Palladium complexes of amido-functionalized N-heterocyclic carbenes as effective precatalysts for the Suzuki–Miyaura C–C cross-coupling reactions of aryl bromides and iodides

A series of air-stable, robust and highly active palladium based precatalysts of amido-functionalized N-heterocyclic carbenes for the Suzuki-Miyaura C-C cross-coupling reaction has been designed. In particular, the [1-R-3-{N-(benzylacetamido)imidazol-2-ylidene](2)PdCl(2) [R = i-Pr (1c) and CH(2)Ph (2c)] complexes efficiently carried out the Suzuki-Miyaura coupling of the aryl bromide and iodide substrates with phenyl boronic acid in good to excellent yields in air at 90 degrees C in 12 h. Quite interestingly, of these palladium precatalysts, the i-propyl derivative (1c) exhibited superior activity as compared to the benzyl derivative (2c). The density functional theory (DFT) studies carried out on the 1c and 2c complexes revealed the strong sigma-donating nature of the NHC ligand as reflected in their high d/b ratio [i.e. forward sigma-donation (d) to backward pi-donation (b)] of these complexes and, thus, point towards greater stability of the Pd-NHC interaction in these complexes. (C) 2009