Influence of the chalcogenocarbonyl group on the structure of heterocyclic analogues of β-tricarbonyl compounds. Synthesis and structural features of Schiff bases derived from 3-formyl-4-thio(seleno)coumarin

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.