Three characteristic reactions of organocobalt compounds in organic synthesis

Organocobalt compounds in organic synthesis have three characteristic reactions. The first occurs because cobalt has a high affinity to carbon–carbon π‐bonds or carbon–nitrogen π‐bonds. The second occurs because cobalt has a high affinity to carbonyl groups. The third is due to cobalt easily tending to form square‐planar bipyramidal six‐coordination structures with four nitrogen atoms or two nitrogen atoms and two oxygen atoms at the square‐planar position, and to bond with one or two carbon atoms at the axial position. The first characteristic reactions are the representative reactions of organocobalt compounds with a mutually bridged bond between the two π‐bonds of acetylene and the cobalt–cobalt bond of hexacarbonyldicobalt. These are reactions with a Co₂(CO)₆ protecting group to reactive acetylene bond, the Nicholas reactions, the Pauson–Khand reactions (2 + 2 + 1] cyclizations), 2 + 2 + 2] cyclizations, etc. These reactions are applied for the syntheses of many kinds of pharmaceutically useful compounds. The second reactions are carbonylations that have been used or developed as industrial processes such as hydroformylation for the manufacture of isononylaldehyde, and carbonylation for the production of phenylacetic acid from benzyl chloride. The third reactions are those reactions with the B₁₂‐type catalysts, and they have recently been used in organic syntheses and are utilized as catalysts for stereoselective syntheses. These reactions have been used as new applications for organic syntheses. Copyright © 2007 John Wiley & Sons, Ltd.