Mechanism of the oxidative addition of aryl halides to bis-carbene palladium(0) complexes

Bis-N-heterocyclic carbenes Pd-0 complexes, Pd-0(NHC)(2), are efficient catalysts in Heck reactions performed with aryl bromides or chlorides. The Pd-0(NHC)(2) that are not stable are generated in situ from Pd-II precursors PdY2(NHC)(2) (Y = halides) after a chemical reduction. The latter procedure can be mimicked by an electrochemical reduction. The transient Pd-0(NHCBn)(2) is generated by electrochemical reduction of PdY2(NHCBn)(2), and its reactivity in oxidative addition to aryl bromides and chlorides is characterized by the same electrochemical technique with the determination of the rate constants. Pd-0(NHCBn)(2) is found to be more reactive than the mixed complex Pd-0(NHCBn)(PPh3). Both are the reactive species in an associative mechanism. Comparison with the isolated Pd-0(NHCtBu)(2) reveals that Pd-0(NHCBn)(2) is more reactive than Pd-0(NHCtBu)(2) even if the latter reacts via the monocarbene Pd-0(NHCtBu)) in a dissociative mechanism. This suggests that the formation of mono-carbene Pd-0(NHC) is not a guarantee for a fast oxidative addition because it is always generated at low concentration in its equilibrium with the related nonreactive bis-carbene Pd-0(NHC)(2).