Cobalt‐Catalyzed [2 + 2 + 2] Cycloaddition Reactions

Abstract

Cobalt‐catalyzed [2 + 2 + 2] cycloaddition reactions are an important tool for the assembly of carbo‐ and hetero‐cyclic ring systems from simple precursor molecules such as alkynes, alkenes, nitriles, and other heterocumulenes such as isocyanates. Cyclopentadienyl (Cp)‐cobalt complexes are still one of the most important classes of catalysts, but in situ generated cobalt catalyst systems are becoming increasingly important for the assembly of achiral as well as chiral target molecules from a large variety of substrates. Substrates comprise simple terminal as well as internal alkynes, diynes, and higher oligoynes and also allow target‐oriented syntheses of unusually large as well as highly substituted molecules as well as natural product synthesis with a cyclotrimerization as the key step of the synthetic sequence. The utilization of olefins as cyclization partners allows the synthesis of products with lesser degree of unsaturation such as cyclohexadienes, cyclohexenes, and dihydropyridines, just to name a view examples. Chiral catalyst systems allow the enantioselective synthesis of biaryls and products with central chirality, and chiral starting materials allow the synthesis of diastereomeric helicenes in an efficient manner. Novel catalysts and reaction engineering allow reaction conditions such as conventional thermal or microwave heating, photochemically assisted cyclizations, and the use of plug‐flow reactors.