Catalysis, by pincer complexes

Catalysis by Pincer Complexes Synthesis of Esters, Amides, and Peptides... 

The wide apphcation of NHCs to catalysis places them along with phosphines and cyclopentadienyls in their utility for organometaUic catalysis. Being much more donor in nature than phosphines, the NHCs occupy a distinct region of the Tolman ligand map [32]. A number of pincer NHC derivatives are catalyticaUy active, but are not discussed here because they have been fully described elsewhere [33]. Some other important recent reviews discuss catalysis by NHC complexes in general [10, 34, 35]. 

Reductive elimination reactions are responsible for the final C-C/C-X bond formation step in catalytic processes. In principle, C-bound or any other anionic pincer ligands could engage in reductive couphng reactions, bringing about the decomposition of the pincer complex. In order to enable catalysis by such complexes, it is mandatory to avoid reductive elimination reactions involving the pincer ligand itself Fortunately, this seems to be usually the case. For example, reductive elimination of ethane from the Pd(IV) pincer complex 18 (Scheme 2.3) is preferred... 

C-C, C-O, and C-B Bond Formation by Pincer Complexes Including Asymmetric Catalysis... 

To study the effect of the chiral backbone on the behavior of the NCN-pincer complex 4 in catalysis, the Michael addition of methyl vinyl ketone to (R/S)-ethyl a-isocyanopropionate was used as a model reaction (Scheme 3). The results of the experiment show 80% conversion after 24 h which is higher than the 38% conversion obtained in the same Michael addition without catalyst. After full conversion, the product and loaded nanocapsules could be recovered separately in almost quantitative yields (> 96%) by dialysis. Product analysis revealed that no enantiomeric excess was found and racemic mixtures were obtained (ee = 0%) (Scheme 3). 

For aU the catalytic systems discussed so far, the mechanistic details are not well understood. It is generally agreed that oxidation of the nickel center should be a critical step in nickel-catalyzed cross-couphng reactions, and therefore an electron-rich metal center would be ideal for catalysis. As argued by some of the studies [20, 23], facile dissociation of one of the pincer arms could be equally important. Degradation of Ni(II) pincer complexes to the more active Ni(0) species is also possible, and may be necessary, although there is no definitive evidence suggesting that this is the case for any of the reactions mentioned above. Moreover,... 

In 2003 The Uses of Pincer Complexes in Organic Synthesis, by Singleton [30] In 2004, PaUadacycles in Catalysis—a Critical Survey, by Beletskaya and Cheprakov [105]... 

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