Aminophenols hydrogen bonds

Interestingly, 2,3-dicarbonyl derivatives formed by the condensation of a-dicarbonyl compounds and 2-aminophenols (80JHC1625) exist in the 2Z/-form (81 Scheme 36). However, when there is an additional carbon atom in the side-chain of the 3-substituent so that hydrogen bonding with the NH group may now occur, as in the benzoxazinone (82), the equilibrium is directed strongly in favour of the 4Z/-tautomer (Scheme 37) (76JHC681). 

It is worth noting that in the 1 1 complex of hydroxy aniline (or aminophenol) with a water molecule, the most stable cluster formed arises from an O—H—O interaction, in which water plays the role of hydrogen bond acceptor275. It is also remarkable that association of ANI with the CF3H molecule yields a weak C—H-N complex, but it induces a blue-shift of the CH stretch frequency276. The latter is blue-shifted by 30 cm-1, and the... 

Infrared spectra of a number of ortfco-aminophenols (and of NHD derivatives) emphasize the importance of the intramolecular hydrogen bond between OH and NH2 groups, as shown in 10. 

Hydroxyquinoline ( oxine ), mp 75°C, contains an intramolecular hydrogen bond and is used as a complexing and precipitating reagent for many metallic ions in analytical chemistry. It is obtained by a Skraup synthesis starting from o-aminophenol. 

Fig. 16. The different styles of packing in the crystalline state of some aminophenols [44-46]. It is shown that p-aminophenol utilizes 0-H---N and N-H---0 hydrogen bonds in crystal packing, while the o- and m-compounds and 2-amino-4-methylphenol also utilize C-H- -O and N-H - 7T interactions...

The position is more complex with derivatives of o- and p-aminophenol. The three reduction waves of o-aminophenol derivatives were attributed to one-electron reduction of quinoid (first wave) and benzenoid (second wave) tautomeric forms of these compounds and to subsequent one-electron reaction of intermediate radical anions (third, total wave). With the p-aminophenol derivatives, reduction of which results in the appearance of two waves on the polarograms, it was assumed that the first wave of the benzenoid form merges with the wave for subsequent reduction of both tautomers. Unfortunately the authors cite no evidence for this tautomer-ism from other methods. In the case of the o-derivatives the intramolecular hydrogen bond could stabilize the benzenoid structure. 

The presence of electron-withdrawing groups positioned on an aromatic nucleus bonded directly to phosphorus can be a source of instability in the phosphorus-carbon bond. This situation is found particularly in (4-nitrophenyl)phosphonic acids. (2-Methoxy-4-nitrophenyl)phosphonic acid can be demethylated in 40% HBr, but cleavage of the carbon-phosphorus bond becomes more pronounced in a reaction with 48% HBr moreover, hydrogenation of the same acid over Raney nickel yields the expected (4-amino-2-methoxyphenyl)phosphonic acid, but a similar reduction of (2-hydroxy-4-nitrophenyl)-phosphonic ac id results in the separation of 3-aminophenol. These and other, similar, reactions have been surveyed by Freedman and Doak ". ... 

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