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

Abstract

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.

Toward understanding tautomeric switching in hydroxynaphthaldehydes: Characterization of electronic absorption spectra

“…The theoretically computed spectra neither of ALD-14 nor its dimer can alone account for the experimentally observed spectrum of ALD-14 17 and in particular for its solvent dependence. Therefore, a major chemical change must take place, which leads to the observed new transition at 370 nm.…”

“…17 Both the position and intensity of this band are solvent dependent. In MeOH, there is an additional small shoulder between 360 nm and 400 nm.…”

“…1) behaves somewhat similar to ALD-14. 17 Exceptions are the almost complete quenching of the 320 nm-340 nm range and the related appearance of a strong band at 360 nm, when changing the solvent in the order toluene, ACN and MeOH. The calculated absorption spectra of ALD-14-pip-E in di®erent media is given in Fig.…”

“…Recently, hydroxynaphthalene derivatives have been examined to elucidate their potential use in photo-molecular switching and signaling devices. [15][16][17] However, many of the fundamental issues underlying their electronic, geometric and topological properties are still unresolved, such that directed design is hampered by lack of basic insights. Here, we focus on 4-hydroxy-1-naphthaldehyde (ALD-14) and its derivative 4-hydroxy-3-(piperidin-1-ylmethyl)-1-naphthaldehyde (ALD-14-pip) shown in Fig.…”

“…18. Recently, Manolova et al 17 investigated both ALD-14 and ALD-14-pip by UV-Vis and nuclear magnetic resonance (NMR) spectroscopy as well as by theoretical methods to explore their tautomerism and protonation/deprotonation in polar solvents. In the case of ALD-14, concentration-dependent deprotonation has been detected in methanol and acetonitrile.…”