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

Abstract

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 cyclization reactions involving 2,3-diaminopyridine gave 7,8-dihydro-6H-pyrrolo[1’,2’:1,2]imidazo[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 benzimidazolide anions to cyclic vinyl anions.