Gold-Catalyzed Synthesis of 1-(Indol-3-yl)carbazoles: Selective 1,2-Alkyl vs 1,2-Vinyl Migration

Gold(III)-catalyzed cycloisomerization of α-bis(indol-3-yl)methyl alkynols selectively affords 1-(indol-3-yl)carbazoles, in a transformation that takes place through a selective 1,2-alkyl vs 1,2-vinyl migration step in the vinyl-gold intermediate generated from the initial 5-endo-spirocyclization. The reaction proceeds well with either tertiary or secondary starting alkynols as well as with a wide variety of alkyne substituents. The key role of the other indol-3-yl substituent for the unexpected selectivity in the 1,2 rearrangement has also been supported by DFT calculations that reveal a low barrier, two-step mechanism in the alkyl migration path where the second indole significantly stabilizes a carbocationic intermediate.