Palladium-Catalyzed Reductive Cross-Coupling Reaction of Aryl Chromium(0) Fischer Carbene Complexes with Aryl Iodides

The
first palladium-catalyzed reductive cross-couplings of aryl chromium(0)
carbene complexes with aryl iodides have been realized. This coupling
reaction shows excellent functional group tolerance and high efficiency.
Mechanistically, aryl chromium(0) carbene complexes undergo transmetalation
with arylpalladium species to generate palladium­(II) carbene intermediates,
which is followed by migratory insertion. The catalytic cycle is then
completed by hydrogen transfer and reductive elimination. Consistent
with the mechanistic hypothesis, density functional theory (DFT) calculations
support the involvement of a palladium carbene intermediate, and carbene
migratory insertion is a facile step with an energy barrier of 5.1
kcal/mol. The carbene transfer step and the hydrogen transfer step
are confirmed as the rate-limiting steps in the catalytic cycle.