Although this has been mentioned and presented during one of our group meetings by a third-year graduate student who specializes in studying Paulson-Khand, I never bothered to further investigate the reaction until now. So what exactly is the Paulson-Khand reaction? Just a bit of the basics, it’s a [2+2+1] cycloaddition of three components: an alkyne, an alkene, and carbon monoxide to produce an alpha, beta cyclopentenone compound. This is catalyzed by a sacrificial (stoichiometric) transition metal catalyst, dicobalt octacarbonyl ((Co)2(CO)8). The order of reactivity of the alkene is strained cyclic > terminal > disubstituted > trisubstituted »» tetrasubstituted. Four different isomers are possible depending on the substituents on the alkyne and alkene components.
This chemistry is great at making bicyclic or even tricyclic compounds; you commonly start with an enyne substrate to allow the intramolecular Paulson-Khand reaction to occur, which also minimizes the issue of regioselectivity. However, this reaction is quite unpleasant because of the use of metal catalysts with carbon monoxide ligands and carbon monoxide gas…(I wouldn’t want to work with that).