Ring-chain tautomerism with participation of pyridine nitrogen: The intramolecular cyclization of 2-pyridinecarboxaldehyde–indandione adducts in acidic medium

• 2-Pyridine-carbaldehyde derivatives of 1,3-indandiones in acidic medium were studied.
• Prototropic ring-chain tautomerism with participation of pyridine nitrogen is found.
• A good agreement is found between experimental (NMR) and theoretical (DFT) results.
• Tautomerization mechanism is suggested.

Keto-enol tautomerism of 2-pyridine carboxaldehyde adducts with ring-substituted 1,3-indandione derivatives observed in neutral solutions, in the presence of trifluoroacetic acid (TFA) is changed by the previously unknown prototropic ring-chain tautomerism with reversible quaternization of pyridine nitrogen. The proposed mechanism of tautomerization includes intramolecular proton transfer from the protonated nitrogen to indandione carbonyl oxygen, with subsequent cyclization of the unstable O-protonated intermediate. Neutralization of TFA leads to recovery of the keto-enol tautomers.

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