C-S Cross-Coupling Reactions Catalyzed by Well-Defined Copper and Nickel Complexes

This review presents well-defined copper and nickel complexes that have been used in the formation of C-S bonds. The use of Cu or Ni catalysts represent an advantage in comparison with those based on precious metals such as Pd, by being two of the most abundant metals in the earth's crust and thus cheaper. As can be expected the catalytic activity of the different Cu and Ni complexes is strongly dependent on the nature of the ligands used. Thus, multidentate and strong electron-donating ligands are of common use to design highly active species since such ligands are capable to stabilize species in high oxidation states which are key intermediates in the reaction mechanisms of these processes. This being particular true in the case of copper, where Cu(III) produces unstable and reactive species that require an "extra stabilization" during the reaction mechanism. Hence, the reaction mechanisms using copper and nickel complexes as catalysts are also discussed in this paper, including the role of the different ligands during the catalytic processes. Although there is an increasing number of reports on C-S cross coupling reactions, due to the relevance of this transformation, they are often difficult to reproduce and not of general use since the actual catalytic species are not identified. Thus, we hope that this report will help to promote the search and synthesis of new ligands for the design of more active well-defined complexes that can be used as catalysts in a more rational manner in the formation of valuable C-S bond-containing species.