Versus electrophilic amination

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The propensity for C-N versus N-H activation correlates well with substituent Hammet parameters groups that increase the basicity of aniline increase the relative rate of N-H activation, suggesting that nucleophilic attack by the amine at an empty d dy orbital of Ta(silox)3 precedes oxidative addition. On the other hand, electron-withdrawing substituents decrease the rate of N-H activation and increase the rate of C-N activation, similarly to the effects observed on electrophilic aromatic substitution. Arylamine N-H versus C-N activation is therefore a consequence of... 

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