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Abstract
In this communication, we present studies of the oxidative homocoupling of arylboronic acids catalyzed by immobilised palladium nanoparticles in aqueous solution. This reaction is of significant interest because it shares a key transmetallation step with the well-known Suzuki-Miyaura cross-coupling reaction. Additives can have significant effects on catalysis, both in terms of reaction mechanism and recovery of catalytic species, and our aim was to study the effect of added halides on catalytic efficiency and catalyst recovery. Using kinetic studies, we have shown that added halides (added as NaCl and NaBr) can increase the catalytic activity of the palladium nanoparticles more than 10-fold, allowing reactions to be completed in less than half a day at 30 °C. However, this increased activity comes at the expense of catalyst recovery. The results are in agreement with a reaction mechanism in which, under conditions involving high concentrations of chloride or bromide, palladium leaching plays an important role. Considering the evidence for analogous reactions occurring on the surface of palladium nanoparticles under different reaction conditions, we conclude that additives can exert a significant effect on the mechanism of reactions catalyzed by nanoparticles, including switching from a surface reaction to a solution reaction. The possibility of this switch in mechanism may also be the cause for the disagreement on this topic in the literature.
Highlights
We have previously reported the synthesis and characterisation of a polypyrrole-palladium pyrrole (Py) by PdCl2 caused the formation of Pd nanoparticles in a nanocomposite with the nanocomposite immobilised on polystyrene microspheres [25,67]
We have further shown that this system functions as a recoverable catalysta recoverable catalyst for the homocoupling reaction of arylboronic acids, allowing quantification of for the homocoupling reaction of arylboronic acids, allowing quantification of recovery of catalytic recovery of catalytic activity through the determination of reaction activity through the determination of reaction rate constants
Mechanistic proposal for for palladium leaching leaching from nanoparticles nanoparticles and subsequent subsequent catalysis of the oxidative homocoupling reaction of arylboronic acids by dissolved palladium complexes
Summary
The Suzuki-Miyaura cross-coupling reaction (SMXR) is one of the best-known C-C bond-forming reactions. Less well-known is its mechanistic cousin, the oxidative homocoupling reaction (OHR) of arylboronic acids (illustrated for water-soluble reactants and products in Scheme 1). Homocoupling reaction (OHR) of arylboronic acids (illustrated for water‐soluble reactants and products in Scheme 1). The palladium catalysed oxidative homocoupling reaction of 4-carboxyphenylboronic acid. The palladium catalysed oxidative homocoupling reaction of 4‐carboxyphenylboronic acid (1) to form 4,40 -dicarboxybiphenyl (2) and 4-hydroxybenzoic acid (3). Added halides often exert a rate-retarding effect on the SMXR and the OHR. The ability to study the transmetallation step in the absence of the halide-releasing catalytic cycle [6]. The ability to study the transmetallation step in the absence of the halide‐releasing oxidative offers offers opportunities opportunities for for improved improved mechanistic mechanistic understanding.
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