Suzuki couplings have proven useful for the preparation of a wide variety of biologically active compounds including functionalized biphenyls. Among all catalysts used for Suzuki coupling reactions, palladium on carbon (Pd/C) is most favorable for industrial applications, because it provides very high catalytic activity and is a heterogeneous catalyst, thus easily removed from the reaction mixture. However, recent studies showed that Pd/C is not a truly heterogeneous catalyst. Instead, palladium leaches from the carbon support and after the reaction is completed, re-adsorbs back on the carbon support. Although Pd-leaching was observed, a detailed understanding of the Pd-leaching mechanism is still lacking, which may help to minimize leaching. Therefore, the objective of this work is to gain a better understanding of the Pd-leaching mechanism for Pd/C-catalyzed Suzuki coupling reactions.
In this work, we first investigated the effects of reaction solvents, bases and aryl-halides on Pd-leaching. Through a filtration test and PVPy analysis, it was found that Pd-leaching is a necessary for Pd-catalyzed aryl-Suzuki coupling reactions. Upon further investigation, oxidation addition of aryl-bromide was found to be the main cause for Pd-leaching. With the help of reactor calorimetric (ASI/Mettler-Toledo RC1), we also proposed an empirical equation to correlate the initial Pd concentrations in the reaction mixture to the initial reaction rates, using the model coupling reaction of biphenylacetic acid.
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