A straight access to functionalized carbazoles by tandem reaction between indole and nitrobutadienes

As a continuation of our research on the synthetic exploitation of the nitrobutadienic building-blocks obtained from the ring-opening of nitrothiophenes, we herein report about their reaction with the π-nucleophilic indole. Thanks to their double Michael-acceptor nature, 2,3-dinitro and 2-nitro-3-phenylsulfonyl substituted 1,3-butadienes produce poly-functionalized carbazoles through a double (inter-+intra-molecular) conjugate addition, followed by aromatization of the newly built ring. Significance is attached to the results obtained in fluorinated solvents such as trifluoroethanol, whereby a mild process, with no need for catalysis, overcomes some practical difficulties otherwise limiting the scope of the reaction. Besides the mechanistic aspects, the reaction encompasses motifs for a synthetic interest, mainly in the field of further-tunable arylcarbazoles endowed with predictable applicative properties, e.g., as fluorescent devices.