NHC as the Guiding Factor in a Copper-Catalyzed Intramolecular C Arylation of Pyrrolylimidazolium Salts: Synthesis of Luminescent Heterotetracyclic Frameworks

3-(2/4-(2-Bromophenyl)-1H-pyrrol-3-yl)-1H-imidazol-3-ium bromides
undergo a copper-catalyzed intramolecular
direct C arylation under mild conditions to give new heterocyclic
frameworks. The cyclizations involve the formation of betaines (imidazoliumylpyrrolides)
under basic conditions and the tautomerizaton of the betaines to the
corresponding NHCs, which are the reactive species responsible for
the selectivity of the arylation via the formation of NHC-Cu complexes.
The primary salt arylation products were dehydrohalogenated to obtain
the first representatives of 7H-imidazo­[2,1-a]­pyrrolo­[3,2-c]­isoquinoline and 1H-imidazo­[2,1-a]­pyrrolo­[3,4-c]­isoquinoline heterocyclic skeletons, which were further transformed
into thermodynamically more stable 1H- and 6H-tautomers, respectively, by removing of the benzyl-PG.
The new heterotetracyclic systems are fluorescent in solutions with
high quantum yields.