Thermal rearrangement of 2-iminocoumarin derivatives into 3-(1,3,4-oxadiazol-2-yl) coumarins: experimental and theoretical analysis of substituents' effects
摘要
Coumarin derivatives have garnered a significant interest and found diverse applications last decades. A unique and efficient pathway of their obtaining is rearrangement in the recyclization reaction occurring at a certain temperature, Tr. Here, we aimed at elucidating the role of substituents in the recyclization of coumarin derivatives. To this end, a set of 2-(2-benzoylhydrazineylidene)-2H-chromene-3-carboxamide derivatives forming 3-(1,3,4-oxadiazol-2-yl)-coumarin derivatives was synthesized, differed both in electrophilicity and position of the substituents. It was established that exploiting butyl alcohol as a solvent for the recyclization is optimal. Thermogravimetry analysis, TGA, revealed a significant shift in Tr (within 100 K), both positive and negative depending on the substituents. Quantum-chemical calculations at PM3(d,p) and 6-31G (d) levels of theory allowed establishing changes in polarity of the double bonds involved in the reaction, which is obviously related to the substituents’ electrophilicity and position. The calculations have also shown strong impact of the substituents on the energy barrier of intramolecular rotation around C = C double bond. The mathematical model developed on the basis of the linear elastic theory has justified analytically the relations observed in the calculation data. Merging the data from TGA measurements, quantum-chemical calculations and mathematical modelling, it has been discovered that the substituents can indirectly impact on the recyclization due to affecting steric hindrances for intramolecular rearrangement. Together, the experimental, computational and theoretical approaches were found to constitute an effective means for elucidating the role of substituents and may see increased application in the future.