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 Co2(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. Co4(CO)12 was also studied as a catalyst source in the PKR. Analysis of the corresponding reaction intermediates by IR demonstrated that Co2(CO)8 and Co4(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.