Inhibiting C-H Bond Activation

Iridium and ruthenium complexes of N-heterocyclic carbenes are particularly sensitive to decomposition by C-H bond activation of the substituents on nitrogen. As described in Section 3.5.2, this reaction can be used productively if reversible. However, it often leads to degradation and loss of catalytic activity. 

Whenever possible, metalations that produce five-membered rings can be expected over those that producing six-membered rings. In the case of aliphatic activation, the substituent that appears most sensitive to metalation is the t-Bu group, likely because it is unable to escape the coordination sphere of the metal. Conformational issues likely also play a role in the increased stability of longer chain alkyl groups, and metalation of iV-Et groups is often observed whereas VV-propyl substituents are inert.  

The introduction of fluorine substituents instead of hydrogen may be a useful strategy for the synthesis of metalation-resistant NHCs. For example, vV-pentafluorophenylmethylene substituted NHC complexes of Ir and Rh showed no evidence of activation of C-H, C-C or C-F bonds. Grubbs and co-workers also prepared a NHC ligand designed to be resistant to orthome-talation by virtue of the presence of ortho fluorine substituents, and the metathesis catalysts prepared from these ligands displayed increased activity when compared to traditional IMes-based catalysts.  

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