Photochemistry of (m2-alkyne)cobaltcarbonyl complexes and their role in the Pauson-Khand reaction

An important method for the production of cyclopentenones is the Pauson - Khand reaction. First reported in 1973, the reactions most important aspect is carbonyl insertion, obtained by reacting an alkene, an alkyne and carbon monoxide which is most often supplied by octacarbonyl dicobalt. However, for many reasons the investigation of the corresponding mechanism is difficult, so that it has remained a subject of intense discussion. This report outlines an investigation of the use of photochemical techniques to generate possible intermediates in order to elucidate the mechanism of the reaction. We have shown that the early stages of such processes can be investigated with the aid of the laser flash photolysis technique, which allows the UV spectroscopic identification of intermediates. Studies on various alkyne dicobalthexacarbonyl complexes have revealed a wavelength dependent photochemistry. Results following high energy photolysis (Xexc = 355nm) have been interpreted in terms of a reversible homolytic cleavage of the Co - Co bond while low energy photolysis (A,exc = 532nm) produces CO loss. Primary photoproducts involved in the reaction of these complexes with coordinating ligands were identified and characterised. Kinetic data for the reaction of these complexes with CO and ligands were obtained. Time resolved IR has been used to investigate the photochemical reactions of alkyne hexacarbonyldicobalt complexes with various olefins. Visible light irradiation and 1 atmosphere of CO has been shown to be sufficient to generate cyclopentenones in a photolytic Pauson - Khand reaction from these alkyne hexacarbonyldicobalt complexes and olefins. This work demonstrates the need for correct selection of excitation wavelength in photo-induced Pauson - Khand reactions