User:Mdlevin/sandbox2

The Pauson–Khand reaction (or PKR or PK-type reaction) is a chemical reaction described as a [ 2+2+1 ] cycloaddition between an alkyne, an alkene and carbon monoxide to form a α,β-cyclopentenone. This reaction was originally mediated by stoichiometric amounts of dicobalt octacarbonyl, but this has since been replaced by newer and more efficient catalyst systems.



With asymmetrical alkenes or alkynes regioselectivity is always an issue, but less so with intramolecular reactions.



The reaction works with both terminal and internal alkynes although internal alkynes tend to give lower yields. The order of reactivity for the alkene is strained cyclic alkene > terminal alkene > disubstituted alkene > trisubstituted alkene. Unsuitable alkenes are tetrasubstituted alkenes and alkenes with strongly electron withdrawing groups.

Variations
Wilkinson's catalyst, based on the transition metal rhodium, also effectively catalyses PK reactions but requires silver triflate as a co-catalyst.



Molybdenum hexacarbonyl is a carbon monoxide donor in PK-type reactions between allenes and alkynes with dimethyl sulfoxide in toluene.



Cyclobutadiene also lends itself to a [ 2+2+1 ] cycloaddition although this reactant is generated in situ from decomplexation of stable cyclobutadiene iron tricarbonyl with CAN.