Ring-chain tautomerism in 2-substituted 1,2,3,4-tetrahydroquinazolines A 1H, 13C and 15N NMR study

In this work 32 1,2,3,4-tetrahydroquinazoline derivatives were synthesized by the reaction of 2-aminomethylaniline with aldehydes and ketones and their ring-chain tautomerism studied by 1H, 13C and 15N NMR spectroscopy. The ketone, as well as the alkyl aldehyde, derivatives were found to favor exclusively ring forms, whereas tautomeric equilibria were observed for aryl aldehyde derivatives. For para-phenyl substituted compounds, good linear correlations were found between the Hammett–Brown σ+ parameter and log K (K=[ring]/[chain]) and also between σ+ and δNC (NC nitrogen chemical shift). An excellent correlation was also found between log K and δNC, which shows that the electronic character of the aryl substituent is carried by the conjugation effect to the environment of the nitrogen atom.