Schiffs bases, keto-enol tautomerism

The various tautomers and rotamers of alloxan have been examined in detail by the MNDO method and it is predicted that the keto form is most important in the gas phase, although in solution the monohydroxy forms are also thought to contribute. A mass spectral study has been used to investigate the enol-keto tautomeric equilibria of a series of substituted salicylaldehyde and 2-hydroxynaphthaldehyde Schiff bases. In neutral, ethanolic solutions, the cis- and trans-tm forms of 4,5-dimethyl-2-(2 -hydroxyphenyl)imidazoles (393) and (394) have been found to exist in equilibrium in the ground state. However, in neutral aqueous solutions, the trans-eao and keto forms (394) and (395) were the only species detected. Deuterium isotope effects on... 

Thus, it was shown that the tautomerization may be of importance in the thermal dehalogenation of chloroaromatics14. The formation of diarylamines in this reaction can be also explained by the presence of aniline tautomers in the reaction mixtures14. One of the well-known types of aniline tautomerism is the enol-imine-keto-amine equilibrium. In effect, it is a reversible conversion of Schiff bases into enamines that is frequently employed for the synthesis of photochromic compounds, as well as of mesomorphic... 

The equilibrium in the case of Schiff bases prepared from salicylaldehyde and 2-amino-, 2,3-diamino-, 2,6-diamino- and 3-aminomethylpyridine was studied by means of NMR, UV and IR spectroscopy and X-ray crystallography It was shown that the enol-imines were the predominant form in non-polar solvents, whereas in polar solvents a rapid tautomeric interconversion between the enol-imines and the keto-enamines as well as a slow hydrolysis were observed. The tendency to tautomeric interconversion was significant for the 2-(3-pyridylmethyliminomethyl)phenol 45 while in the case of other Schiff bases it was very low. 

Nuclear magnetic resonance studies were also made of Schiff bases obtained from amines and aliphatic /9-diketones. These compounds can exist in any of three tautomeric forms, the keto-imine (10), the keto-enamine (11) and the enol-imine (12) . It was found that in... 

Tautomerism is an important phenomenon and an example of this process is the equilibrium in Schiff bases. These bases can exist as an equilibrium between two species, keto — enol [Scheme 1]. This process is affected by a number of factors such as... 

A. (2006) Enol-keto tautomerism of aromatic photochromic Schiff base N,N -bis(salicylidene)-/i-phenylenediamine ground state equilibrium and excited state deactivation studied by solva-tochromic measurements on ultrafast time scale. /. Chem. Phys., 124, 124518. 

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