Abstract
The equilibrium between Z and E isomers of Schiff bases derived from 3-formyl-4-thio(seleno)coumarin 1, 2 resulting from the hindered rotation around the exocyclic carbon–carbon bond in the stable ketoamine tautomeric form has been studied by means of IR and 1D and 2D (NOESY)1H NMR spectroscopy and ab initio and semiempirical calculations. The ratio between the Z and E forms in solution was found to lie in the range 3.5:1–5.8:1 (XS) and 2.4:1–3.1:1 (XSe). The activation energy for Z⇌E interconversion was estimated for 1a and 2a using dynamic 1H NMR spectroscopy to be 45.7 and 35.3 kJ mol–1, respectively. The process was found to be catalysed by trifluoroacetic acid. The relative stability of both tautomeric and conformational isomers calculated theoretically at MP2/6–31G* level for the model molecules of 3-methylimino-4-hydroxy(thio, seleno)-2-pyrone is in good agreement with experimental data. Estimated activation barriers for the rotational mechanism are higher than experimentally obtained which may indicate a contribution from acid-promoted reaction pathways.
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