Pd-based heterogeneous cataysts for Suzuki coupling

(eng) The Suzuki coupling is a crucial reaction for the preparation of many high value compounds, especially for the synthesis of pharmaceuticals. Pd-based heterogeneous catalysts are highly regarded for achieving this reaction. However, the nature of the active phase for this type of catalysts in the Suzuki reaction has not yet been well elucidated and investigations on these subjects lead to contradictory conclusions. Furthermore, the knowledge of the Suzuki coupling mechanism with this type of catalysts is relatively limited. A systematic investigation of a large variety of Pd-based heterogeneous catalysts in the Suzuki coupling has permitted to suggest that we are in the presence of a real heterogeneous catalysis, homogeneous one is not excluded. The active species is Pd2+. It has also been determined that the optimal Pd loading is about 5 wt. % and that a high specific surface area and micropores bring nothing more to the catalytic activity. The number of acid sites at the catalyst surface is a key point to improve the performances in the Suzuki coupling. More precisely, an optimum of the number of weak acid sites allows increasing the reaction rate. These weak acid sites likely participate in the adsorption of the activated phenylboronic acid. Based on a kinetic study, the transmetallation of the activated phenylboronic acid on an intermediate species formed by the adsorption of the 4-bromotoluene onto the Pd, has been identified as the rate limiting step of the Suzuki coupling. So, for improving the catalytic performances, the rate of the transmetallation step has to be increased. Finally, according to the whole results obtained during this PhD thesis, a realistic mechanism of the Suzuki coupling reaction with a heterogeneous catalyst has been proposed.