Hetrocycles from nitrile imines. Part III. Substituent effect on ring-chain tautomerism of 1,2,3,4-tetrahydro-s-tetrazines

Selected series of 6-substituted 4-aryl-3-heteroaryl-1-methyl-1,2,3,4-tetrahydro-s-tetrazines (V;1-12) were synthesized via direct interaction between appropriate nitrile imine and model methylhydrazones. The extent of "ring-chain" tautomerism, as evidenced from nmr spectral data of V, is dependent on the nature of substituents at N-4, C-3, and C-6. The results reveal that: (i) The concentration of the cyclic tautomer tends to decrease as the basicity of the arylated N-4 decreases, (ii) For C-3 substituents, the cyclic tautomer ratio decreases in the order: phenyl > pyridyl > furyl > thienyl. (iii) For C-6 substituents, the cyclic ratio decreases in the order: C₆H₅> CO₂Me > COMe. These trends are interpreted in terms of conjugative and electronic effects.