Ir-Catalyzed Cascade C–H Fusion of Aldoxime Ethers and Heteroarenes: Scope and Mechanisms

A concise synthesis of phenanthridine derivatives is
achieved by
an iridium-catalyzed direct fusion of oxime ethers and heteroarenes,
which is a successful example of a cascade C–H/C–H cross-coupling/cyclization
strategy for polycyclic heteroaromatic synthesis. By subtle tuning
of the reaction conditions, both benzo- and simple five-membered heteroarenes
are suitable substrates under similar but different conditions. The
key is the right choice of a silver salt. The detailed mechanistic
study discloses that the first C–H/C–H cross-coupling
step involves an [IrII]–[IrIV] catalytic
cycle, which needs Ag2O as the oxidant. For the second
cyclization step, a radical process takes control in the reactions
of benzoheteroarenes and Ag2O is required; however, a C–H
cyclization functions in the reactions of simple five-membered heteroarenes
involving an [IrI]–[IrIII] catalytic
cycle, and AgTFA is necessary.