Abstract
The case of extremely slow (on the NMR time scale) annular prototropy rare in azole systems has been observed. According to 1H NMR spectral data, both NH tautomers are observed in solutions of 6(7)-formyl- and 6(7)-acetyl-2-trifluoromethylperimidines in CDCl3 and C6D6 at room temperature and even on heating to 60-70°C. The rate of interconversion of the tautomers in DMSO-d6 and CD3CN is sufficiently great that only averaged signals are observed in their 1H NMR spectra. The main reason of the slow tautomerism is the cooperative effect of the 2-CF3 group and the carbonyl substituent which decreases the basicity of the pyridine hetero atom to such an extent that a transfer of a proton between molecules in a nonpolar medium becomes strongly hindered.
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