4-Hydroxy-1-naphthaldehydes: proton transfer or deprotonation.

A series of naphthaldehydes, including a Mannich base, have been investigated by UV-Vis spectroscopy, NMR and theoretical methods to explore their potential tautomerism. In the case of 4-hydroxy-1-naphthaldehyde concentration dependent deprotonation has been detected in methanol and acetonitrile. For 4-hydroxy-3-(piperidin-1-ylmethyl)-1-naphthaldehyde (a Mannich base) an intramolecular proton transfer involving the OH group and the piperidine nitrogen occurs. In acetonitrile the equilibrium is predominantly at the OH-form, whereas in methanol the proton transferred tautomer is the preferred form. In chloroform and toluene, the OH form is completely dominant. Both 4-hydroxy-1-naphthaldehyde and 4-methoxy-1-naphthaldehyde (fixed enol form) show dimerization in the investigated solvents and the crystallographic data, obtained for the latter, confirm the existence of a cyclic dimer.

1 Solid state analysis of 3.
The question arises where the acid function arises from, especially at this relative amount of 40%. Obviously, there is oxidation of the aldehyde by oxygen from air. To verify this, two analyses were carried out. The IR spectrum of the solid of compound 3 evolves over time when the powder is kept in air, and new bands at 2750, 1700, 1640 and 1180 cm -1 appear ( Figure S6). This may well correspond to further H-bonding, C=O and C-O stretching vibrations due to presence of acid. Analysis of the evolution of the X-ray single crystal data over 3 days shows an increase of the relative amount of the acid with respect to the aldehyde in the solid state. This let us to the conclusion that the presence of the acid can be explained by the slow oxidation of the aldehyde in the solid state.     Figure S6. Solid state IR spectra of 3 as it is (blue line) and after 3 days x-ray irradiation (red line). 6 2 Isomers of 1, 2 and 4.
The isomers of 1, 2 and 4 were studied theoretically in respect of the orientation of the C=O group and in respect of the position of the piperidine units in the case of 2. As seen the effect of the solvent as PCM model is negligible in both relative stability of the isomers and barriers of rotation. The preference of the form of 2 with intramolecular hydrogen bonding was proven by NMR spectroscopy. Figure S7. Rotation of the carbonyl group in 1 in gas phase from syn (with respect of the second aromatic ring, the dihedral angle is 0 o ) to anti isomer (the dihedral angle is 180 o ). More details are given in Table S1.  Figure S8. Rotation of the carbonyl group in 2 in gas phase from syn (with respect of the second aromatic ring, the dihedral angle is 0 o ) to anti isomer (the dihedral angle is 180 o ). The lines correspond to the availability of intramolecular hydrogen bonding: solid line -with intramolecular hydrogen bonding (a, the nitrogen atom faces the OH group, from Figure 8); dashes -without intramolecular hydrogen bonding (c-like structure, the nitrogen atom faces the carbonyl group). The curves are shifted by the difference between with and without isomers of 2 (8.08 kcal/mol). More details are given in Table  S2 and the discussion about NMR results below. .72 a C 8a -C 1 -C 9 -O 1 angle = 90 o ; b C 8a -C 1 -C 9 -O 1 angle = 270 o . Figure S9. Partial 1 H-1 H NOESY spectrum of 2 in CDCl 3 .
The steric preferences of the ligand 2 were additionally studied by NOESY NMR experiment ( Figure  S9). From one side, clean interactions between the singlet for CH-2 and spacer CH 2 (C10) were detected, while no interactions with CH 2 -N groups of piperidine were observed (Scheme S2). The latter can be an indication for fixed by H-bonding conformation with piperidine ring on the hydroxyl side. However, it has to be mentioned that piperidine methylene groups appear as broad signals, which can result in absence of interactions in a reasonable time-scale. From the other side, the interactions of aldehyde proton with CH-2 and CH-8 indicate that the rotation barrier of C-CHO bond is very low. Scheme S1. Figure S10. Rotation of the carbonyl group in 4 in gas phase from syn (with respect of the second aromatic ring, the dihedral angle is 13.3 o ) to anti isomer (the dihedral angle is 147.9 o ).
3 Deprotonation of 4. Figure S11. Deprotonation of 4 in gas phase (lines), interacting with acetonitrile in acetonitrile environment (dashes) and methanol molecule in methanol environment (points).