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

Two new trans ‐ and cis ‐[(NHC) 2 PdX 2 ] (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} 2 PdBr 2 ] ( 3 ) and cis ‐[{1‐benzyl‐3‐( N ‐ tert ‐butylacetamido)imidazol‐2‐ylidene} 2 PdCl 2 ] ( 4 ) complexes effectively catalyzed the Sonogashira cross‐coupling reaction of aryl iodides with substituted acetylenes in air in a mixed solvent (DMF/H 2 O, 3:1 v/v) under amine‐free conditions. Interestingly, these trans ‐ and cis ‐[(NHC) 2 PdX 2 ] (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)X 2 ] (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)PdCl 2 ], whereas the corresponding PEPPSI analogues 3 a and 4 a were obtained directly from the imidazolium halide salts by reaction with PdCl 2 in pyridine in the presence of K 2 CO 3 as base.