Selective Synthesis of Benzo[a]Carbazoles and Indolo[2,1-a]-Isoquinolines via Rh(III)-Catalyzed C-H Functionalizations of 2-Arylindoles with Sulfoxonium Ylides

A highly chem- and regioselective synthesis of diversely substituted benzo[a]carbazoles and indolo[2,1-a]-isoquinolines through Rh(III)-catalyzed cascade reactions of 2-arylindoles with sulfoxonium ylides is presented. To be specific, treatment of 2-arylindoles, 2-arylindole-3-carbaldehydes, 2-arylindole-3-carbonitriles or 2-aryl-3-methylindoles with sulfoxonium ylides under the catalysis of Rh(III) led to the selective formation of 6-aryl/alkyl benzo[a]carbazoles, 5-acylbenzo[a]carbazoles, 6-amino-5-acylbenzo[a]carbazoles or 12-methylindolo[2,1-a]isoquinolines, respectively. Mechanistically, the formation of the title compounds involves a cascade process including metalation of the inert C(sp(2))-H bond, migratory insertion of ylide into the carbon-metal bond via an insitu carbenoid formation, protodemetalation, and condensation. To our knowledge, this is the first example in which -carbonyl sulfoxonium ylides were used as stable carbene precursors and bifunctional C2 synthons to afford benzo[a]carbazoles and indolo[2,1-a]isoquinolines.