Title : Innovative flow reactor to study the nature of active species in Suzuki Miyaura reaction of iodoacetophenone with phenylboronic acid in ethanol
The Suzuki-Miyaura reaction (SMR) and more generally the carbon-carbon (C-C) coupling reactions catalyzed by Pd [1, 2] have been the subject of numerous and prolific research in organic chemistry catalysis. Indeed, it was crowned by the awarding of the Nobel Prize in Chemistry to Richard Heck, Ei- ichi Negishi and Akira Suzuki in 2010.
Several studies have been conducted to determine the nature and origin of the active species in the mechanism of this reaction when using a heterogeneous catalyst based on palladium but, this question remains open. Some authors postulating a purely heterogeneous mechanism and others a homogeneous mechanism in which a catalytically active molecular complex is formed from the palladium nanoparticle. Moreover, the understanding of this mechanism will help to revolutionize the implementation of the SMR reaction which is widely used industrially.
An innovative flow reactor was used for the determination of the nature of the mechanism. The idea is to build a column filled with a heterogeneous catalytic precursor, and add a tubular reactor just after the column. The catalytic solid is only in the column. So, if the mechanism is heterogeneous, the conversion at the outlet of the tubular reactor must be equal to that at the outlet of the column. ICP-MS is used to estimate the amount of leached palladium.
The results showed a large homogeneous contribution in the tubular reactor which confirms the formation of homogeneous catalytic species from supported Pd. Modeling on MATLAB©,using two different homogeneous and heterogeneous models has permitted to demonstrate that the catalytic activity in the tube is equal to that in the column, and that only the leached Pd is catalytic (no or negligible heterogeneous contribution in the column).
ICP-MS analysis has shown that a very small amount of palladium (0.1ppm) can catalyze the Suzuki reaction under mild conditions (T= 60 ° C). This amount of Pd corresponds to a TOF ≥ 300,000 h-1 in the tubular reactor.
In this work an innovative continuous reactor was used to verify the nature of the active species that catalyzes the Suzuki reaction of iodoacetophenone with phenylboronic acid, using three commercial catalysts known in the literature to have a heterogeneous mechanism. The obtained results demonstrate that these heterogeneous precursors of Pd are in fact a production sources of highly active species in a homogeneous medium that catalyze the SMR reaction of iodoacetophenone without the catalytic contribution of palladium supported on the solid. Albeit this demonstration is only valid for the reagents used, i.e. iodoacetophenone, future work is devoted to extend it by studying other reagents based on bromides and aryl chlorides which are less active than iodo-aryls.