Quinolines, hydroxy-, tautomerism

Attempts have been made to deduce the structure of the predominant form of a potentially tautomeric compound from the shifts which occur in the ultraviolet spectrum of the compound in question on passing from neutral to basic or acidic solutions. The fact that no bathochromic shifts were observed for 2- and 4-hydroxy quinoline and 1-hydroxyisoquinoline under these conditions was taken as evidence that they existed in the oxo form [similar work on substituted quinol-4-ones led to no definite conclusions ]. A knowledge of the dissociation constants is essential to studies of this type, and the conclusions can, in any case, be only very tentative. A further dif-... 

The infrared spectra of 2- and 4-hydroxy quinoline 1-oxide have been interpreted to favor forms 55 and 56,. respectively. Lehm-stedt has discussed the tautomerism of 10-hydroxyacridone (57 58) but reached no definite conclusions. Igeta investigated the... 

Nuvole et al. stated that the cyclization of 974 and its desmethyl derivative gave tautomeric 9-hydroxy-1,2,3-triazolo[4,5-/]quinoline-8-carboxyl-ates (e.g., 975) or 9-oxo-6,9-dihydro-l,2,3-triazolo[4,5-/]quinoline-8-car-boxylates (e.g., 976, depending on the cyclization conditions (in xylene on the action of polyphosphoric acid (PPA) vs. in DMF on the action of PPA and in Dowtherm A under boiling vs. in polyphosphate (89FES609, 89FES619). 

Vasudevan, D. Dorley, P.J. Zhuang, X. (2001) Adsorption of hydroxy pyridines and quinolines at the metal oxide-water interface Role of tautomeric equilibrium. Environ. Sci. 

Derivatives of Quinoline.—(i) Hydroxy quinolines. These are exactly analogous to the hydroxy pyridines and like them are assigned tautomeric formulas. 

All bands in the IR spectra of several substituted 1,5-naphthyridines have been assigned the spectrum of 1,5-naphthyridine 1 was compared with the spectrum of 5-substituted quinoline (1963MI1). IR spectroscopy was used to study the keto-enol tautomerism of 4-hydroxy-1,5-naphthyridine in solution and in the solid state it was found that the keto form predominates in polar solvent and the enol form in nonpolar solvent (1967MI2). 

Figure 13.5 Tautomerization vs. proton transfer in hydroxy-quinolines and their N-Oxides.

The monodentate function in 2-hydroxypyridines is commonly described by O-coordination in the pyridinol tautomeric form 21 or N-coordination in the pyridinone tautomer 22. The N,0-chelation is observed in both pyridinol 23 and pyridinone 24. Bridging function of 2-pyridones can be of two principal types 25 and 26 (95CCR313, 97ACR89). The bridging function 27 in 4-hydroxypyridine derivatives is quite common. A similar situation is observed in hydroxy quinoline analogues. 

In general, these properties of hydroxy substituted derivatives will be present in pi-deficient nitrogen heterocycles with benzo fusion or with multinitrogen substitution. For example, the 4-OH group on quinoline tautomerizes to the 4-quinolone structure (Scheme 6.36). 

The energetics of this tautomerism is affected by substituents on and especially within the arene ring. An important class of hgands, the 2-hydroxy pyridines and quinolines, exemplify this (Eq. 6.2). 

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