Cyclization of alk-4-ynals with o-diaminoarenes as a selective one-pot synthesis of arylmethylidene-substituted 2,3-dihydro-1H-pyrrolo[1,2-a] benzimidazoles and 7,8-dihydro-6H-pyrrolo[1′,2′:1,2] imidazo[4,5-b] pyridines

A one-pot synthesis of arylmethylidene-substituted 2,3-dihydro-1H-pyrrolo[1,2-a]-benzimidazoles based on the reaction of alk-4-ynals with 1,2-diaminobenzenes in DMSO at sequential catalysis with NH4Br and bases was suggested. The use in these processes of KOH as a base led to the selective formation of E-isomers of the final products in 48-66% yields, whereas less basic K2CO3 gave the corresponding Z-isomers in 32-82% yields. Similar cycliza-tion reactions involving 2,3-diaminopyridine gave 7,8-dihydro-6H-pyrrolo[1 ',2 ' : 1,2] imid-azo[4,5-b] pyridines in up to 56% yields. The distinguishing feature of the processes under study is proceeding of their key step that is intramolecular hydroamination of the triple bond, as a 5-exo-dig-cyclization. The necessity for the application of drastic conditions required for these processes is in agreement with the results of quantum chemical calculations of PES of their most probable rate-determining step, namely, the cyclization of the corresponding benzimid-azolide anions to cyclic vinyl anions.