Chiral diaminocarbene palladium(II) complexes: synthesis, reduction to Pd(0) and activity in the Mizoroki–Heck reaction as recyclable catalysts

A preparation of chiral palladium(II) bis-diaminocarbene complexes was developed from the corresponding silver(I) diaminocarbenes and bis(acetonitrile)dichloropalladium(II). Crystal structure details of trans-diiodo bis[(4R,5R)-1,3-dibenzyl-4,5-di-tert-butylimidazolin-2-ylidene] palladium(II) are presented. Mixed diaminocarbene–phosphine complexes were also prepared in two steps from the corresponding imidazolidinium salts and palladium acetate. The very high air, moisture and thermal stability of such complexes were confirmed. These two families of complexes were tested in the Mizoroki–Heck reaction. Their ability to be reduced into Pd(0) which undergoes oxidative addition with aryl halides was studied by cyclic voltammetry. The ability of different agents to generate Pd(0) complexes and the reactivity of these latters with aryl halides were also studied in situ. The obtained results allowed us to perform Heck reactions at temperatures starting from 40 °C in DMF, by activation of the palladium(II) catalyst precursor. Moreover, in the case of the bis-diaminocarbene complexes, the Pd(0) catalysts were totally re-oxidized during the work-up and the starting Pd(II) complexes could be quantitatively recovered.