Thermal Isomerization of 6π-Electron Five-Membered Aromatic Heterocycles C4H3FX (X = NH, O, S)

Abstract Possible steric structures of transition states in thermal isomerization of five-membered aromatic heterocycles C 4 H 3 FX (X = NH, O, S) in an inert atmosphere have been simulated on the basis of analysis of isospectral molecular graphs. The formation of transition states is ensured by stabilization effect of the fragmented parts of the initial conjugated π-electron system. The geometric parameters and energies of the ground and transition states of C 4 H 3 FX (X = NH, O, S) have been calculated by the DFT/B3LYP/6-31G( d ) method. Possible thermal isomerization pathways of 2-fluoro-1 H -pyrrole, 2-fluorofuran, and 2-fluorothiophene molecules have been proposed, and mechanisms of [2,3]-rearrangement of substituents in these heterocycles have been established. Thermal [2,3]-rearrangements of 2-fluoro-1 H -pyrrole, 2-fluorothiophene, and 2-fluoro­furan involve intermediate formation of nonaromatic isomers belonging to the Dewar benzene type.