The Pauson-Khand Reaction: A Gas-Phase and Solution-Phase Examination Using Electrospray Ionization Mass Spectrometry

A series of dicobalt hexacarbonyl complexes with charged alkyne ligands were prepared to enable the study of the Pauson–Khand reaction using ESI-MS. The hexacarbonyl complexes can be activated in the gas phase through removal of a CO ligand. The resulting pentacarbonyl ions react readily with alkenes, and no discrimination between alkenes was found for this step, indicating that alkene association is not rate determining in the intermolecular reaction. Solution-phase ESI-MS studies on a system set up for intramolecular reactivity revealed only the hexacarbonyl complex as a detectable intermediate, and the reaction was shown to have a large enthalpy and entropy of activation, consistent with ligand dissociation being rate limiting in the reaction.