High-Valent Ni and Ni Species Relevant to C-C and C-Heteroatom Cross-Coupling Reactions: State of the Art.

Ni catalysis constitutes an active research arena with notable applications in diverse fields. By analogy with its parent element palladium, Ni catalysts provide an appealing entry to build molecular complexity via cross-coupling reactions. While Pd catalysts typically involve a M/M redox scenario, in the case of Ni congeners the mechanistic elucidation becomes more challenging due to their innate properties (like enhanced reactivity, propensity to undergo single electron transformations vs. 2e redox sequences or weaker M-Ligand interaction). In recent years, mechanistic studies have demonstrated the participation of high-valent Ni and Ni species in a plethora of cross-coupling events, thus accessing novel synthetic schemes and unprecedented transformations. This comprehensive review collects the main contributions effected within this topic, and focuses on the key role of isolated and/or spectroscopically identified Ni and Ni complexes. Amongst other transformations, the resulting Ni and Ni compounds have efficiently accomplished: C-C and C-heteroatom bond formation C-H bond functionalization; and N-N and C-N cyclizative couplings to forge heterocycles.