMR spectroscopy in their structural investigations of diverse 1,2,4-triazole nitro derivatives [566-581], In the series of 5-substituted l-methyl-3-nitro-1,2,4-triazoles the correlation between the A-methyl group proton chemical shifts induced by 5-substituents (A8) and the substituent Hammett constants has been found to divide into two branches [577], This nonordinal event is explained by impossibility of any additional contribution to the shielding of substituents having two and more lone electron pairs [577],... 

The energies of activation increase from about 21.5 to 34 kcal.mole and the heats of reaction vary from —11.84 kcal.mole" to about +2.20 kcal.mole" For aryl substituted amino-1,2,3-triazoles there is a good correlation between the logarithm of the rates of isomerization and Hammett s a value for substituents. 

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