NHC-palladium complexes in catalysis

Carbon arbon cross-coupling reactions dominate catalytic applications of NHC Pd complexes, demonstrating the importance of this class of reaction and the unique success of Pd compounds in these transformations.In the 

N-Heterocyclic Carbenes From Laboratory Curiosities to Efficient Synthetic Tools 

The Mizoroki Heck reaction (Equation (9.1)) is a valuable synthetic tool for the catalytic formation of a range of aryl-olefins, and other related building blocks, widely used in the pharmaceutical and fine chemicals industries. Many Pd-based compounds are known to catalyse the reaction to varying extents. In the past 10 12 years, NHC-based systems were added to the list, and seemed to provide some advantages with respect to other catalysts. 

Several studies reported the application of established five-membered ring (imidazol-2-ylidene type) NHC ligands in Heck chemistry. Whilst most authors generally focus on catalytic activity, Jutand and co-workers examined the mechanism of oxidative addition of aryl halide to NHC-Pd complexes. Complexes [Pd L2] (L = saturated NHC) were generated electrochemically and tested in situ for oxidative addition. They proposed that either an associative or dissociative mechanism could operate, depending on steric and electronic properties of the NHC ligand. 

The Hahn group also investigated the catalytic behaviour of phosphine- and picoline-functionalised benzimidazol-2-ylidene-Pd complexes, 24-26. Complexes 24 (R = Et, -Pr or -Bu) and 25 catalysed the coupling of 4-bromo-benzaldehyde with styrene in good yield (1 mol% [Pd], 110°C, 2h), but 25 

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