Skeletal and Mechanistic Diversity in Ir-Catalyzed Cycloisomerizations of Allene-Tethered Pyrroles and Indoles.

Pyrroles and indoles bearing N-allenyl tethers participate in a variety of iridium-catalyzed cycloisomerization processes initiated by a C-H activation step, to deliver a diversity of synthetically relevant azaheterocyclic products. By appropriate selection of the ancillary ligand and the substitution pattern of the allene, the reactions can diverge from simple intramolecular hydrocarbonations to tandem processes involving intriguing mechanistic issues. Accordingly, a wide range of heterocyclic structures ranging from dihydro-indolizines and pyridoindoles to tetrahydroindolizines, as well as cyclopropane-fused tetrahydroindolizines can be obtained. Moreover, by using chiral ligands, these cascade processes can be carried out in an enantioselective manner. DFT studies provide insights into the underlying mechanisms and justify the observed chemo- regio- and stereoselectivities.