Self-Assembly of N-Phenyl-2,5-dimethylpyrrole from Acetylene and Aniline in KOH/DMSO and KOBu^t/DMSO Superbase Systems: A Quantum-Chemical Insight

By employing the contemporary B2PLYP(D3)/6-311+G**//B3LYP/6-31+G* method of quantum chemistry, we unraveled the mechanism of a recently discovered cascade assembly of N-phenyl-2,5-dimethylpyrrole from one molecule of aniline and three molecules of acetylene activated by KOH/DMSO and KOBut/DMSO superbase systems. For the established mechanism, we compare and analyze the activity of these two superbases. The reaction is found to be activated by the interaction of aniline with acetylene, and the barrier associated with this interaction turns out to be the limiting one. Another barrier close in value to the limiting one is found along the cascade assembly pathway and relates to ethynylation of N-but-3-yn-2-yl-aniline by the ethynide ion. The cascade reaction turns out to be selective with respect to propyne-allene rearrangements. The alternative mechanism of N-phenyl-2,5-dimethylpyrrole self-assembly, which involves a preliminary stage of acetylene oligomerization, is shown to be kinetically less favorable.