2-Hydroxy five-membered heterocycles, tautomerism

The application of spectroscopic methods to the study of tautom-erism proved especially fruitful. The tautomerism of hydroxy and amino derivatives of isoxazole is of great interest to the chemistry of isoxazole this subject, as well as the tautomerism of functional derivatives of other five-membered heterocycles, has been reviewed by Katritzky and Lagowski. We shall therefore only... 

The presence of a hydroxy-, thio-, or amino-substituent in five-membered heterocycles, leads to various tautomeric forms. In most cases, one of the tautomeric forms is not aromatic and the degree to which the tautomeric form exists depends on the loss in resonance energy. Thus, compounds which are less aromatic would be more prone to tautomerize than the more aromatic ones. 

The reliability of semi-empirical methods (AMI, PM3, and MNDO) for the treatment of tautomeric equilibria has been tested for a series of five-membered nitrogen heterocycles, including 1,2,3-triazole and benzotriazole. The known tendency of MNDO to overestimate the stability of heterocycles with two or more adjacent pyridine-like lone pairs is also present in AMI and to a somewhat lesser extent in PM3. Tautomers with a different number of adjacent pyridine-like nitrogens cannot be adequately treated by these semi-empirical methods. Both AMI and PM3 represent major improvements over MNDO in the case of lactam-lactim tautomerism. The stability of N-oxides as compared to N-hydroxy tautomers is overestimated by PM3 method. All three methods give reliable ionization potentials and dipole moments (90ZN(A)1328). 

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