A Quantum Chemical Study on the Mechanism of Base‐Mediated Cyclodimerization of Benzoylethynylpyrrole

Abstract Possible mechanisms of cyclodimerization of benzoylethynyl pyrrole to give 2,2′‐(dipyrrolo[1,2‐a:1′,2′‐d]pyrazine‐5,10‐diylidene)bis(1‐phenylethanone) have been investigated by using a DFT B2PLYP‐D3/6–311+G**//B3LYP‐D3/6–31+G*+PCM/MeCN approach. This study shows that the cyclodimerization induced by 1‐methylimidazole, and that observed in the presence of strong bases (NaOH or KOH) occur by different mechanisms. The reaction pathway involving 1‐methylimidazole, which starts with the formation of a zwitterionic adduct, was compared with the cyclization pathway as expected by analogy with the acylethynyl pyrrole reaction with 1‐pyrroline. The C─N bond formed by the attack of the acetylene triple bond with the 1‐methylimidazole molecule is maintained throughout the reaction, and the elimination of the imidazole molecule is eliminated only in the final step after pyrazine cycle formation. The formation pathways for the Z , Z ‐, Z , E ‐, and E , E ‐isomers of the product were analyzed. An explanation for the low yield of the final product in the presence of strong bases is proposed and the possible pathways for oligomerization of the initial benzoylethynyl pyrrole are considered.