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Author(s): Ray L; Barman S; Shaikh MM; Ghosh P
Title: Highly convenient amine-free Sonogashira coupling in air in a polar mixed aqueous medium by trans-and cis-[(NHC)(2)PdX2] (X = Cl, Br) complexes of N/O-functionalized N-heterocyclic carbenes
Source: CHEMISTRY-A EUROPEAN JOURNAL 14 (22): 6646-6655
Date: 2008
Document Type: Journal : Article
DOI:
Language: English
Comment:
Address: Indian Inst Technol, Dept Chem, Bombay 400076, Maharashtra, India.
Indian Inst Technol, Natl Single Crystal Xray Diffract Facil, Bombay 400076, Maharashtra, India. Reprint: Ghosh, P, Indian Inst Technol, Dept Chem, Bombay 400076, Maharashtra,
India. E-mail: pghosh@chem.iitb.ac.in
Author Keywords: carbenes; cross-coupling; heterocycles; palladium; Sonogashira reaction
KeyWords Plus: RING-OPENING POLYMERIZATION; PHENYLENE ETHYNYLENE OLIGOMERS; AB-INITIO
PSEUDOPOTENTIALS; COPPER-FREE; PALLADIUM COMPLEXES; ARYL BROMIDES;
PHOTOPHYSICAL PROPERTIES; TERMINAL ACETYLENES; TRANSITION-ELEMENTS;
ROOM-TEMPERATURE
Abstract: Two new trans-and cisI(NHC)(2)PdX2] (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)PdBr2] (3) and cis-[{1-benzyl-3-(Ntert-butylacetamido)imidazol-2-ylid-ene}(2)PdCl2] (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)(2)PdX2] (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)PdCl2], whereas the corresponding PEPPSI analogues 3a and 4a 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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