INFLUENCE OF THE CHALCOGENOCARBONYL GROUP ON THE STRUCTURE OF HETEROCYCLIC-ANALOGS OF BETA-TRICARBONYL COMPOUNDS - SYNTHESIS AND STRUCTURALFEATURES OF SCHIFF-BASES DERIVED FROM 3-FORMYL-4-THIO(SELENO)COUMARIN

The equilibrium between Z and E isomers of Schiff bases derived from 3 -formyl-4-thio(seleno)coumarin 1, 2 resulting from the hindered rotati on around the exocyclic carbon-carbon bond in the stable ketoamine tau tomeric form has been studied by means of IR and 1D and 2D (NOESY) H-1 NMR spectroscopy and ab initio and semiempirical calculations. The ra tio between the Z and E forms in solution was found to lie in the rang e 3.5:1-5.8:1 (X=S) and 2.4:1-3.1:1 (X=Se). The activation energy for Z half arrow right over half arrow left E interconversion was estimate d for 1a and 2a using dynamic H-1 NMR spectroscopy to be 45.7 and 35.3 kJ mol-1, respectively. The process was found to be catalysed by trif luoroacetic acid. The relative stability of both tautomeric and confor mational 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 w hich may indicate a contribution from acid-promoted reaction pathways.