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Copper-free and amine-free Sonogashira coupling in air in a mixed aqueous medium by palladium complexes of N/O-functionalized N-heterocyclic carbenes
Authors:Manoja K Samantaray  Mobin M Shaikh  Prasenjit Ghosh  
Institution:aDepartment of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India;bNational Single Crystal X-ray Diffraction Facility, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
Abstract:Highly convenient copper-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal acetylenes under amenable conditions in air and in a mixed aqueous medium are reported using several new, user friendly and robust palladium precatalysts (15) of N/O-functionalized N-heterocyclic carbenes (NHCs). In particular, the precatalysts, 1 and 2, were synthesized from the imidazolium chloride salts by the treatment with PdCl2 in pyridine in presence of K2CO3 as a base while the precatalysts, 35, were synthesized from the respective silver complexes by the treatment with (COD)PdCl2. The DFT studies carried out on the 15 complexes suggest the presence of strong NHC–Pd σ-interactions arising out of deeply buried NHC–Pd σ-bonding molecular orbitals (MOs) that account for the inert nature of the metal–carbene bonds and also provide insights into the exceptional stability of these precatalysts.
Keywords:Carbenes  Functionalized NHC  Sonogashira coupling  Organometallic catalysis  Palladium–  NHC complex
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