Tautomerism anils

Photochromism Based on Tautomerism. Several substituted anils of saHcylaldehydes are photochromic but only in the crystalline state. The photochromic mechanism involves a proton transfer and geometric isomerization (21). An example of a photochromic anil is /V-sa1icylidene-2-ch1oToani1ine [3172-42-7] C H qCINO. 

A particular effect of an intramolecular hydrogen bond occurs in /V-salicylideneaniline derivatives119 and anils of hydroxynaphthaldehydes120, and their tautomerism (shown... 

Intensive investigations have been conducted to elucidate the nature of the mechanism of thermochromism of salicylidene Schiff bases. Different techniques or methods suitable for the study of the tautomeric equilibrium between the end form and the (Z)-keto form have been used, including X-ray diffraction, NMR, infrared (IR) and Raman spectroscopy, and theoretical calculations. In the anil-... 

Grignard reagents do not add to ketimines in the manner described above.234 Aliphatic and alkyl aryl ketone anils react as if they existed in the tautomeric enamine form ... 

The formation of the diene precursor from the anil above requires a tautomerization step. Such additions following tautomerization are not unique as seen in the reaction of crotonaldehyde with naphthoquinone and dimerization of crotonaldehyde each forming a carbocyclic ring. In such a case, an acid or base catalyst should be present. Berner obtained a 40% yield... 

Aldehydes and ketones react with primary amines to form azomethines which are usually known as Schiff bases, or sometimes, if the amine is aromatic, as anils. Stable Schiff bases are formed with aromatic aldehydes and with aliphatic and aromatic ketones, those formed from ahphatic aldehydes are often subject to aldol-type polymerization and are not suitable for group protection. The Schiff bases formed from aliphatic ketones are potentially tautomeric with the corresponding enamines, but they exist as azomethines unless there is some other structural feature present to stabilize the enamine form (section 2.1.3.2). The condensation reaction by which these derivatives are formed is acid-catalyzed and easily reversible, thus this method of oup protection is only applicable under neutral or alkaline conditions. The condensation using aromatic aldehydes or aliphatic ketones take place readily without solvent or in refluxing ethanol, those with aryl-alkyl or diaryl ketones may require catalysis or azeotropic removal of the water formed in the reaction. 

Ledbette JW (1966) Spectroscopic evidence for enol imine-keto enamine tautomerism of N-(0- and P-hydroxybenzylidene) anils in solution. J Phys Chem 70 2245... 

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