Nickel N‑Heterocyclic Carbene-Catalyzed C–Heteroatom Bond Formation, Reduction, and Oxidation: Reactions and Mechanistic Aspects

The chemistry of nickel N-heterocyclic carbene species has blossomed at the beginning of this century with a particularly rapid acceleration in the last 5 years, as indicated by the fact that more than 65% of the discussed research articles in this comprehensive review date from the period 2010–2015. The rapid evolution of this chemistry has led to an increasing number of applications in the field of catalytic C–heteroatom bond formation, reduction, and oxidation where the heteroatom is nitrogen, sulfur, oxygen, or boron. Thus, in addition to the development of aryl aminations, aryl thiolations, alkyne hydrothiolations, and transfer hydrogenations, which are the most ancient reactions of this type known for these systems, the last five years have seen the appearance of a number of novel interesting Ni­(NHC)-catalyzed transformations such as the dehydrogenative cross-coupling of aldehydes and amines or alcohols, the hydroamination of alkenes, the hydroimination of alkynes, a one-step indoline synthesis, (cross-) Tishchenko reactions, the borylation of unsaturated C–C bonds and arenes, the borylative cleavage of C–N bonds, the hydrosilylation of C–O and C–N multiple bonds, the reductive cleavage of aromatic C–O bonds, and the anaerobic oxidation of alcohols. This rapid expansion calls for a review of the state of the art at this time with a special emphasis on mechanistic data to allow new insights into catalyst improvement.