Cobalt-Catalyzed Cross-Coupling Reactions of Aryl Halides

Transition metal-catalyzed C–C cross-coupling reactions are among the most powerful transformations known to organic chemists, and have received considerable attention over the last decades. For this reason, the development of mild chemo-, regio-, and stereoselective syntheses for the formation of carbon–carbon bonds catalyzed by metal complexes is of great interest. These methods have had a profound impact on the synthesis of natural products, supramolecular complexes, and biologicallyactive compounds. Classically, most approaches require the use of an organometallic reagent such as organomagnesium, organoboron, organostananes, or organozinc reagents in conjunction with an organic or pseudo halide. The most common implementations of this approach use palladium and nickel complexes, which provide excellent compatibility with many functional groups. However, these common catalysts have some disadvantages, such as the high cost of palladium and the high toxicity of nickel catalysts. The development of broadly applicable, more economical catalysts for cross-coupling is an increasingly important goal in modern synthetic organic chemistry. Fortunately, inexpensive alternative catalysts are available in the form of simple, effective iron and cobalt salts. Although there have been elegant developments in iron-based catalysis, cobalt catalysts can sometimes present a higher reactivity for various carbon– carbon bond-forming reactions. The use of this catalyst for cross-coupling reactions has recently been shown, mainly by Cahiez, Knochel, Oshima, Cheng, and our group; their use for ring formation reactions has also been shown by Vollhardt, Hilt, and Aubert and Malacria. Cobalt-catalyzed carbon–carbon bond-forming reactions have received particular attention since the pioneering work of Kharasch on the metal-catalyzed homocoupling reaction of aromatic Grignard reagents in the middle of the 20th century. The generally mild reaction conditions, high functionalgroup tolerance, and broad availability of cobalt halides contribute to the growing success of this catalyst. The present microreview summarizes our progress over the last few years in cobalt-catalyzed cross-coupling reactions using aryl compounds. In this account, we review our achievements in cobalt-catalyzed reactions involving a functionalized stoichiometric organometallic compound especially an arylzinc species. We also present novel carbon– carbon bond formation reactions mediated by catalytic organocobalt reagents.