Poly-NHCs metal complexes reactions

Cross-coupling reactions were the first reactions in which poly-NHC metal complexes (and NHC metal complexes altogether) were successfitUy employed as catalysts. Considering also the popularity gained by such reactions over the last 20 years, further corroborated by the award of the Nobel Prize in Chemistry for their discovery in 2010, it cannot be deemed surprising that cross-coupling reactions have become the favorite tool to probe the catalytic efficiency of poly-NHC metal complexes, especially palladium(II) complexes. However, in spite of the great number of complexes of this kind that have been evaluated, litde advance has been made in the discovery of more efficient catalytic systems, as poly-NHC complexes... 

Finally, an asymmetric variant of the Suzuki reaction was developed using as catalyst a poly-NHC metal complex with hgand 2 (see also Section 2.1). The complex exhibited a rather low catalytic activity toward aryl iodides and bromides as substrates. However, chiral binaphthyls were prepared with ees up to 70%. ... 

C—H activation/functionalization reactions represent one of the classes of chemical transformations that have been most intensively studied in recent years. Poly-NHC metal complexes have a long-standing tradition of involvement as catalysts in these processes, which can be tracked back to their stability under the relatively harsh reaction conditions that are often needed to drive such reactions to completion in a reasonable time. For example, the group of Strassner reported already in 2002 that paUadium(II) complexes with chelating dicarbene ligands can catalyze the... 

The catalytic performance of NHC-Pd complexes in this reaction is however usually less satisfactory than that of catalytic systems based on other ligands, such as, for example, phosphane hgands, and poly-NHC Pd complexes make no exception to this general trend. Consequently, although reports on the catalysis of the Heck reactions by these complexes have been numerous, they are often hmited to the reaction of aryl iodide or electron-poor, activated aryl bromide substrates, whereas electron-neutral or electron-rich aryl bromides require high temperatures for satisfactory yields. Reactivity of aryl chlorides is confined to electron-poor substrates and is observed only under very drastic conditions (T> 150 °C with tetra-alkylammonium salts as promoters). " An exception is represented by the work of Huynh and Guo on dipalladium(II) complexes with the Janus-type ditz ligand of structure 68." In this case, Heck reactions of activated aryl chlorides were possible already at 120 °C without addition of promoters the authors ascribe this result to the cooperative effect of the two metal centers. Poly-NHC complexes of type 69 have been found to catalyze also the less common diarylation (i.e., double Heck reaction) of ethyl acrylate with aryl bromides, albeit at 120 °C and with tetrabutylammonium bromide as additive. " Simple Pd species such as Pd(OAc)2 and PdCl2 were found to be quite ineffective under these conditions. 

Reports on poly-NHC complexes as catalysts for the Kumada coupling (Eq. 4) have been quite rare, despite the successful results often achieved with mono-NHC metal catalysts for this reaction ... 

At the end of this section, a special mention should be made of the intensive work performed by Peris on the development of poly-NHC-based hetero-bimetallic complexes. In these complexes, the two metal centers are capable of catalyzing two mechanistically distinct reactions on the same substrates, thereupon enabfing overall tandem synthetic process. Several examples have been provided, including Ir/Rh, Ir/Pd, Ir/Pt, and Ru/Pd systems (see also Section 2.3), and several combinations of tandem processes with these metal centers as catalysts. This subject has been thoroughly reviewed in 2014 by... 

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