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

✍ Scribed by Jutand, Anny (author);Pytkowicz, Julien (author);Roland, Sylvain (author);Mangeney, Pierre (author)

• Book ID: 120157433
• Publisher: Walter de Gruyter GmbH
• Year: 2010
• Tongue: English
• Weight: 400 KB
• Volume: 82
• Category: Article
• ISSN: 0033-4545
• DOI: 10.1351/PAC-CON-09-09-22

✦ Synopsis

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 PdY~2~(NHC)~2~ (Y = halides) after a chemical reduction. The latter procedure can be mimicked by an electrochemical reduction. The transient Pd^0^(NHC~Bn~)~2~ is generated by electrochemical reduction of PdY~2~(NHC~Bn~)~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^(NHC~Bn~)~2~ is found to be more reactive than the mixed complex Pd^0^(NHC~Bn~)(PPh~3~). Both are the reactive species in an associative mechanism. Comparison with the isolated Pd^0^(NHC~__t__Bu~)~2~ reveals that Pd^0^(NHC~Bn~)~2~ is more reactive than Pd^0^(NHC~__t__Bu~)~2~ even if the latter reacts via the mono-carbene Pd^0^(NHC~__t__Bu~) 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~.