Twenty-one a-amino aza heterocycles were reacted with (ethoxymethylene)malononitrile. UV and 1H and 13C NMR studies indicated that in solution the structures of the condensation products can be described as chain and ring tautomers. Investigations were also made of the solvent and temperature dependence of the position of equilibrium of the ring and chain tautomer forms and the effect of protonation on the ring-chain tautomerism of the 2-aminopyridine derivative. The proportion of the ring form is increased by the presence of a substituent in position 2 of the pyridopyrimidine skeleton type ring tautomer and also by electron-donating substituents in position 7 or 8, while electron-accepting groups increase the content of the chain tautomer. A substituent in position 6 sterically favors the chain form, while substituent in position 9 influences the equilibrium between the ring and chain tautomer forms through its electronic and steric properties and its hydrogen bond forming ability. The 1-aminoisoquinoline derivative is present in ring form in both CDC13 and Me2SO-d6. The derivatives of 2-aminoquinoline, 3-aminoisoquinoline, 2-aminopyrimidine, and 2-aminopyrazine predominantly exist as chain tautomers. The derivatives of 7r-excess five-membered heterocycles—of 2-aminothiazole, 3-aminopyrazole, 2-aminobenzthiazole, and 2-aminobenzimidazoles—favor the ring form. In the case of 3-aminopyrazole, the chain and ring forms could be separated.
ASJC Scopus subject areas
- Organic Chemistry