Nickel and Platinum NHC Catalysts

Herrmann and co-workers reported the activation of aryl C-F bonds by [(NHC)Ni] systems under Kumada-Corriu-type coupling conditions. Screening of catalytic conditions revealed that yields with [(IPr)2Ni] were poorer than when the catalyst was generated in situ from [Ni(acac)2]/IPr-HBF4. Interestingly, the electronic influence of the aryl fluorides on the reaction rate as demonstrated by Hammett ct -values indicated different pathways for the NHC-modifled catalytic system versus the analogous Ni-phosphine systems. 

Ni-catalyzed aryl aminations involving NHC ligands are best performed at a Ni/NHC ratio of 1 1. To ensure the efficient generation of Ni species in situ from the Ni° precursor, NaH/t-BuOH were employed in THF. Additional NaOt-Bu generated with NaH served to generate the free carbene from the imidazolium chloride. Isolated [(NHC)2Ni°] species demonstrated better activity in this reaction with certain substrates, leading the authors of the study to conclude that these are very close to the active species in solution.  

The use of NHCs as ancillary ligands in iridium-catalyzed Oppenauer-type oxidation of alcohols to carbonyls has led to some of the most active catalysts for this class of transformation. In 2005, Yamaguchi and co-workers reported the synthesis of a number of [(Cp )Ir] complexes featuring NHCs as the ancillary ligands.In addition to neutral complexes of the formula 

Wasserscheid and T. Welton, Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, 2007 (b) T. Welton, Chem. Rev., 1999,99, 2071-2084. 

Kaufhold, A. Flores-Figueroa, T. Pape and F. E. Hahn, Organome-tallics, 2009, 28, 896-901. 

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