Catalytic Pauson Khand and related cycloaddition reactions

The Pauson Khand reaction is a powerful synthetic tool that yields cyclopentenones by the 2+2+1 cycloaddition of an alkyne, an alkene, and carbon monoxide. The reaction is widely practiced stoichiometrically and requires an equivalent of dicobalt octacarbonyl to effect the cycloaddition. Recent advances in the development of catalysts have extended the application of the reaction. Catalysts include complexes of cobalt, rhodium, iridium, ruthenium, and titanium. With ruthenium catalysts it has been discovered that unsaturated substrates containing heteroatoms, such as imines and aldehydes, can be used in 2+2+1 cycloadditions. Catalysts and proposed mechanisms for the Pauson Khand reaction are reviewed here.