A general and efficient synthesis of 4,9-dihydro-
1H-carbazoles from 3-allenylmethylindoles is reported. The
process, catalyzed by a cationic gold(I) complex, involves a
formal C2−H bond activation of the indole unit by reaction
with the allene. The nature of the substituents at the allylic and
terminal positions of the allene moiety has a crucial effect on
the regioselectivity of the cyclization, which is also influenced by the catalyst and the solvent employed. Moreover, some evidence
of the contribution of different reaction routes is provided, which led us to propose a plausible multipathway mechanism
consistent with all of the results described.
