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

作者：Ray Lipika; Barman Samir; Shaikh Mobin M; Ghosh Prasenjit^*

来源：Chemistry - A European Journal, 2008, 14(22): 6646-6655.

DOI：10.1002/chem.200800301

• carbenes
• cross-coupling
• heterocycles
• palladium
• Sonogashira reaction

Two new trans- and cisI(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 rans-[{1-benzyl-3-(3,3-dimethyl-2-oxobutyl)imidazol-2-ylid-ene}(2)PdBr(2)] (3) and cis-[{1-benzyl-3-(Ntert-butylacetamido)imidazol-2-ylid-ene}(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, 3a and 4a, 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 3a and 4a are attributed to more-clectron-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 3a and 4a 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.

• 出版日期2008