Kinetic studies on the cobalt-catalyzed norbornadiene intermolecular pauson-khand reaction

The kinetics for the cobalt-catalyzed intermolecular Pauson-Khand reaction (PKR) between (trimethylsilyl)acetylene and norbornadiene (NBD) at a constant CO pressure has been studied by means of in situ FT-IR. The rate dependence on catalyst and substrate concentrations was examined, and it was found that the process is -1.9 order with respect to CO pressure, zero order with respect to acetylene, 0.3-1.2 order with respect to NBD, and 1.3 order with respect to the Co-2(CO)(8) catalyst. Catalytic reaction intermediates were examined by their corresponding metal carbonyl IR frequencies. By a one-pot consecutive Pauson-Khand experiment, the NBD-dicobalt hexacarbonyl complex was identified as a catalytically active complex. Co-4(CO)(12) was also studied as a catalyst source in the PKR. Analysis of the corresponding reaction intermediates by IR demonstrated that Co-2(CO)(8) and Co-4(CO)(12) provide identical intermediate profiles upon reaction with TMSC2H. The experimental measured kinetics are consistent with the alkene insertion being the rate-limiting step in the catalytic PKR. Finally, the effect of phosphine substitution on the catalyst and the use of Lewis acid additives were shown to have a deleterious effect on the reaction rate.