Regioselective Synthesis of 2-Alkenylindoles and 2-Alkenylindole-3-carboxylates through the Cascade Reactions of N-Nitrosoanilines with Propargyl Alcohols

In this paper, a novel and efficient synthesis of 2-alkenylindoles and 2-alkenylindole-3-carboxylates via the cascade reactions of N-nitrosoanilines with propargyl alcohols is presented. Mechanistically, the formation of the title compounds is triggered by a Rh(III)-catalyzed C(sp(2))-H alkenylation of N-nitrosoaniline with propargyl alcohol followed by the simultaneous intramolecular amination/cyclization, NO extrusion, and dehydration. With this method, a variety of diversely substituted 2-alkenylindole derivatives were prepared in good efficiency and exclusive site-selectivity. To the best of our knowledge, this is the first example in which the indoyl and the alkenyl units are formed sequentially in one pot from halide-free substrates through a redox neutral C-H bond activation. In addition, the utility of the 2-alkenylindole products thus obtained was remarkably showcased by their facile transformations into synthetically and biologically significant compounds such as 2-acylindole, benzocarbazole, indoloquinoline, pyridoindolone, cyclopentaindolone, and indenoindolone.