Insertion of Bulky Substituent to Control the Reactivity

It is accepted that this reaction involves the formation of the alkynedicobalt hexacarbonyl complex from an alkyne and Co2(CO)s by the evolving of two CO ligands, followed by the alkene coordination at one of the two enantiotopic Co atoms with concomitant CO insertion, and final reductive elimination of the metal to an a,(3-unsaturated cyclopentenone. In the traditional protocol, the reaction mixture is heated in toluene at 110°C, or tertiary amine A-oxides are added to promote the reaction at ambient temperature. For the purpose of stereochemical control, many Pauson-Khand reactions are designed as intramolecular reactions P " or using cyclic alkenes, such as norbornene. It has been found that the reactivity of cyclic alkenes is in the order of cyclohexene < cyclopentene < norbornene. For the intermolecular Pauson-Khand reaction, alkene is positioned adjacent to the less bulky acetylenic substituent during coordination because of steric hindrance, and a subsequent C-C bond forms between an alkenic... 

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