Nickel coupling mechanistic considerations

Following the early developments in nickel-catalyzed cycloadditions and cross-coupling processes, continued sophistication has been demonstrated over the years, and recent developments have elevated these classes of transformations into robust and useful catalytic processes. The recent burst of activity in these areas can be attributed to a better appreciation of both ligand effects and mechanistic considerations. Additionally, recent developments in reductive couplings, carbonyl additions, and C-H and C-C functionalization processes have demonstrated new reactivity trends and have suggested many new directions for continued development. Based on the current trajectory and recent fundamental findings, one can expect increased use of the types of reactions described herein as well as the discovery of many types of entirely new processes. 

As for mechanistic considerations of homogeneous zerovalent nickel-induced biaryl synthesis, several possibilities have been discussed. Semmelhack et al. [126] suggested that the coupling of aryl halides using bis(l,5-cyclooctadiene)-nickel(0) in DMF occurs as in Scheme 7.4. 

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