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Highly Convenient Amine-Free Sonogashira Coupling in Air in a Polar Mixed Aqueous Medium by trans- and cis-[(NHC)2PdX2] (X=Cl, Br) Complexes of N/O-Functionalized N-Heterocyclic Carbenes



Two new trans- and cis-[(NHC)2PdX2] (X=Cl, Br) complexes of N/O-functionalized N-heterocyclic carbenes employed in a highly convenient amine-free Sonogashira cross-coupling reaction in air in a polar mixed aqueous medium are reported. Specifically, the trans-[{1-benzyl-3-(3,3-dimethyl-2-oxobutyl)imidazol-2-ylidene}2PdBr2] (3) and cis-[{1-benzyl-3-(N-tert-butylacetamido)imidazol-2-ylidene}2PdCl2] (4) complexes effectively catalyzed the Sonogashira cross-coupling reaction of aryl iodides with substituted acetylenes in air in a mixed solvent (DMF/H2O, 3:1 v/v) under amine-free conditions. Interestingly, these trans- and cis-[(NHC)2PdX2] (X=Cl, Br) complexes, with two N-heterocyclic carbene ligands, exhibited superior activity compared with the now popular PEPPSI (pyridine enhanced precatalyst preparation, stabilization and initiation)-themed analogues, trans-[(NHC)Pd(pyridine)X2] (X=Cl, Br), 3 a and 4 a, with one N-heterocyclic carbene ligand and a “throw away” pyridine ligand in a trans disposition to each other. The higher activities of 3 and 4 compared with PEPPSI analogues 3 a and 4 a are attributed to more-electron-rich metal centers, as revealed by DFT studies, in the former complexes and is in concurrence with a more electron-rich metal center being effective in facilitating the oxidative addition of aryl halide, often a rate-determining step in palladium-mediated cross-coupling reactions. Complexes 3 and 4 were prepared from the corresponding silver analogues by transmetalation with [(cod)PdCl2], whereas the corresponding PEPPSI analogues 3 a and 4 a were obtained directly from the imidazolium halide salts by reaction with PdCl2 in pyridine in the presence of K2CO3 as base.

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