Tautomerism in Hydroxynaphthaldehyde Anils and Azo Analogues: a Combined Experimental and Computational Study

The enol imine ⇄ enaminone tautomerization constants, K T, and thermodynamic parameters, Δ H T and Δ S T, of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the equilibrium toward the quinone form (for the unsubstituted derivative 1c, K T = 0.20 (cyclohexane) and K T = 1.49 (ethanol)). Both donor (MeO, NMe 2 ) and acceptor (CN, NO 2 ) substituents lead to a decreased K T independent of solvent polarity. In apolar solvents, for all derivatives 1a − 1e, the enol imine ⇄ enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of Δ G T to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated Δ G T values. Solvent effects on 1c are also successfully reproduced by the calculations. Geometric (O···N distance) and energetic criteria (conformer energy differences, homodesmotic reactions) establish the importance of intramolecular hydrogen bonding for the tautomerism of these compounds. The results obtained for 1a − 1e are compared with tautomeric properties of the isomeric naphthaldehyde anils 2 − 4, the monocyclic analogues 5 and 6, the corresponding azo derivatives 7 − 9, and the N -alkyl derivative 10 .