Thermal Isomerization of 6 π-Electron Five-Membered Aromatic heterocycles CHFX (X = NH, O, S)

Based on the analysis of isospectral molecular graphs, possible spatial structures of the transition states of thermal isomerization reactions of CHFX (X = NH, O, S) molecules in an inert atmosphere were determined. The formation of transition states of aromatic systems is provided by the stabilization effect of fragmented parts of the initial conjugated π-electron system. The structural and energetic characteristics of the ground and transition states of CHFX (X = NH, O, S) molecules were calculated using the DFT/B3LYP/6-31G(d) method. The proposed pathways of thermal isomerization of 2-fluoropyrrole, 2-fluorofuran, and 2-fluorothiophene molecules were constructed, and the mechanisms of [2,3]-substituent rearrangements in the considered heterocycles were determined. The studies showed that thermal [2,3] rearrangement reactions of 2-fluoropyrrole, 2-fluorothiophene, and 2-fluorofuran proceed via the formation of non-aromatic isomers belonging to the Dewar benzene structural type.