Imidazol-2-ylidene type NHC ligands

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. 

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

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