Tautomerism in hydroxynaphthaldehyde anils and azo analogues: a combined experimental and computational study

The enol imine reversible arrow enaminone tautomerization constants, K-T, and thermodynamic parameters, DeltaH(T) and DeltaS(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, NMe2) and acceptor (CN, NO2) substituents lead to a decreased K-T independent of solvent polarity. In apolar solvents, for all derivatives 1a-1e, the enol imine reversible arrow enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of DeltaG(T) to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated DeltaG(T) values. Solvent effects on 1c are also successfully reproduced by the calculations. Geometric ((ON)-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.