Quantum Chemical Study of the Formation Mechanism of 5-Substituted N -Methylthiophen-2-amine and N -Methyl-1 H -pyrrol-2-thiol from Allenylimidothioates

The reaction mechanism for the formation of thiophene and pyrrole rings from N-methylallenylimidothioate with various substituents (R = NMe2, H, OMe, Me) was investigated using quantum chemical methods. Taking NMe2 as an example, we demonstrated that thiophene formation is kinetically favored, while pyrrole formation occurs via its recyclization. The relatively small difference in the activation barriers for thiophene and pyrrole ring formation from N-methylallenylimidothioate (with R = NMe2) allows for the selective synthesis of both products under appropriate experimental conditions. The activation barrier to pyrrole formation for R = OMe, Me, H is prohibitively high under experimental conditions, explaining the absence of pyrrole in the reaction products.