Cross-couphng catalysis

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,... 

An additional advantage of Freeh s palladium pincer cross-couphng catalyst includes its well-defined fate after catalysis. Treatment of 10 with aqueous hydrochloric acid and thus under work-up conditions led to a rapid and complete catalyst degradation, accompanied by the formation of phosphonate, piperidinium salts, 1,3-diaminobenzene, and other insoluble paUadium-containing products, which were easily separable from the coupling products-an important issue to be considered in particular for pharmaceutical applications. All these advantages make, [ C H3-2,6-(NHP(piperidinyl)2)2 Pd(Cl)] (10) attractive for industrial applications. Indeed, 10 was successfully applied in industry, and nowadays is commercially available. 

Negishi, E. Van Horn, D. E. 1977. Selective carbon-carbon bond formation via transition metal catalysis. 4. A novel approach to cross-couphng exemplified by the nickel-catalyzed reaction of alkenylzirconium derivatives with aryl halides. J. Am. Chem. Soc. 99 3168-3170. 

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