The Pauson-Khand Reaction is a [2+2+1] cycloaddition of an alkyne, an alkene and carbon monoxide.
Mechanism of the Pauson-Khand Reaction
The following mechanism is postulated, although only the stable alkyne Co2(CO)6 complex has been isolated. (P. Magnus, Tetrahedron Lett., 1985, 26, 4851. DOI)
The stereochemistry of the complexation of the alkene at cobalt is guided by steric repulsions between the R and R' groups, so that isomers 1 and 2 are favored.
The insertion of the alkene is followed by insertion of carbon monoxide and reductive elimination of one Co unit:
Dissociation of the second Co unit gives the resulting cyclopentenone product:
Effect of Lithium Chloride on Tuning the Reactivity of Pauson-Khand Reactions Catalyzed by Palladium-Tetramethylthiourea
L.-J. Deng, J. Liu, J.-Q. Huang, Y. Hud, M. Chen, Y. Lan, J.-H. Chen, A. Lei, Z. Yang, Synthesis, 2007, 2565-2570.