NHC as the Guiding Factor in a Copper-Catalyzed Intramolecular C Arylation of Pyrrolylimidazolium Salts: Synthesis of Luminescent Heterotetracyclic Frameworks
2016-12-21T21:29:26Z (GMT) by
3-(2/4-(2-Bromophenyl)-1<i>H</i>-pyrrol-3-yl)-1<i>H</i>-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 7<i>H</i>-imidazo[2,1-<i>a</i>]pyrrolo[3,2-<i>c</i>]isoquinoline and 1<i>H</i>-imidazo[2,1-<i>a</i>]pyrrolo[3,4-<i>c</i>]isoquinoline heterocyclic skeletons, which were further transformed into thermodynamically more stable 1<i>H</i>- and 6<i>H</i>-tautomers, respectively, by removing of the benzyl-PG. The new heterotetracyclic systems are fluorescent in solutions with high quantum yields.