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

The reaction mechanism for the formation of thiophene and pyrrole rings from <i>N</i>-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 <i>N</i>-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.