The Pauson-Khand Reaction

The transition metal-catalyzed [2+2+1] cycloaddition of alkynes, alkenes, and carbon monoxide leading to the cyclopentenone skeleton, commonly called the Pauson-Khand reaction when dicobalt octacarbonyl is used as the promoter, has become an indispensible synthetic tool in organic chemistry. This chapter will first describe the characteristic features and scope of the original Pauson-Khand reaction together with its limitations. Then, the following efforts to complement the original protocol will be discussed using promoters or catalysts based on either cobalt or other metals. It includes the efforts to improve the reaction conditions to expand the substrate scope and to develop catalytic- and catalytic-enantioselective versions. For the demonstration of its power in synthesis, applications of the reaction to the synthesis of natural and/or unnatural products over the decades are reviewed.